EQEP-2018 Main Topics and Organizerseqep-workshop.eu/wp-content/uploads/2018/11/Book... · in quantum dot spatial positions after growth, location techniques that enable the creation

EQEP-2018 Main Topics and Organizers

The aim of the 6th workshop on “Engineering of Quantum Emitter Properties” is to create a

forum to discuss challenges and opportunities for the exploitation of sources of non-classical

light in quantum information science and technology. It addresses topics that range from

single and entangled photon generation and detection, engineering of quantum emitter

properties via light-matter interaction and external perturbations, hybrid natural-artificial

atomic interfaces.

Topics:

Quantum sources of non-classical light

Quantum photonics and circuitry

Long-distance quantum communication

Tailoring the properties of quantum light

2D materials

Quantum information

Organization Committe:

Rinaldo Trotta, Sapienza University of Rome

Simone Luca Portalupi, University of Stuttgart

Klaus Jöns, KTH Stockholm

Armando Rastelli, Johannes Kepler University Linz

Local Organization:

Rinaldo Trotta and Alba Perrotta,

Department of Physics, Sapienza University of Rome

EQEP-2018 Sponsors

The EQEP2018 workshop has received funding from:

The European Research Council (ERC) under the European

Union’s Horizon 2020 research and innovation programme,

project SPQRel - Entanglement distribution via

Semiconductor- Piezoelectric Quantum-Dot Relays, Grant

Agreement No. 679183.

The IQST (Integrated Quantum

Science and Technology) center,

financially supported by the Ministry of

Science, Research and Arts Baden-

Württemberg.

The QuantERA ERA-NET Cofund in Quantum

Technologies implemented within the European

Union's Horizon 2020 Programme, project HYPER-

U-P-S, Hyper-entanglement from ultra-bright photon

pair sources.

Department of Physics, Sapienza University of

Rome.

EQEP-2018 Venue and Dinner

The workshop will be held in the Rectorate Building of the Sapienza University of Rome,

Piazzale Aldo Moro 5, 00185 Roma. The building is located at the center of the university

campus (see map and photo below). It is close to the main railway station Roma Termini (15

minutes walking distance), which is linked to Fiumicino airport by bus or train and to

Ciampino airport by bus.

All the talks will take place at the Sala Organi Collegiali in the Rectorate Building, located at

the ground floor of the building. Coffee breaks and lunches will be served in the area in front

of the lecture hall.

The social dinner on Thursday 6th December will be hosted at the “Casa dell'Aviatore”,

located in the Italian Air Force headquarters close to the main entrance of the University

campus.

EQEP-2018 Accommodation

The invited speakers will stay at the Best Western Globus Hotel in Viale Ippocrate 119,

00161 Roma (+39 06 4457001; [email protected]).

The hotel can be conveniently reached

from the main railway station Roma

Termini. Take the Bus 310 to Viale

Ippocrate, or the Metro Line B, direction

Rebibbia or Jonio, and stop at

Policlinico. The hotel is within walking

distance of the workshop venue.

EQEP-2018 Scientific Program – Talks – December 6th

08:45 - 09:00 Opening

09:00 - 09:30 Peter Michler

On-chip quantum photonics on a GaAs platform

09:30 - 10:00 Kartik Srinivasan

Integrated quantum photonics with single, optically-located

quantum dots

10:00 - 10:30 Christian Schimpf

All-photonic quantum teleportation using on-demand solid-state

quantum emitters

10:30 - 11:00 Coffee break

11:00 - 11:30 Niels Gregersen

Designing Single-Photon Sources: Towards Unity

11:30 - 12:00 Jean-Michel Gerard

Nanowire single-photon sources: Mechanics matters

12:00 - 12:30 Pascale Senellart

Generation of quantum light in a photon-number superposition

12:30 - 13:30 Lunch

13:30 - 14:30 Poster session

14:30 - 15:00 Fabio Sciarrino

Machine learning for certification of photonic quantum

information

15:00 - 15:30 Giuseppe Vallone

Long-distance quantum communication in Space

15:30 - 16:00 Eden Figueroa

Building scalable room temperature quantum technologies

16:00 - 16:30 Costanza Toninelli

Organic molecules for quantum technologies



17:00 - 17:30 Richard Warburton

A quantum dot exciton deep in the strong coupling regime of

cavity-QED

17:30 - 18:00 Jonathan Finley

Quantum emission dynamics and hole-spin dephasing in single

quantum dots

18:00 - 18:30 Christoph Becher

Spin properties and quantum control of group-IV vacancy

centers in diamond

20:00 Social dinner at “Casa dell’Aviatore”


09:00 - 09:30 Brian Gerardot

Coulomb blockade in atomically thin quantum dots

09:30 - 10:00 Christian Schneider

Light-matter coupling with atomically thin WSe2

10:00 - 10:30 Marco Felici

Hydrogen-assisted fabrication of site-controlled light-emitting

micro/nanodomes in bulk transition-metal dichalcogenides


11:00 - 11:30 Gregor Weihs

Photon triplet creation in and coherent control of nanowire

quantum dots

11:30 - 12:00 Ana Predojevic

Analysis of (hyper-) entanglement in quantum dot systems

12:00 - 12:30 Val Zwiller

Generating, manipulating and detecting quantum states of light

at the nanoscale

12:30 - 13:30 Lunch

13:30 - 14:30 Poster session

14:30 - 15:00 Stephan Reitzenstein

Electrically Triggered Emission of Indistinguishable Photons by

Resonant Microlaser Excitation

15:00 - 15:30 Glenn Solomon

Quantum dot resonance fluorescence in an integrated cavity-

waveguide device

15:30 - 16:00 Miroslav Jezek

High-fidelity photon statistics detection and quality assessment

of single-photon and entanglement sources



16:30 - 17:00 Emanuele Pelucchi

Engineering opportunities (and struggles) with site-controlled

Pyramidal quantum dots

17:00 - 17:30 Stefano Sanguinetti

High Temperature Droplet Epitaxy Technique for Quantum

Photon Sources

17:30 - 18:00 Michael Reimer

Towards perfect photon entanglement with a quantum dot

18:00 Departure

EQEP-2018 Scientific Program – Posters

Arash Ahmadi

On-demand entangled photon pair generation

Marco Avesani

Secure heterodyne-based quantum random number generator at 17 Gbps

Christian Dangel

Dephasing dynamics of optically active electron and hole spin qubits in selfassembled

quantum dots

Emil V. Denning

Cavity-waveguide interplay in optical resonators and its role in optimal

single-photon sources

Ali Elshaari

Strain Tuning of Hybrid Quantum Photonic Circuits

Lukas Hanschke

Generation of single-photon and two-photon pulses from a self-assembled

quantum dot

Tobias Huber

Towards a solid-state quantum repeater using highly efficient single photon sources

Sascha Kolatschek

Deterministic fabrication of circular Bragg gratings around pre-selected quantum dots for high

performance light sources

Zhe X. Koong

Coherent and incoherent scattering from resonantly driven quantum dots

Thomas Lettner

Bright and tunable single-photon sources for quantum optics

Pietro Lombardi

Planar optical antennas as efficient single-photon sources for free-space

quantum optics operation

Morgan Mastrovich

Implementing and characterizing resonant two photon excitation in quantum dot nanowires

Magdalena Moczala-Dusanovska

Towards Scalable Quantum Dot Quantum Technologies: Spatially and Spectrally

Deterministic Technologies

EQEP-2018 Scientific Program – Posters

Markus Müller

Quantum-Dot Single-Photon Sources for Entanglement Enhanced Interferometry

Markus Rambach

Hectometer Revivals of Quantum Interference

Eva Schöll

Bright Single InAsP Quantum Dots at Telecom Wavelengths in Position-Controlled InP

Nanowires

Lukas Schweickert

Coherent Control of On-demand Single Photons from a Quantum Dot in a Hybrid Quantum

Network

Alessia Scriminich

Hong-Ou-Mandel interference between two weak coherent pulses retrieved from room-

temperature quantum memories

Nachiket Sherlekar

All-electric on-demand single/entangled photon source with high emission rate

Artur Tuktamyshev

Droplet Epitaxy GaAs/AlGaAs QD nucleation regimes on vicinal GaAs(111)A substrates

Simone Varo

Pyramidal Quantum Dots: from artificial atoms to systems

Katharina Zeuner

Reconfigurable Modulation of a Quantum Light Source in the C-Band

6th international workshop on “Engineering of Quantum Emitter Properties” – Rome, Italy

On-chip quantum photonics on a GaAs platform

M. Schwartz1, E. Schmidt2, U. Rengstl1, F. Hornung1, S. Hepp, S. L. Portalupi1, K. Ilin2, M.

Jetter2, M. Siegel2 and P. Michler1

1 Institut für Halbleiteroptik und Funktionelle Grenzflächen, Center for Integrated Quantum

Science and Technology (IQST) and SCoPE, University of Stuttgart, Allmandring 3, 70569

Stuttgart, Germany

2 Institute of Micro- and Nanoelectronic Systems, Karlsruhe Institute of Technology (KIT),

Hertzstrasse 16, 76187 Karlsruhe, Germany

Electronic address: [email protected]

Photonic quantum technologies such as quantum cryptography, photonic quantum metrology,

photonic quantum simulators and computers will largely benefit from highly scalable and small

footprint quantum photonic circuits. To perform fully on-chip quantum photonic operations,

three basic building blocks are required: single-photon sources, photonic circuits and single-

photon detectors. Highly integrated quantum photonic chips on silicon and related platforms

have been demonstrated incorporating only one or two of these basic building blocks. Previous

implementations of all three components were mainly limited by laser stray light, making

temporal filtering necessary or required complex manipulation to transfer all components onto

one chip. So far, a monolithic, simultaneous implementation of all elements demonstrating

single-photon operation remains elusive. Here, we present a fully-integrated Hanbury-Brown

and Twiss setup on a micron-sized footprint, consisting of a GaAs waveguide embedding

quantum dots as single-photon sources, a waveguide beamsplitter and two superconducting

nanowire single-photon detectors (see Fig.1) [1].

Figure.1: Sketch of the on-chip Hanbury-Brown and Twiss setup.

This enables a second-order correlation measurement at the single-photon level under both

continuous-wave and pulsed resonant excitation.

[1] M. Schwartz et al., Fully on-chip single-photon Hanbury-Brown and Twiss experiment on

a monolithic semiconductor-superconductor platform, arXiv:1806.04099, 2018.


Integrated quantum photonics with single, optically-located quantum dots

Kartik Srinivasan1, Marcelo Davanço1, and Jin Liu2

1. National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA

2. School of Physics, Sun-Yat Sen University, Guangzhou, 510275 China

To reach their full potential as quantum light sources, single epitaxially-grown quantum dots

require integration within photonic structures that efficiently out-couple quantum dot emission

into desired collection channels, ideally while enhancing the quantum dot radiative rate and

without introducing adverse dephasing and spectral diffusion processes. Given the uncertainty

in quantum dot spatial positions after growth, location techniques that enable the creation of

suitably aligned photonic nanostructures for achieving the aforementioned performance goals

are needed. In this talk, we will start by briefly reviewing our approach for optically locating

single quantum dots through wide-field photoluminescence imaging [1,2]. We will then

describe the application of this technique to the creation of single-photon and entangled-photon

sources using geometries such as micropillar and circular Bragg grating cavities [1,3,4], how

the technique can be used to study the influence of etched surfaces on quantum dot behaviour

[5], and future extensions that can enable its incorporation within advanced photonic circuits

that combine active quantum dot regions with passive low-loss waveguide networks [6].

Finally, we will briefly review recent experiments in which single photons based on single,

optically-located quantum dots in micropillar cavities are frequency-converted using silicon

nanophotonic resonators [7].

[1] L. Sapienza, M. Davanço, A. Badolato, and K. Srinivasan, Nat. Comm., 6: 7833 (2015)

[2] J. Liu, M.I. Davanço, L. Sapienza, K. Konthasinghe, J.V.D.M. Cardoso, J.D. Song, A.

Badolato, and K. Srinivasan, Rev. Sci. Instr., 88, 023116 (2017)

[3] Y.M. He, J. Liu, S. Maier, M. Emmerling, S. Gerhardt, M. Davanço, K. Srinivasan, C.

Schneider, and S. Höfling, Optica, 4(7), 802-808 (2017).

[4] J. Liu et al, submitted (2018).

[5] J. Liu, K. Konthasinghe, M. Davanço, J. Lawall, V. Anant, V. Verma, R. Mirin, S.W.

Nam, J.D. Song, B. Ma, Z.S. Chen, H.Q. Niu, Z.C Niu, and K. Srinivasan, Phys. Rev. Appl.,

9, 064019 (2018).

[6] M. Davanço, J. Liu, L. Sapienza, C.-Z. Zhang, J.V.D.M. Cardoso, V. Verma, R. Mirin,

S.W. Nam, L. Liu, and K. Srinivasan, Nat. Comm., 8: 889 (2017).

[7] A. Singh et al, submitted (2018).


All-photonic quantum teleportation using on-demand solid-state quantum

emitters

M. Reindl1*, D. Huber1, C. Schimpf1, S.F. Covre da Silva1, M. Rota2, H. Huang1, V. Zwiller3,

K.D. Jöns3, A. Rastelli1 and R. Trotta2

1. Institute of Semiconductor of Solid State Physics, Johannes Kepler University, 4040 Linz,

Austria

2. Department of Physics, Sapienza University of Rome, 00185 Rome, Italy

3. Department of Applied Physics, Royal Institute of Technology, 106 91 Stockholm, Sweden

* Electronic address: [email protected]

All-optical quantum teleportation represents a pivot concept in quantum communication

science and technology. This quantum phenomenon is build up around the non-local

properties of entangled states of light that, in perspective of real-life applications, should be

encoded on photon pairs on demand [1]. Despite the recent advances, however, the

exploitation of deterministic quantum light sources in unconditional quantum teleportation

schemes remains a major open challenge [2,3]. In this work, we show that photon pairs

generated on-demand by GaAs quantum dots [4] can be used to implement a teleportation

protocol with a fidelity violating the classical limit by more than five standard deviations, for

arbitrary input states. Moreover, we develop a theoretical framework that predicts the

experimental observations and which determines the degree of entanglement and

indistinguishability needed to overcome the classical limit for any input state. Our results

emphasize that deterministic solid-state quantum emitters represent one of the most promising

candidates for implementing quantum teleportation in practical quantum networks.

Figure 1: Experimentally extracted average teleportation fidelity as a function of the

entanglement fidelity fE (correlation between the two photons emitted by the biexciton-exciton

decay cascade) and the two-photon interference visibility VHOM (of single exciton photons) for

all investigated quantum dots. The deployed on-demand all-photonic teleportation scheme

agrees well with the theoretical predictions.

[1] D. Huber, et. al., Physical Review Letters 121, 033902 (2018).

[2] J. Nilsson, et al., Nature Photonics 7, 311-315 (2013).

[3] R. M. Stevenson, et al., Nature Communications 4, 2859 (2013).

[4] M. Reindl, et al., Nano Letters 17, 4090-4095 (2017).


Designing Single-Photon Sources: Towards Unity

N. Gregersen1*, E. V. Denning1 and J. Mørk1

1. DTU Fotonik, Department of Photonics Engineering

Technical University of Denmark, Kongens Lyngby, DK-2800, Denmark


A key building block within optical quantum information technology is the single-photon

source. The key figures of merit are the efficiency, defined as the number of photons detected

by the collection optics per trigger, as well as the indistinguishability describing the coherence

properties of the emitted photons. Spontaneous parametric down-conversion (SPDC) has been

the main workhorse for generating single photons within quantum optics for many years;

however SPDC is inherently probabilistic limiting the number of photons involved in an

experiment to a handful.

Recently, the semiconductor quantum dot (QD) has emerged as an alternative to SPDC. By

integrating the QD into a microstructure, the light emission can be controlled and extraction

efficiency of around 0.7 [1,2] have been achieved. Furthermore, using resonant excitation and

careful control of the neighbouring charge environment, indistinguishability up to 0.99 [2,3]

has been demonstrated. However, future progress within QD-based single-photon sources will

require the combination of high efficiency with high indistinguishability.

In this presentation, I will discuss the physical limitations of present-day design schemes,

which must be overcome to combine near-unity efficiency and indistinguishability. I will

discuss new promising QD-based design schemes and I will discuss the challenges ahead.

Figure 1: Major single-photon source design strategies: (a) The micropillar cavity, (b) the

photonic nanowire and (c) the photonic “trumpet” geometry.

[1] J. Claudon et al., Nat. Photonics 4, 174–177 (2010). M. Munsch et al., Phys. Rev. Lett.

110, 177402 (2013). O. Gazzano et al., Nat. Commun. 4, 1425 (2013).

[2] X. Ding et al., Phys. Rev. Lett. 116, 020401 (2016).

[3] N. Somaschi et al., Nat. Photonics 10, 340–345 (2016)

mailto:[email protected]


Nanowire single-photon sources: Mechanics matters

Jean-Michel Gérard1, Alberto Artioli1, Saptarshi Kotal1, and Julien Claudon1

1. Univ. Grenoble Alpes, CEA, INAC, PHELIQS, “Nanophysique et

semiconducteurs" » group, F-38000 Grenoble, France

* [email protected]

Tapered photonic nanowires provide an efficient interfacing between an embedded

semiconductor quantum dot (QD) and a free-space optical beam. Such photonic nanostructures

find applications in solid-state quantum optics, for example to realize bright sources of non-

classical states of light [1,2]. At the same time, a nanowire is also a mechanical system, which

supports high Q vibration modes. Mechanical oscillation generates a stress field, which

modifies the bandgap energy of the QD [3]. This effect provides a very efficient coupling

between optical and mechanical degrees of freedom in this hybrid optomechanical system, with

appealing applications including the mapping of QD locations on the nanometric scale [4,5],

quantum non-demolition measurements of the QD state using nanowire position measurement,

or the preparation of quantum mechanical states using resonant optical driving of the QD.

However, this large optomechanical coupling is also potentially detrimental for quantum optics

applications. Indeed, recent experiments on photonic trumpets have shown that thermal motion

significantly broadens the QD emission line [5], hence preventing the emission of

indistinguishable photons.In this contribution, we perform a systematic theoretical investigation

of this process. We consider ‘standard’ nanowire geometries, and determine the impact of the

relevant set of vibration modes. We propose modified designs which enable to quench the

broadening of the QD line induced by the Brownian motion of the nanowire, without

compromising its photonic properties [6]. These results pave the way toward efficient source

of indistinguishable photons based on nanowire antennas. Finally, we discuss also the impact

of mechanical vibrations on quantum light sources based on micropillar cavities.

Fig 1: Sketch of the optomechanical coupling (b) in a QD-nanowire antenna system (a).

[1] J. Claudon, et al., Nature Photon. 4, 174 (2010)

[2] M. Munsch, et al., Phys. Rev. Lett. 110, 177402 (2013)

[3] I. Yeo, et al., Nature Nanotech. 9, 106 (2014)

[4] P.L. de Assis et al, Phys. Rev. Lett. 118, 117401(2017)

[5] M. Munsch, et al., Nat. Commun. 8, 76 (2017)

[6] A. Artioli et al., in preparation


Generation of quantum light in a photon-number superposition

J. C. Loredo,1 C. Antón,1 B. Reznychenko,2 P. Hilaire,1 A. Harouri,1 C. Millet,1 H.

Ollivier,1 N. Somaschi,3 L. De Santis,1 A. Lemaître,1 I. Sagnes,1 L. Lanco,1, 4

A. Auffèves,2 O. Krebs,1 and P. Senellart1, †

1. CNRS Centre for Nanoscience and Nanotechnology, Université Paris-Sud, Université

Paris-Saclay, 91120 Palaiseau, France

2. Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France

3. Quandela SAS, 86 rue de Paris, 91400 Orsay, France

4. Université Paris Diderot, Paris 7, 75205 Paris CEDEX 13, France

The ability to generate light in pure quantum states is central to the development of quantum-

enhanced technologies. Although controlling the photon number is the backbone of many

applications, the generation of pure quantum superpositions in the photon-number basis has

remained elusive. Light fields with zero and one photon can be generated by single atoms.

However, it has so far been limited to the generation of incoherent superpositions or to

coherent superpositions but with a vanishing one-photon part. Here, we report on the

generation of light pulses in a pure quantum superposition of zero, one-, and even two-

photons, using a single artificial atom-a semiconductor quantum dot [1].

Performing pulsed coherent control of the atomic population, a pure quantum superposition of

vacuum and one-photon is generated with a full control of their relative populations by the

laser intensity. Driving the system even stronger, with 2-pulses, a coherent superposition of

vacuum, one- and two-photons is generated, with the two-photon part exceeding the one-

photon part---a state that shows phase super-resolving interferometry. These observations of

text-book idealized quantum optics in semiconductor devices open new paths for quantum

technologies with access to the photon-number degrees-of-freedom.

[1] J. C. Loredo, C. Anton, et. al, arXiv:1810.05170


Machine learning for certification of photonic quantum information

F. Sciarrino

Dipartimento di Fisica, Sapienza Università di Roma, 00185 Roma, Italy

*Electronic address: [email protected]

Photonic technologies provide a promising platform to address at a fundamental level the

connection between quantum information and machine learning. We will exploit machine

learning as a tool to validate quantum devices such as Boson Samplers [1]. Indeed, the difficulty

of validating large-scale quantum devices poses a major challenge for any research program

that aims to show quantum advantages over classical hardware. To address this problem, we

propose a novel data-driven approach wherein models are trained to identify common

pathologies using supervised and unsupervised machine learning.

First, we propose a novel data-driven approach wherein models are trained to identify common

pathologies using unsupervised machine learning methods [1]. We train a classifier that exploits

K-means clustering to distinguish between Boson Samplers that use indistinguishable photons

from those that do not. We train the model on numerical simulations of small-scale Boson

Samplers and then validate the pattern recognition technique on larger numerical simulations

as well as on photonic chips in both traditional Boson Sampling and scattershot experiments.

The effectiveness of such method relies on particle-type-dependent internal correlations present

in the output distributions. This approach performs substantially better on the test data than

previous methods and underscores the ability to further generalize its operation beyond the

scope of the examples that it was trained on.

As complementary approach, we experimentally identify genuine many-body quantum

interference via a recent efficient protocol, which exploits statistical signatures at the output of

a multimode quantum device [3]. We successfully apply the test to validate three-photon

experiments in an integrated photonic circuit, providing an extensive analysis on the resources

required to perform it. Moreover, drawing upon established techniques of machine learning, we

show how such tools help to identify the—a priori unknown—optimal features to witness these

signatures. Our results provide evidence on the efficacy and feasibility of the method, paving

the way for its adoption in large-scale implementations. Our results provide evidence on the

efficacy and feasibility of this approach, paving the way for its adoption in large-scale

implementations.

[1] F. Flamini, N. Spagnolo, F. Sciarrino, “Photonic quantum information processing: a

review”, [arXiv:1803.02790] Reports on Progress in Physics (in press)

[2] I. Agresti, N. Viggianiello, F. Flamini, N. Spagnolo, A. Crespi, R. Osellame06863], N.

Wiebe, F. Sciarrino, “Pattern recognition techniques for Boson Sampling validation”,

[arXiv:1712.06863]

[3] T. Giordani, F. Flamini, M. Pompili, N. Viggianiello, N. Spagnolo, A. Crespi, R. Osellame,

N. Wiebe, M. Walschaers, A. Buchleitner, F. Sciarrino. “Experimental statistical signature of

many-body quantum interference”. Nature Photonics 12, 173 (2018).


Long-distance quantum communication in Space

G. Vallone*, C. Agnesi, L. Calderaro, D. Dequal, F. Vedovato, M. Schiavon, A. Santamato, V. Luceri, G. Bianco, P. Villoresi

1. Dip. di Ingegneria dell'Informazione, Università di Padova, Padova, Italy 2. Matera Laser Ranging Observatory, Agenzia Spaziale Italiana, Matera, Italy

3. e-GEOS SpA, Matera, Italy


We here review our results on satellite quantum communications, by exploiting different degrees of freedom of the photon, such as polarization [1] or time-bin [2] and using Low Earth Orbit Satellites and Matera Laser Ranging Observatory on ground. The exchange of single photons was recently extended to MEO [3] (Medium Earth Orbit) and GNSS [4] (Global Navigation Satellite System) satellites up to a distance of 20000 km. We finally present our recent realization of the Wheeler’s delayed-choice Gedankenexperiment along a space channel [5]. This experiment extends the validity of wave-particle duality at the spatial scale of LEO satellites. Our results pave the way for new applications of quantum technologies and fundamental experiments of physics exploiting quantum communication at large distance.

Figure 1: Illustrative representation (to scale) of the typical distance (≈ 20000 km) between GLONASS terminals and the MLRO ground station in Italy.

[1] G. Vallone, D. Bacco, D. Dequal, S. Gaiarin, V. Luceri, G. Bianco, P. Villoresi, Experimental Satellite Quantum Communications, Phys. Rev. Lett. 115, 040502 (2015)

[2] G. Vallone, D. Dequal, M. Tomasin, F. Vedovato, M. Schiavon, V. Luceri, G. Bianco, P. Villoresi, Interference at the Single Photon Level Along Satellite-Ground Channels, Phys. Rev. Lett. 116, 253601 (2016)

[3] D. Dequal, G. Vallone, D. Bacco, S. Gaiarin, V. Luceri, G. Bianco, P. Villoresi, Experimental single photon exchange along a space link of 7000 km, Phys. Rev. A 93, 010301(R) (2016)

[4] L. Calderaro, C. Agnesi, D. Dequal, F. Vedovato, M. Schiavon, A. Santamato, V. Luceri, G. Bianco, G. Vallone, P. Villoresi, Towards Quantum Communication from Global Navigation Satellite System, [arXiv:1804.05022]

[5] F. Vedovato, C. Agnesi, M. Schiavon, D. Dequal, L. Calderaro, M. Tomasin, D. G. Marangon, A. Stanco, V. Luceri, G. Bianco, G. Vallone, P. Villoresi, Extending Wheeler's delayed-choice experiment to Space, Science Advances 3, e1701180 (2017)


Building scalable room temperature quantum technologies

E. Figueroa*

Quantum Technology Laboratory, Stony Brook University

* [email protected]

Quantum engineering is the design and testing of the novel devices needed to communicate

and transform quantum bits of information. Some of these fundamental components store and

retrieve the qubits (quantum memories), while others are geared towards their manipulation

(quantum gates). Successfully interconnecting many of these devices is the key to construct

the first generation of quantum technology systems, namely, quantum computers and

quantum-protected communication networks.

In the first part of the talk I will show how to build the atomic instruments needed to store,

modify and distribute light-based qubits, using optical engineering of the properties of room

temperature atomic clouds. In the second part I will present our recent experiments in which

several of these quantum devices are already interconnected forming one of the largest

quantum processing networks in the world. I will discuss the prospects of using this unique

system to build the first generation of long-distance quantum cryptographic communication

networks and programmable light-based quantum computers.


Organic molecules for quantum technologies

C. Toninelli1 1CNR-INO and LENS, Istituto Nazionale di Ottica, Via Carrara 1, 50019 Sesto F.no, Italy

* [email protected]

Organic molecules of polyaromatic hydrocarbons were the first system in the solid state to show single photon emission [1,2]. However they are still considered unconventional sources of non-classical light. I will try to unveil part of the mystery behind such quantum emitters and show how they could effectively contribute to integrated quantum photonic platforms.

I will report on fluorescence coupling from a single molecule to a planar optical antenna [3] and a single-mode dielectric waveguide [4] (Fig. 1, left), discuss the integration of single quantum emitters into hybrid dielectric-plasmonic devices [5] and the coupling with 2D materials [6]. I will present our recent results about the fabrication of single-molecule doped nancrystals, preserving the optical properties of the bulk system, i.e. negligible blinking and spectral diffusion [7] (Fig.1, right). Eventually, I will report on ultrafast time-resolved transient spectroscopy on a single molecule [8].

Figure 1: Left, concept for the device showing single molecule emission into an integrated photonic waveguide. Right, optical characterization of DBT-doped anthracene Nanocrystals.

[1] W. E. Moerner and L. Kador, Phys. Rev. Lett. 62, 2535 (1989). [2] M. Orrit and J. Bernard, Phys. Rev. Lett. 65, 2716 (1990). [3] S. Checcucci et al., Light: Science and Applications 6, e16245 (2017) [4] P. Lombardi et al., ACS Photonics 5, 1, 126-132 (2017) [5] G. Kewes et al., Sci. Rep. 6, 28877 (2016). [6] K. Schaedler et al., submitted [7] S. Pazzagli et al., ACS Nano 12, 4295−4303 (2018) [8] M. Liebel et al., Nat. Phot. 12, 45-49 (2017)


A quantum dot exciton deep in the strong coupling regime of cavity-QED

R. J. Warburton

Department of Physics, University of Basel, Klingelbergstrasse 82,

CH4056 Basel, Switzerland


The strong coupling regime of cavity quantum electrodynamics (cQED) represents a fully

quantum light-matter interaction. It results in non-linearities at the level of a single photon.

Achieving strong coupling is key to creating coherent atom-atom couplings and single photon

transistors. Three parameters are typically used in its description: the atom-photon coupling rate

g, the atom decay rate , and the photon loss rate . True strong coupling is achieved only when

g exceeds both loss rates by a large margin.

An exciton in a quantum dot represents, in the best case, a close-to-ideal “atom” [1]. In

particular, a quantum dot has a large optical dipole moment. This potentially leads to large

coupling strengths g in cQED provided the cavity has a small mode volume. The conundrum is

that a small mode-volume cavity is typically fabricated with nano-fabrication, a process which

often leads to a large via scattering-induced losses and also an increased via additional

dephasing channels.

We report here an experiment in which g is much larger than both and . We use a quantum

dot embedded in a highly miniaturized, fully tunable Fabry-Pérot microcavity [2,3]. This gives

reasonably large values of g, and, crucially, a way to miniaturize without increasing and .

The quantum dot is embedded in a charge-tunable heterostructure which gives close-to-

transform limited optical linewidths, in situ tuning via the dc Stark effect, and control of the

quantum dot charge via Coulomb blockade. The output mode is a simple Gaussian beam. We

achieve g/2=3.31 GHz, 0.34 GHz and 0.63 GHz (Q-factor=510,000), resulting in

a cooperativity C=2g2/=100.

Resonant laser spectroscopy shows a very clear avoided crossing at the quantum dot exciton-

cavity resonance. The splitting between the two peaks, the coupled exciton-photon modes, is a

factor of 7 larger than the individual polariton linewidths. The intensity correlation function

g(2)(t=0) exhibits antibunching when the laser is tuned to one of the polariton resonances. On

detuning the laser, we observe pronounced oscillations in g(2)(t), unambiguous evidence of a

coherent exciton-photon interaction. Detuned from the polariton resonances, g(2)(t=0)~80.

This work was performed with Daniel Najer, Immo Söllner, Matthias Löbl, Daniel Riedel,

Benjamin Petrak and Natasha Tomm at the University of Basel; and Sascha Valentin, Rüdiger

Schott, Andreas Wieck and Arne Ludwig at Ruhr-University, Bochum.

[1] Andreas V. Kuhlmann, Julien Houel, Arne Ludwig, Lukas Greuter, Dirk Reuter, Andreas

D. Wieck, Martino Poggio, and Richard J. Warburton, Nature Physics 9, 570 (2013).

[2] Russell J. Barbour, Paul A. Dalgarno, Arran Curran, Kris M. Nowak, Howard J. Baker,

Denis R. Hall, Nick G. Stoltz, Pierre M. Petroff, and Richard J. Warburton, J. Appl. Phys. 110,

053107 (2011).

[3] Lukas Greuter, Sebastian Starosielec, Andreas V. Kuhlmann, and Richard J. Warburton,

Phys. Rev. B 92, 045302 (2015).


Quantum emission dynamics and hole-spin dephasing in single quantum

dots

K. Müller1, L. Hanschke1, C. Dangel1, T. Simmet1, F. Sbresny1, S. Appel1, J. Wierzbowski1,

K. Boos1, J. Vuckovic2, K. A. Fischer2, D. Lukin2, S. Sun2 and R. Trivedi2 and J. J. Finley1*

1. Walter-Schottky-Institut and Physik Department, Technische Universität München, 85748 Garching, Germany

2. E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94306, USA

* Electronic address: [email protected] , kai.mü[email protected]

Spins in individual quantum dots (QDs) are an attractive resource for encoding quantum

information and mediating entanglement between coherently scattered light fields. In this

contribution, we explore single and multi-photon generation from resonantly driven QDs and

explore the coherent dynamics of electron and hole spin-qubits. For on-demand single-photon

generation, the QD is typically excited using a resonant laser pulse of area π to prepare the two-

level system in its excited state from where it spontaneously emits a single photon. However,

emission that occurs already during the presence of the laser pulse allows for re-excitation of

the system and multi-photon emission that spoils the single-photon character and increases the

measured degree of second-order coherence g(2)(0) [1]. In contrast, when exciting the system

with 2π-pulses the system is expected to be returned to the ground state. However, emission

which occurs during the presence of the pulse is most likely to occur when the system is in its

excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation

with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity

probability and the emission of a second photon within the excited state lifetime [2]. We will

discuss the dynamics of processes leading to the generation of single photons and two-photon

pulses [3], paying attention to the influence of the pulse length, pulse shape and dephasing

arising from coupling to acoustic phonons [4]. In related experiments, we explore electron and

hole spin dynamics in the fluctuating magnetic environment provided by the nuclear spin bath.

For electron spins we observe the usual Overhauser dynamics that results in characteristic spin

dephasing over timescales of ∼2ns [5]. In contrast, for holes we observe 10-100x slower spin

dephasing with no observable dynamics up to ~100ns followed by a monotonic exponential

relaxation, which remains incomplete even for timescales longer than ~5µs. Spin echo

measurements reveal revivals of up to several microsecond timescales and point to electronic

noise ass being the current limit of hole spin coherence [6].

[1] K. A Fischer, K. Müller et al., New J. Phys. 18, 113053 (2016)

[2] K. A. Fischer et al., Nature Physics 13, 649-654 (2017)

[3] K. A. Fischer et al-, Quantum Sci. Technol. 3, 014006 (2017)

[4] L. Hanschke et al. arxiv 1801.01672 (2018)

[5] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015)

[6] T. Simmet et al., in preparation (2018)


mailto:kai.mü[email protected]


Spin properties and quantum control of

group-IV vacancy centers in diamond

C.Becher

Fachbereich Physik, Universität des Saarlandes, Saarbrücken, Germany


Color centers in diamond, i.e. atomic-scale, optically active defects in the diamond lattice,

have received large recent attention as versatile tools for solid-state-based quantum

technologies ranging from quantum information processing to quantum-enhanced sensing and

metrology. They provide individually addressable spins with very long coherence times,

narrow optical spectra and bright single-photon emission. However, identifying a spin

impurity which combines all of these favorable properties still remains a challenge.

I will present the example of the Silicon vacancy (SiV) center which allows for optical

addressing [1] and ultrafast all-optical coherent manipulation [2,3] of its orbital and spin

states. However, this color center reaches long spin coherence times only in the limit of very

low temperatures (<100mK) due to phonon-induced decoherence processes [3,4]. A potential

resort are vacancy defects with a heavier group-IV impurity atom, such as GeV, SnV and PbV

centers, featuring a larger ground state splitting and thus less susceptibility against phonon-

induced decoherence. Here, I will report on spectroscopy of SnV centers which show

promising optical and single-photon emission properties.

[1] T. Müller et al., Nature Commun. 5, 3328 (2014).

[2] J.N. Becker et al., Nature Commun. 7, 13512 (2016).

[3] J.N. Becker et al., Phys. Rev. Lett. 120, 053603 (2018).

[4] D.D. Sukachev et al., Phys. Rev. Lett. 119, 223602 (2017).


Coulomb blockade in atomically thin quantum dots

Brian D. Gerardot

Institute for Photonics and Quantum Sciences, SUPA, Heriot-Watt University,

Edinburgh EH14 4AS, UK


Gate-tunable tunnel coupling between a semiconductor quantum dot and a nearby Fermi sea

has underpinned many advances in quantum spintronics and solid-state quantum optics. Due

to Coulomb blockade, such devices enable direct control over the quantum dot charge state

(loading electrons or holes one at a time). In this talk, I will present single quantum dots in an

atomically thin semiconductor that exhibit Coulomb blockade. This is achieved by embedding

monolayer WSe2 in a heterostructure device with a gate-tunable tunnel interaction between the

quantum dots and a nearby Fermi sea in few-layer graphene. Magneto-optical spectroscopy of

the positively charged, neutral, and negatively charged excitons reveals the nature of the WSe2

quantum emitters. Further, due to a strong tunnel interaction between the quantum dots and

Fermi sea, we observe distinct Kondo-like many-body interactions between the localized states

and the continuum of electron or hole states. Next generation heterostructure devices offer the

potential to further investigate and engineer either isolated single spins or strongly coupled

many-body states in a two-dimensional platform.


Light-matter coupling with atomically thin WSe2

C. Schneider1, O. Iff1, N. Lundt1, S.H. Kwon2, A. Kavokin3, S. Höfling1

1. Technische Physik, University of Würzburg, Germany

2. Dept. of Physics, Chung-Ang University, Seoul, Korea

3. WIAS, Hangzhou, China

* Electronic address: Christian.schneider@uni –wuerzburg.de

Monolayers of transition metal dichalcogenides have emerged as a modern material platform

to study manybody effects and quantum phenomena via optical spectroscopy.

The high stability and large oscillator strength of quasi-particle excitations emerging in

monolayers of WSe2 (and likewise, MoS2, MoSe2 and WS2) enables the direct observation of

excitons, trions, and more complex manybody states via standard spectroscopy. I will first

discuss the fundamental modification of the linear emission and absorption spectrum of a single

layer of WSe2 in a microcavity, clearly yielding polaritonic behaviour at ambient conditions

[1]. At cryogenic temperatures, one intriguing feature, which is particularily well-pronounced

in WSe2 thin sheets, is the emergence of tightly localized excitons, which act as single photon

sources. The formation of these sources can be manipulated by surface strain, and their

proximity to the surface can be harnessed to coupled them to surface resonances, e.g. in metallic

nanoparticles. I will specifically address recent developments and first implementations of

cavity-quantum electrodynamics experiments based on such localized excitons in WSe2

quantum emitters [2] and discuss advantages (and disadvantages) with regard to more

established solid-state single photon sources.

Figure 1: Artistic Illustration of a strain engineered WSe2 single

photon source

[1] C. Schneider et al. Nat. Comms. 2695 (2018)

[2] L. Tripathi et al. ACS Photonics, 1919 (2018)


Hydrogen-assisted fabrication of site-controlled light-emitting micro/nanodomes in bulk transition-metal dichalcogenides

M. Felici,1* D. Tedeschi,1 E. Blundo,1 G. Pettinari,2 T. Yildrim3, E. Petroni,1 S. Sennato,4 B. Li,5 C. Zhang,5 Y. Zhu,5 Y. Lu,5 A. Polimeni1

1. Dipartimento di Fisica, Sapienza Università di Roma, 00185 Roma, Italy.

2. Institute for Photonics and Nanotechnologies, National Res. Council, 00156 Roma, Italy.

3. College of Chemistry and Environmental Engineering, Shenzhen University, P. R. China

4. Institute for Complex Systems, National Research Council, 00185 Roma, Italy.

5. Research School of Engineering, College of Engineering and Computer Science, The Australian National University, Canberra, ACT2601, Australia.


At the few-atom-thick limit, transition-metal dichalcogenides (TMDs) exhibit strongly interconnected structural and optoelectronic properties. The possibility to tailor the latter by controlling the former is guaranteed to have a great impact on applied and fundamental research. Here, we demonstrate that low-energy hydrogen irradiation deeply affects the surface morphology of bulk TMD crystals. Protons penetrate the bulk top layer, resulting in the formation and progressive accumulation of H2 in the first interlayer region. The trapped gas coalesces into micro-/nano-bubbles, leading to the blistering of one-monolayer thin domes [see Fig. 1(a)] that protrude from the crystal surface and locally turn the dark bulk material into an efficient light emitter. These domes host strong, non-trivial strain fields that cause unprecedented major changes in the band structure of the material, including a hitherto unobserved direct-to-indirect band gap transition on going from the dome’s edge to its top [see Fig. 1(b)]. The domes can be produced with well-ordered positions [see Figs. 1(c-d)] and sizes tunable from the nm to the µm scale, with important prospects for nanophotonics.

Figure 1: (a) AFM image of a WS2 dome having ~3 µm radius. (b) Micro-photoluminescence spectra recorded on the dome shown in (a), taken at different distances from the dome center (r=0). A (I) indicates the direct (indirect) excitonic peak. The shaded spectra were recorded at the positions marked by dots in panel (a). (c) AFM image of an array of WS2 domes obtained after H+ irradiation of a flake patterned with an H-opaque mask (mask opening diameter S=3 µm). (d) Same as (c), but for S=1 µm.


Photon triplet creation in and coherent control of nanowire quantum dots

Lukas Kirchner1, Max Prilmüller1, Milad Khoshnegar2, Hamed Majedi2, Tobias Huber3, Ana

Predojević4, Dan Dalacu5, Jean Lapointe5, Xiaohua Wu5, Philip Poole5 and G. Weihs1

1. Institut für Experimentalphysik, Universität Innsbruck,

Technikerstr. 25, 6020 Innsbruck, Austria

2. Department of Electrical and Computer Engineering, University of Waterloo,

Waterloo, Ontario, Canada N2L 3G1

3. Technische Physik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany

4. Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden

5. National Research Council of Canada,

1200 Montreal Road, Ottawa, Ontario, Canada K1A 0R6


Quantum dots embedded in nanowires are attractive as single photon sources [1], because the

waveguiding properties of the nanowire help adapt the emission mode to the receiving optical

system (see Fig. 1a). Because of the wurtzite structure, higher symmetry and thus reduce fine-

structure splitting enables a high degree of polarization entanglement in the emitted biexciton-

exciton photon pairs.

In this talk, we will present our results on photon triplet emission from a stack of two coupled

quantum dots in a nanowire [2]. A triexciton complex in this quantum dot molecules decays

in a cascade emitting three photons at an overall high rate surpassing other sources of multiple

correlated photons.

For our final goal of entangled photon triplets we have been investigating coherent control of

the quantum dot molecule. In this talk we will demonstrate that high-fidelity coherent two-

photon excitation of one of the dots in the molecule enables strongly time-bin entangled

photon pairs.

[1] Dalacu, D. et al. Ultraclean emission from InAsP quantum dots in defect-free wurtzite

InP nanowires, Nano Lett. 12, 5919–5923 (2012).

[2] M. Khoshnegar et al., A solid state source of photon triplets based on quantum dot

molecules, Nat. Commun. 8, 15716 (2017).

Figure 1: Nanowire

quantum dot molecule. a)

the taper structure expands

the nanowire waveguide

mode. b) SEM picture of a

nanowire c) TEM image

with false-colored quantum

dot insertions.


Analysis of (hyper-) entanglement

in quantum dot systems

M. Prilmüller1, G. Weihs1 and A. Predojević2

1. Institut für Experimentalphysik, University of Innsbruck,

Technikerstrasse 25d, 6020 Innsbruck, Austria

2. Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden

* Electronic address: [email protected].

Entanglement of photons is an essential tool of quantum information science. It’s applications

stretch from tests of fundamental quantum-mechanical principles to quantum communication

protocols. Most often the photons are entangled in only one degree of freedom, for example

polarization. Nonetheless, the versatility of entanglement can be more optimally used and

explored if the involved parties are entangled simultaneously in more than one degree of

freedom – hyperentangled.

Here, we showed that the hyperentanglement can also be achieved in quantum dot systems.

We demonstrated simultaneous entanglement in polarization and time-bin degree of freedom

that are characterized by concurrence of 0.71(7) and 0.76(7), respectively. In terms of fidelity

with the maximally entangled state our result yields 0.80 (3) and 0.84 (3) for polarization and

time-bin entanglement, respectively.


Generating, manipulating and detecting quantum states of light at the

nanoscale

V. Zwiller*, L. Schweickert, K. Zeuner, T. Lettner, J. Zichi, A. Elshaari, K. Jöns

Department of Applied Physics, KTH, Stockholm, Sweden

*E-mail: [email protected]

With the aim of realizing complex quantum networks, we develop quantum devices based on

nanostructures to generate quantum states of light with semiconductor quantum dots, single

photon detectors based on superconducting nanowires and on-chip circuits based on

waveguides to manipulate light.

The generation of single photons can readily be performed with single quantum dots. We

demonstrate a very high single photon purity exceeding 99.99% generated at 795 nm with GaAs

quantum dots [1], these quantum emitters also allow for interfacing with atomic ensembles. To

enable long distance communication, we are also developing devices based on single InAs

quantum dots able to emit at telecom frequencies [2].

To allow for complex architectures, on-chip integration is desirable. We will demonstrate

filtering and routing of single photons with tunable ring resonators on a chip and discuss the

scalability of this approach [3].

Generation and manipulation of quantum states of light would be useless without single photon

detectors. We are therefore developing high-performance single photon detectors based on

superconducting nanowires and will present state-of-the-art performance in terms of detection

efficiency and time resolution [4].

[1] L. Schweickert et al., On-demand solid-state single-photon source with 99.99% purity,

Appl. Phys. Lett. 112, 093106 (2018).

[2] K. D. Zeuner et al., A stable wavelength-tunable triggered source of single photons

and cascaded photon pairs at the telecom C-band, Appl. Phys. Lett. Accepted (2018).

[3] A. W. Elshaari et al., On-chip single photon filtering and multiplexing in hybrid

quantum photonic circuits, Nat. Commun. 8, 379 (2017).

[4] I. Esmaeil Zadeh et al., Single-photon detectors combining ultra-high efficiency,

detection-rates, and timing resolution, APL Photonics 2, 111301 (2017).

https://arxiv.org/abs/1801.01518


Electrically Triggered Emission of Indistinguishable Photons by

Resonant Microlaser Excitation

S. Kreinberg1, T. Grbešić1, M. Strauß1, A. Carmele2,

M. Emmerling3, C. Schneider3, S. Höfling3,

X. Porte1, S. Reitzenstein1

1Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany 2Institut für Theoretische Physik, Technische Universität Berlin, 10623 Berlin, Germany

3Technische Physik, Julius-Maximilians-Universität Würzburg, 97074 Würzburg, Germany

*Electronic address: [email protected]

Two-level emitters constitute core elements of photonic quantum systems and exploring their

physics is at the heart of quantum optics. Of special interest is the strict-resonant coherent

optical excitation of such emitters to generate quantum light with close to ideal properties.

Our concept (c.f. Fig. 1) for resonant excitation of a quantum emitter is based on an electrically

driven high-β quantum dot micropillar laser which resonantly drives a single QD embedded in

a planar microcavity. The experiments are performed under continuous wave (CW) and pulsed

excitation of the electrically driven microlaser to observe Mollow-triplet spectra, Rabi

oscillations and the triggered emission of single photons with excellent quantum properties,

respectively. We investigate the second-order photon autocorrelation g(2)(τ) by means of a

Hanbury Brown & Twiss setup and measure the photon indistinguishability via Hong-Ou-

Mandel style two-photon quantum interference. We obtain single photons with strong multi-

photon suppression (g(2)(0) = 0.02) and high photon indistinguishably (V = 57 ± 9%) under

pulsed excitation with a repetition rate of 156 MHz [1].

Figure 1: Schematic illustration of the experimental concept: Emission of the electrically driven

microlaser in cryostat 1 is fiber-coupled to resonantly excite a single QD in cryostat 2.

[1] S. Kreinberg, T. Grbesic, M. Strauß, A. Carmele, M. Emmerling, C. Schneider, D. Höfling,

X. Porte, and Reitzenstein, Quantum-optical spectroscopy of a two-level system using an

electrically driven micropillar laser as resonant excitation source, Light: Science &

Applications 7, 41 (2018).


Quantum dot resonance fluorescence in an integrated cavity-waveguide device

G. S. Solomon1,2, T, Huber1, Y, Shuai1, M, Davanco2, O, Gazzano1

1. Joint Quantum Institute, National Institute of Standards and Technology, & University of Maryland, College Park, MD, USA

2. National Institute of Standards and Technology, Gaithersburg, MD, USA


Semiconductor quantum dots (QD) embedded in micro-pillar cavities are excellent emitters of single photon light when resonantly pumped. Often, the same spatial mode is used to resonantly excite a quantum dot and to collect the emitted single photons, requiring cross-polarization to reduce the uncoupled scattered laser light. This inherently reduces the source brightness to 50 %. For some quantum applications the total efficiency—from generation to detection—must to be greater than 50 %. Many in the community are actively working to increase the brightness efficiency of light sources. Here, we continue along the path of an orthogonal pumping-detection scheme, developed by ourselves and other groups, to increase the brightness efficiency.

In this talk we discuss a recent device we have fabricated and tested based on micro-pillar cavities containing QDs connected to ridge waveguides; see Fig. 1. This geometry allows us to resonantly excite single quantum-dot states via the waveguide which is orthogonal to the vertical, off-chip collection axis, eliminating the need for cross-polarization. Highly anti-bunched light is measured with no filtering or spectrometer in cw. We will discuss implementing this prototype device into larger chip-scale quantum photonics devices.

Figure 1.Scanning-electron microscopy images of a device. (a) Cleaved edge, showing cavity and mirror pairs, used to couple the pump laser into the waveguide—here, 5.5 μm wide and adiabatically. (b) The waveguide connecting micro-pillar cavities—5 shown here. (c) Single micro-pillar cavity, 2.8 μm in diameter, where the waveguide is 1.25 μm wide.

2µm

10 µm

a

b

c

2µm


High-fidelity photon statistics detection and quality assessment

of single-photon and entanglement sources

J. Hloušek, I. Straka, R. Hošák, L. Lachman, R. Filip, M. Ježek*

Department of Optics, Palacký University, 17. listopadu 12, 77146 Olomouc, Czech Republic


Accurate evaluation of statistics of light at the level of individual photons is a critical

component of many advanced photonic and biomedical applications. We report a

measurement workflow free of systematic errors consisting of a reconfigurable photon-

number-resolving detector (PNRD) and a novel data processing method based on expectation-

maximization iterative algorithm weakly regularized by maximum-entropy principle. We

achieve the exceptionally accurate photon statistics measurement for various classical and

non-classical sources with mean photon number up to 20 and non-negligible multi-photon

content up to 30 photons. Coherent, chaotic, multi-photon-subtracted, and multi-mode

sources, as well as clusters of single-photon emitters, are characterized with the normalized

intensity correlation g(2) ranging from 5×10-3 to 2. The maximum discrepancy of g(2)

parameter computed from the measured photon statistics and the corresponding ideal statistics

is found to be less than a few percents across dozens of tested photonic signals, and is caused

primarily by imperfections of the sources. We also present continuous statistics transition

from highly non-classical states to a coherent state and further to chaotic states of light.

Furthermore, the reconfigurability of the presented PNRD allows for direct measurement of

non-classicality and quantum non-Gaussianity (QNG) witnesses [1-2]. QNG is a fundamental

quantum property that exhibits higher resilience to losses than the negativity of Wigner

function [3]. It was recently proven to be a sufficient condition for discrete-variable quantum

key distribution security, an efficient tool for quality assessment of single-photon sources [3-

4], and possibly a tool for emitters counting based only on their collective emission. The

photon statistics and, consequently, the QNG features are also crucial for entanglement

sources. Even a small multi-photon contribution can significantly deteriorate the generated

photonic entangled state. We discuss an application-oriented evaluation of high-quality

entanglement sources and analyze the correlations between the underlying photon statistics

and the ultimate entanglement distribution distance.

[1] I. Straka, et al., Quantum non-Gaussian multiphoton light, npj Quant. Inf. (2018).

[2] L. Lachman, et al., Faithful hierarchy of genuine N-photon quantum non-Gaussian light,

arXiv:1810.02546 [quant-ph] (2018).

[3] I. Straka, et al., Quantum non-Gaussian depth of single-photon states, Phys. Rev. Lett.

113, 223603 (2014).

[4] A. Predojević, et al., Efficiency vs. multi-photon contribution test for quantum dots, Opt.

Express 22, 4789 (2014).


Engineering opportunities (and struggles) with site-controlled Pyramidal

quantum dots

E. Pelucchi*, G. Juska, S. T. Moroni, S. Varo, and A. Gocalinska

1. Tyndall National Institute, UCC, Lee Maltings, Dyke Parade, Cork, Ireland


The requirements for practical applications of QDs in quantum information are very

demanding. Among them are site-control, emission uniformity, spectral purity, high QD

symmetry, high photon conversion/extraction efficiency, and many others depending on the

specific application. Herein we would like to navigate through some aspects of site-controlled

InGaAs QDs grown by Metalorganic Vapor Phase Epitaxy (MOVPE) in inverted pyramidal

recesses. Important achievements (wavelength uniformity, spectral purity, entangled-photon

emission) and potential for practical applications have been already discussed [e.g. 1-3]. The

system’s most recent highlight has been polarization-entangled photon emission from site-

controlled µLED exploiting a selective current injection scheme (Fig. 1a).

We will address now some recent advances based on the pyramidal QD (PQD) system,

highlighting its engineering flexibility and some challenging open issues. We present our

recent advances in the tackling of two photon resonant pumping, as well as the application of

a stress field to suppress the residual fine-structure splitting, facing some hurdles linked to the

non-planarity of the system. We will discuss a number of open challenges and the possible

technical solutions currently explored (in collaboration with JKU, Linz). On the other hand,

further engineering possibilities include the ability to stack an arbitrary number of precisely

designed QDs, possibly building well-reproducible QD-molecules (Fig. 1b), and transferring

pyramids (potentially one by one) on external substrates and surfaces, such as flexible films or

onto the core of an optical fiber allowing, e.g., decoupling the quantum-light source from a

photonic chip (Fig. 1c).

[1] L. O. Mereni et al., Appl. Phys. Lett. 94, 223121 (2009). [2] G. Juska et al., Nature Phot.

7, 527 (2013); [3] M. A. M. Versteegh et al., Phys. Rev. A 92, 033802 (2015).

Figure 1: (a) A lower band-gap region – a vertical quantum wire (VQWR) – along the

epitaxial pyramidal structure to inject carriers into a PQD. (b) Emission energy dependence

on the separation between two stacked QDs. (c) Pyramid transfer procedure which enables

integration of pyramidal structures on the external surfaces. Spectrum and single photon

emission detected from an optical fiber with an integrated pyramidal QD attached to it.

(a) (b) (c)


High Temperature Droplet Epitaxy Technique for Quantum Photon

Sources

Stefano Sanguinetti1*, Francesco Basso Basset2, Sergio Bietti1, Alexey Fedorov3, Rinaldo

Trotta2

1. L-NESS and Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-

Bicocca, Italy

2. Dipartimento di Fisica, Università di Roma, Italy

3. L-NESS and CNR‐IFN, Como, Italy

*Corresponding author: [email protected]

Epitaxial quantum dots (QDs) are a promising alternative to parametric down-converters as QDs

are on-demand photons emitters with high efficiency and compatible with semiconductor

technology. The use of QD as sources of entangled photons in real-life technologies requires the

overcome of two main roadblocks. The first is related to the difficulty of consistently finding

emitters capable to generate highly entangled photon pairs. The second concerns the wavelength

of operation of the quantum source, which must be compatible with other components of a

quantum network, such as storage media and detectors. To achieve reproducible entangled

photon generation, it is necessary to deal with the in-plane anisotropies in the confinement

potential that induce a fine structure splitting (FSS) between the bright exciton states through

electron-hole exchange interaction.

In order to comply with these requirements, we introduced a quantum dot fabrication procedure

by Droplet Epitaxy (DE) on (111)A substrates, in which the Ga droplets are crystallized, and

the subsequent barrier layer is deposited, at high substrate temperature (500 °C), close to the

temperature used for the growth of high quality GaAs and AlGaAs on (111)A [1]. This QD self-

assembly regime becomes accessible thanks to the specific choice of (111)A substrate

orientation, which promotes Vapour Liquid Solid crystallization processes within the droplet

respect to layer-by-layer growth in the droplet surroundings. The use of (111)A orientation as

substrate for the growth results in QDs with high in-plane symmetry, which is an essential

requirement to achieve low FSS and entangled photon emission. In addition the high

temperature DE growth procedure has the effect to improve the crystallinity of the QDs by

reducing the concentration of point defects typical of the low growth temperature usually used

in DE QD fabrication processes that degrade the QD emission properties (spectral broadening,

low efficiency etc.).

The QDs realized by high temperature DE show and extremely narrow emission linewidth (≈ 9

μeV) and entangled photon source operation in the 780 nm range, which allows for frequency-

matching of our QDs with Rb-based quantum memories, an important target for the realization

of quantum repeaters and long distance qubit teleportation. Fidelity measurements under two-

photon resonant excitation yielded a value of 0.8, which already reaches the state-of-the-art for

the more studied In(Ga)As/GaAs QDs in absence of postselection or external tuning, thus

confirming the intriguing potentiality of this material system. We finally obtained a fraction

higher than 95% of the QDs emitting in the strategical wavelength range that shows compliance

with the criteria for polarization-entangled photon generation.

[1] F. Basso Basset, S. Bietti, M. Reindl, L. Esposito, A. Fedorov, D. Huber, A. Rastelli, E.

Bonera, R. Trotta, and S. Sanguinetti, Nano Lett. 18, 505


Towards perfect photon entanglement with a quantum dot

M.E. Reimer

Institute for Quantum Computing and Department of Electrical & Computer Engineering,

Waterloo, ON, Canada


The on-demand generation of bright entangled photon pairs is an essential resource in quantum

optics, quantum communication and quantum sensing. However, an entangled photon source

with near-unity fidelity and efficiency is currently lacking. In this talk, I will present the

collaborative work of A. Fognini, A. Ahmadi, M. Zeeshan, S.J. Daley, N. Sherlekar, D. Dalacu,

P.J. Poole, K.D. Jöns, and V. Zwiller on the generation of dephasing-free entangled photon

pairs from a nanowire quantum dot with high collection efficiency [1]. We prove through our

research that it is possible to reach perfect entanglement fidelity with current technology by

also considering the detection process in addition to the generation process, even in an indium

rich quantum dot with a large nuclear spin.

Finally, I will present our exciting advances towards surpassing the need to resort to time-gating

techniques, as these reduce the source efficiency by post-selecting the desired Bell state for

applications. Two approaches will be presented. First I will discuss our novel gating strategy

with a quadrupole electrostatic potential, which shows the fine structure splitting can be erased

for any quantum dot dipole orientation without comprising the quantum dot brightness [2].

Second, I will present our all-optical approach to compensate the fine structure splitting by

employing a fast rotating waveplate emulated by a high frequency shifter [3]. This latter

approach has the unique advantage that the fine structure splitting of quantum dots in nanowires

and micropillars can be directly compensated for without the need for further sample

processing.

In this presentation I will discuss these two important points: dephasing free entangled photons

and no fine structure splitting, both of which lead us further towards the perfect source of

entangled photons. With this work we make great strides forward as a quantum dot community

to transition out of the lab into practical and powerful real-world applications, such as

information security for the day to day user as well as quantum radar which positively

influences strategies of national defence.

[1] A. Fognini et al., Path to perfect photon entanglement with a quantum dot,

arXiv:1710.10815 (2017).

[2] M. Zeeshan et al., Quadrupole electric field for erasing the fine structure splitting in a

single quantum dot, arXiv:1809.02538 (2018).

[3] A. Fognini et al., Universal fine-structure eraser for quantum dots, Opt. Express 26,

24487-24496 (2018).


On-demand entangled photon pair generation

Arash Ahmadi*1, Andreas Fognini2, Morgan Mastrovich1, Mohammad Zeeshan3, Val

Zwiller4, Michael E. Reimer3

1. Institute for Qauntum Computing and Department of Physics & Astronomy,

University of Waterloo, ON, Canada

2. Kavli Institute of Nanoscience Delft,

Delft University of Technology, Delft, The Netherlands (corresponding author)

3. Institute for Qauntum Computing and Department of Electrical & Computer

Engineering,

University of Waterloo, ON, Canada

4. Department of Applied Physics, Royal Institute of Technology (KTH), Stockholm,

Sweden


Long-distance quantum communication requires entangled photon sources that meet stringent

criteria including brightness, near-unity fidelity, and single-photon purity (i.e., no multiphoton

emission). A quantum light source that meets all of these requirements at once is yet to be

developed. In this work we present a source of entangled photon pairs based on quantum dots

in photonic nanowires with a clear route to meet all of these criteria. We demonstrate that our

source exhibits no dephasing and as a result, the measured fidelity of the entangled state is only

limited from reaching perfect entanglement by multiphoton emission and the timing jitter of the

detection system. The latter has been largely disregarded in the literature and other features of

the quantum dots, such as fine-structure splitting and nuclear spins, have been reported to be

the main reasons for the reduction of the measured entanglement fidelity. The model we

developed indicating that the entanglement generation process is dephasing free will be

discussed. We will also present a novel universal method to erase the fine-structure splitting of

the emitted photon pair after they have been emitted.

Our results provide more insight into the nature of the two-photon biexciton-exciton cascade

and demonstrate a clear path towards reaching perfect entanglement in semiconductor quantum

dots in the future for the first time.

[1] A. Fognini, A. Ahmadi et al., “Path to Perfect Entanglement”, arXiv:1710.10815 (2017).

[2] A. Fognini, A. Ahmadi et al., "Universal fine-structure eraser for quantum dots", Opt.

Express 26, 24487-24496 (2018)

https://arxiv.org/abs/1710.10815


Secure heterodyne-based quantum random number generator at 17 Gbps

M. Avesani1, D.G. Marangon1, G.Vallone1,2 and P. Villoresi1,2

1. Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, Italia

2. Istituto di Fotonica e Nanotecnologie – CNR, Padova, Italia


Random numbers are an invaluable resource in many different fields, ranging from simulations

in fundamental science to security applications. Quantum random number generators (QRNGs),

exploit the intrinsic randomness of quantum mechanics for the generation of genuine random

numbers. A promising approach, that combines the speed of commercial QRNG and the

security of Device-Independent QRNG, is given by Semi-Device-Independent (Semi-DI)

protocols: with respect to common trusted QRNG, they require weaker assumptions on the

devices (e.g, only on the source or on the measurement side), but they can achieve a generation

rate dramatically larger than DI-QRNG [1,2].

Figure 1: Schematic

representation of the

experimental setup

In our work we introduce a QRNG belonging to the family of the Semi-DI generators [3]: in

particular, we describe a novel source-device independent (SDI) protocol based on generic

Positive Operator Valued Measurements (POVM).We exploit the structure of the POVM to

naturally bound the private extractable randomness without any assumption on the source,

which can be even fully controlled by an adversary. The analysis takes into account both

classical and quantum side information. Unlike previous secure QNRG, the amount of

extractable randomness does not depend on the data, but only on the structure of the

measurement. Moreover, the bound on the min-entropy is valid also in the non-asymptotic

regime, i.e. for finite block size. All previously known Semi-DI or DI protocols needed to

randomly switch between two basis, thus requiring an external randomness source. Being free

of the strong assumption on the given external randomness, our protocol removes a critical side-

channel, improving the security and making it able to operate as a standalone random number

generator. Then, we experimentally implemented the protocol, using for the first time

continuous variable (CV) and heterodyne measurement. Combining our protocol with high

bandwidth telecom components, we were able to experimentally demonstrate a secure

generation rate greater than 17 Gbit/s: to our knowledge, the fastest random generation rate for

a Semi-DI QRNG obtained so far. Hence, our QRNG combines simplicity, ultrafast-rates and

high security with low cost components compatible with standard photonic integration

technologies, paving the way to new practical solutions for integrated random number

generation.

[1] D. G. Marangon, G. Vallone, and P. Villoresi, Source-Device-Independent Ultrafast Quantum Random

Number Generation,Physical Review Letters, vol. 118, (2017).

[2] J. B. Brask, A. Martin,W. Esposito, R. Houlmann, J. Bowles, H. Zbinden, and N. Brunner, Megahertz-rate

semidevice-independent quantum random number generators based on unambiguous state discrimination, Phys.

Rev.Applied, vol. 7, (2017)

[3] M. Avesani, D. G. Marangon, G. Vallone, and P. Villoresi, Secure heterodyne-based quantum random

number generator at 17 Gbps, Preprint at arXiv:1801.04139 (2018)


Dephasing dynamics of optically active electron and hole spin qubits in self-

assembled quantum dots

C. Dangel1*, T. Simmet1, K. Müller1, W. Rauhaus1, M. Kremser1, F. Li2, N. Sinitsyn2 and J. J.

Finley1

1. Walter Schottky Institute and Physik Department, Technische Universität München, 85748

Garching, Germany

2. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545

USA


Using solid-state spin qubits for quantum information processing requires a detailed

understanding of the decoherence mechanisms. For electron spins in quantum dots (QDs),

considerable progress has been achieved in strong external magnetic fields; however,

decoherence at very low magnetic fields remains puzzling when the magnitude of the Zeeman

energy becomes comparable with intrinsic couplings. Phenomenological models of

decoherence currently recognize two types of spin relaxation; fast ensemble dephasing due to

the coherent precession of spin qubits around nearly static but randomly distributed hyperfine

fields (∼2ns) and a much slower process (>1μs) of irreversible relaxation of the spin

polarization due to nuclear spin co-flips with the central spin. Here, we demonstrate that not

only two but three distinct stages of decoherence can be identified in the relaxation.

Measurements and simulations of the spin projection without an external field clearly reveal an

additional decoherence stage at intermediate timescales (∼750ns) [1]. The additional stage

corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath

itself induced by quadrupolar coupling of nuclear spins to strain driven electric field gradients,

leading to a relatively fast but incomplete non-monotonic relaxation of the central spin. For

hole spins we observe a one to two orders of magnitude slower dephasing due to the reduced

hyperfine interaction of the p-like Bloch wave function. In addition, time domain measurements

of T2* show faster dephasing rates with increasing external magnetic field. We attribute this to

electronic noise, which broadens the distribution of Zeeman frequencies via the linear coupling

of the hole g-tensor to the local electric field. Strategies to counteract this noise source as well

as measurements of T2 (via spin-echo) are discussed [2].

[1] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015)

[2] T. Simmet et al., in preparation


Cavity-waveguide interplay in optical resonators and its role in optimal

single-photon sources

E.V. Denning1,*, J. Iles-Smith1,2, A.D. Osterkryger1, N. Gregersen1 and J. Mork1

1. Department of Photonics Engineering, DTU Fotonik, Technical University of Denmark,

Building 343, 2800 Kongens Lyngby, Denmark

2. School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester

M13 9PL, United Kingdom

* [email protected]

Interfacing solid-state emitters with photonic structures is a key strategy for developing highly

efficient photonic quantum technologies. Such structures are often organized into two distinct

categories: nanocavities and waveguides. However, any realistic nanocavity structure

simultaneously has characteristics of both a cavity and waveguide, which is particularly

pronounced when the cavity is constructed using low-reflectivity mirrors in a waveguide

structure with good transverse light confinement. In this regime, standard cavity quantum optics

theory breaks down, as the waveguide character of the underlying dielectric is only weakly

suppressed by the cavity mirrors. By consistently treating the photonic density of states of the

structure, we provide a microscopic description of an emitter including the effects of phonon

scattering over the full transition range from waveguide to cavity [1]. This generalized theory

lets us identify an optimal regime of operation for single-photon sources in optical

nanostructures, where cavity and waveguide effects are concurrently exploited.

FIG. 1. (a) Schematic of a two-level emitter in a waveguide structure with two mirrors forming a

Fabry-Pérot cavity. (b)–(d) Optical LDOS vs frequency, scaled with the free spectral range (FSR), at

the position of the emitter for mirrors with weak, intermediate, and high reflectivity, respectively.

[1] E. V. Denning, J. Iles-Smith, A. D. Osterkryger, N. Gregersen and J. Mork, Phys. Rev. B

98, 121306(R) (2018)

a.

b. c. d.


Strain Tuning of Hybrid Quantum Photonic Circuits

Ali W. Elshaari †,*, Efe Büyüközer ‡, Iman Esmaeil Zadeh₼, Thomas Lettner†, Peng Zhao₸ ,

Eva Schöll†, Samuel Gyger†, Michael E. Reimer§, Dan Dalacu∥, Philip J. Poole∥, Klaus D.

Jöns†, Val Zwiller†1.

† Quantum Nano Photonics Group, Department of Applied Physics, Royal Institute of

Technology (KTH), Stockholm 106 91, Sweden

‡ Department of Mechanical and Process Engineering, ETH Zurich, CH - 8092 Zurich,

Switzerland

₼ Optics Group, Delft University of Technology, Delft 2628 CJ, The Netherlands

₸Department of Electronic Engineering, Tsinghua National Laboratory for Information

Science and Technology, Tsinghua University, Beijing, China

§ Institute for Quantum Computing and Department of Electrical & Computer Engineering,

University of Waterloo, Waterloo, ON N2L 3G1, Canada

∥ National Research Council of Canada, Ottawa, ON K1A 0R6, Canada* Electronic address:

*Corresponding author: Ali W. Elshaari [email protected]

Semiconductor quantum dots are crucial parts of the photonic quantum technology toolbox, as

they show excellent single photon emission properties in addition to their potential as solid state

qubits. Recently, there has been an increasing effort to deterministically integrate single

semiconductor quantum dots into complex photonic circuits[1-6]. Despite rapid progress in the

field, it remains challenging to manipulate the optical properties of waveguide-integrated

quantum emitters, in a deterministic, reversible, and non-intrusive manner. Here we

demonstrate a new class of hybrid quantum photonic circuits combining III-V semiconductors,

silicon nitride, and piezoelectric crystals[7-10]. Using a combination of bottom-up, top-down,

and nanomanipulation techniques, we realize strain tuning of a selected, waveguide-integrated,

quantum emitter and a planar integrated optical resonator. Our findings are an important step

toward realizing reconfigurable quantum integrated photonics, with full control over the

quantum sources and the photonic circuit.

References

[1] Elshaari, A.W., et al., Nature Communications, 2017. 8.

[2] Zadeh, I.E., et al., Nano letters, 2016. 16(4): p. 2289-2294.

[3] Kim, J.-H., et al., Nano Letters, 2017. 17(12): p. 7394-7400.

[4] Ellis, D.J.P., et al., Applied Physics Letters, 2018. 112(21): p. 211104.

[5] Katsumi, R., et al., Optica, 2018. 5(6): p. 691-694.

[6] Davanco, M., et al., Nature Communications, 2017. 8(1): p. 889.

[7] Zeuner, K.D., et al., Applied Physics Letters, 2018. 112(17): p. 173102.

[8] Kremer, P.E., et al., Physical Review B, 2014. 90(20): p. 201408.

[9] Chen, Y., et al., Applied Physics Letters, 2016. 108(18): p. 182103.

[10] Jöns, K.D., et al., Physical Review Letters, 2011. 107(21): p. 217402.



Generation of single-photon and two-photon pulses from a self-assembled

quantum dot

L. Hanschke1*, K. A. Fischer2, J. Wierzbowski1, S. Appel1, D. Lukin2, S. Sun2, R. Trivedi2,

M. Kremser1, T. Simmet1, C. Dory2, J. Vuckovic2, J. J. Finley1, K. Müller1

1Walter Schottky Institut and Physik Department, Technische Universität München, 85748

Garching, Germany

2E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94306, USA


Quantum two-level systems in the solid state are poised to serve the pivotal role of an on-

demand single-photon source. More recently, multi-photon quantum state generators have

nucleated additional interest for many quantum applications. Here, we investigate the dynamics

of generating single photons from a resonantly-driven two-level system and demonstrate that it

can surprisingly also operate in a two-photon bundling regime. Moreover, we demonstrate the

generation of single-photons with ultra-low multi-photon error rates based on a three-level

system.

Our two-level system of choice is a trion transition in a self-assembled quantum dot. For on

demand single-photon generation, it is typically excited with a resonant laser pulse of area π.

This prepares the two-level system in its excited state from where it spontaneously emits a

single photon. However, emission that occurs already during the presence of the laser pulse

allows for re-excitation of the system and, thus, multi-photon emission which degrades the

single-photon purity [1,2].

In contrast, when exciting the system with a pulse of area 2π, the system is expected to be

returned to the ground state. However, emission that occurs during the presence of the pulse is

most likely to occur when the system is in its excited state – exactly after an area of π has been

absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which

leads to re-excitation with near-unity probability and the emission of a second photon within

the excited state lifetime [2,3].

In addition, we investigate a two-photon excitation scheme based on a three-level system

formed by the bi-exciton - exciton cascade in a self-assembled quantum dot and demonstrate

that it improves the multi-photon error rate by several orders of magnitude while maintaining a

simple implementation [4].

[1] K. A Fischer, et al., New J. Phys. 18, 113053 (2016)

[2] K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017)

[3] K. A. Fischer, L. Hanschke, et al., Nature Physics 13, 649-654 (2017)

[4] L. Hanschke, K. A. Fischer, npj Quantum Information 4, 43 (2018)

https://arxiv.org/find/quant-ph/1/au:+Hanschke_L/0/1/0/all/0/1

https://arxiv.org/find/quant-ph/1/au:+Fischer_K/0/1/0/all/0/1


Towards a solid-state quantum repeater using highly efficient single photon

sources

T. Huber1,*, C. Schneider1, Ł. Dusanowski1, M. Moczała-Dusanowska1, S. Gerhardt1,

J. Jurkat1, and S. Höfling1

1 Technische Physik, Universität Würzburg, Würzburg, Germany


Modern classical cryptography relies on mathematical complexity and is likely to become

unsecure with future developments in quantum computing. The security in communication can

be recovered using quantum communication, whose security is solely based on physical laws.

Photons, which are used for encoding, change their quantum state upon measurement and thus

a possible eavesdropper can be easily detected. Unfortunately, this feature also disables the use

of amplifiers in the classical sense, where the signal that lost strength in the channel is measured

and reamplified to cover bigger distances. To circumvent the problem of amplification while

enabling the possibility to cover large distances the concept of a quantum repeater [1] was

introduced.

Recently, several groups demonstrated in a proof of principle experiment that two distant

quantum dot (QD) ground state spins can be entangled [2, 3], which is a prerequisite to use QDs

as quantum repeater nodes. To develop a quantum repeater with QDs, they have to be

competitive with other technologies, such as NV centers in diamond, atoms or ions. To achieve

this, the spin coherence times will need to be addressed [4] as well as the detected photon rates

must be increased. We propose to embed the QDs into a micropillar cavity to deal with the

latter. While we know that this will increase the detected rates, methods for tuning QDs in

micropillar cavities must be developed and spin properties in the modified photonic

environment have to be understood.

On the long road to develop a quantum repeater, we present some first results on the interference

from photons from distant QDs, where one of the QDs can be piezo tuned inside a micropillar

cavity.

[1] H.-J. Briegel, W. Dür, J. I. Cirac, and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998).

[2] A. Delteil, Z. Sun, W. Gao, E. Togan, S. Faelt, and A. Imamoglu, Nature Physics 12, 218

(2016)

[3] R. Stockill, M. J. Stanley, L. Huthmacher, E. Clarke, M. Hugues, A. J. Miller, C.

Matthiesen, C. Le Gall, and M. Atatüre, Phys. Rev. Lett. 119, 010503 (2017)

[4] L. Huthmacher, R. Stockill, E. Clarke, M. Hugues, C. Le Gall, and M. Atatüre, Phys. Rev.

B 97, 241413 (2018)

Figure 1: Schematics of a quantum repeater node using two distant quantum dots embedded into micropillar cavities.

6th

international workshop on “Engineering of Quantum Emitter Properties” – Rome, Italy

Deterministic fabrication of circular Bragg gratings around pre-selected

quantum dots for high performance light sources

S. Kolatschek, S. Hepp, M. Sartison, M. Jetter, P. Michler, and S. L. Portalupi

Institut für Halbleiteroptik und Funktionelle Grenzflächen (IHFG), Center for Integrated

Quantum Science and Technology (IQST

) and SCoPE, University of Stuttgart, Allmandring 3,

70569 Stuttgart


Highly efficient single-photon sources are a crucial component for quantum information

processes. Semiconductor quantum dots (QDs) have been proven to be excellent candidates

due to the capability of emitting photons on-demand and the possibility of integrating them

into photonic structures to modify their emission properties. Among different strategies to

increase light extraction, the use of cavity quantum electrodynamics enables the shortening of

the radiative lifetime via the Purcell effect. Together with increased brightness and the

funneling of photons into the cavity mode, also the photon indistinguishability can benefit

from a reduced lifetime. Here, we show a novel deterministic fabrication method for the

integration of preselected QDs into optimized circular Bragg grating cavities by combining

in-situ low-temperature optical lithography [1] and standard electron-beam lithography for

optimal spatial overlap. The used circular Bragg grating cavity exhibits, despite the relatively

low Q-factor, a significant Purcell factor. FDTD simulations are performed in order to ensure

the desired spectral frequency of the cavity mode. A 2-fold Purcell enhancement for a

deterministically positioned circular Bragg grating cavity is shown also when the transition

line is situated on the flank of the cavity [2]. Measuring the modification of the lifetime for

different emitter-cavity mode detunings shows that even under larger detunings between

cavity mode and transition line (36 times the QD linewidth) a non-negligible Purcell factor

can be achieved. This points towards the ability of this cavity to simultaneously enhance

Biexciton and Exciton for the generation of bright entangled photon pairs. Furthermore,

investigations on the bending of the cavity membrane and the effects on the cavity mode and

QD emission are presented.

Figure 1: SEM image of a fabricated circular Bragg grating

[1] M. Sartison et al., Scientific Reports 7, 39916 (2017)

[2] S. Kolatschek et al., submitted (2018)


Coherent and incoherent scattering from resonantly driven quantum dots

Z. X. Koong1*, D. Scerri1, M. Rambach1, E. M. Gauger1 and B. D. Gerardot1

1Institute of Photonics and Quantum Sciences, SUPA, Heriot-Watt University, UK


Semiconductor quantum dots (QDs) can mimic the behaviour of few-level atomic systems. The

optical transitions can be driven resonantly for coherent manipulation, to generate transform-

limited photons, or to address a single spin. However, a QD interacts with its solid-state

environment, coupling to mesoscopic dephasing mechanisms not present in the case of a single

atom. The departure from the natural atomic picture impacts the potential for exploiting the

beneficial aspects of solid-state artificial atoms for future quantum technologies, and hence

approaches to alleviate these detrimental environmental couplings become important. One

approach is to use weak resonant excitation. For a two-level atomic system, the fraction of

elastic (coherently) scattered photons approaches unity in the weak-driving regime. Similarly,

for a three-level spin-Λ-transition atomic system, spin-flip Raman scattered photons inherit the

coherence of the ground spin-state in the weak-driving regime. It is expected that these

behaviours translate to their QD counterpart systems [1,2]. In particular, for minimal population

of the excited state it is expected that the phonon-sideband is eliminated. However, it has

recently been shown theoretically that non-Markovian relaxation leads to an incoherent

phonon-sideband independent of the excitation power and excited state population for a two-

level QD [3]. Here, we investigate the non-Markovian relaxation for a two-level QD and extend

the analysis to a spin-Λ system. We obtain experimental data using resonance fluorescence from

the neutral and negatively charged exciton states in a charge-tunable QD device and apply a a

non-Markovian model derived from solving the master equation in the polaron picture. We

show that the ratio of the zero-phonon line to the total emission, 𝛼𝐷𝑊, is constant regardless of

the power and excitation scheme. As such, we find that the coherent fraction of resonance

fluorescence spectra is limited by 𝛼𝐷𝑊, which is intrinsic for all solid-state emitters. We will

discuss these results in the context of generating indistinguishable single photons.

Figure 1: Resonance fluorescence spectra at (a) 𝛺 ≪ 𝛺𝑠𝑎𝑡 and (b) 𝛺 ≫ 𝛺𝑠𝑎𝑡 (where 𝛺𝑠𝑎𝑡 is the

saturation Rabi frequency) from a two-level QD system. The insets show high resolution spectra of the

zero-phonon line. We find 𝛼𝐷𝑊 = 0.93 in both cases.

[1] C. Matthiesen, A.N. Vamivakas, M. Atature. Subnatural linewidth single photons from a quantum

dot. Phys. Rev. Lett. 108, 093602 (2012); [2] Z Sun, A Delteil, S Faelt, A Imamoğlu, Measurement of

spin coherence using Raman scattering. Phys. Rev. B 93, 241302R (2017); [3] J. Iles-Smith, D. P. S.

McCutcheon, J. Mørk, and A. Nazir, Limits to coherent scattering and photon coalescence from solid-

state quantum emitters. Phys. Rev. B 95, 201305R (2017).


Bright and tunable single-photon sources for quantum optics

T. Lettner1*, K. D. Zeuner1, H. Huang2, S. Scharmer1, S. F. Covre da Silva2, E. Schöll1,

L. Schweickert1, A. Rastelli2, K. D. Jöns1 and V. Zwiller1

1. Quantum Nano Photonics, Department of Applied Physics, KTH Royal Institute of

Technology, Albanova University Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

2. Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz,

Altenbergerstr. 69, 4040 Linz, Austria


Optically active semiconductor quantum dots (QDs) are excellent single-photon sources [1]

with tailorable optical properties [2]. We work with highly symmetric QDs of gallium arsenide

(GaAs) infilled holes obtained by aluminium (Al) droplet etching in Al0.4Ga0.6As [3].

In order to utilize those QDs we develop new structures to efficiently couple the single photons

out of the semiconductor material and into the collection optics of our micro-

photoluminescence (μ-PL) experiment. For this, we employ a low-Q microcavity with a

metallic gold backside mirror. Precise control of the microcavity sidewall curvature allows us

to achieve a parabolic backside mirror shape and enhanced μ-PL intensity (Fig. 1) with an

estimated extraction efficiency of 12.5%.

Furthermore, we integrate our QD microcavity structures onto 200 μm thick PMN-PT

piezoelectric substrates using a polymer-based bonding process. The piezo allows us to induce

a large in-plane biaxial strain into the semiconductor material at low temperature. With our

devices, we tune the emission of the QDs with planar (parabolic) metallic backside mirror by 1

meV (0.4 meV) for 400 V applied to the piezo, in a dynamic, reversible and linear way.

Figure 1: a) Scanning electron microscope image of circular paraboloids obtained by dry

reactive ion etching in an inductively coupled plasma. The viewing angle is 75°. b) μ-PL of a

sample with single quantum dots in parabolic (solid line) and planar metallic backside mirror

(dashed line) microcavity structures. A dark blue drop represents the quantum dot.

[1] P. Senellart et al., Nat. Nanotech. 12, 1026-1039 (2017).

[2] A. Rastelli et al., Phys. Status Solidi B 249, 687-696 (2012).

[3] Y. H. Huo et al., Appl. Phys. Lett. 102, 152105 (2013).



Planar optical antennas as efficient single-photon sources for free-space

quantum optics operation

P. Lombardi1,2*, H. Schauffert3, M. Colautti2, S. Pazzagli2, and C. Toninelli1,2

1. CNR-INO & LENS, via N. Carrara 1, Sesto Fiorentino, Italy

2. LENS & Dip. di Fisica, Università di Firenze, via G. Sansone 1, Sesto Fiorentino, Italy

3. Atominstitute, TU Wien, Wien, Austria


Practical implementations of quantum technologies, ranging from optical quantum computing

to metrological measurements, suffer from the lack of high-rate, on-demand sources of

indistinguishable single photons.

We will discuss a simple and versatile planar optical antenna, showing both theoretical and

experimental evidence of low-loss (< 20%) beaming of the radiation from a single quantum

emitter into a narrow cone of solid angles in free space, which allows in principle up to 50%

coupling into a single-mode fiber.

In particular, we will present an experimental implementation of the design operated at room

temperature, exploiting Dibenzoterrylene molecules (DBT) hosted in an anthracene crystalline

matrix (Ac) [1]. The DBT:Ac system is particularly suitable for this task, due to its outstanding

photo-physical properties (i.e. long-term photostability both at room and cryogenic

temperature, lifetime-limited emission at cryogenic temperatures, 780 nm operating

wavelength) demonstrated in 50 nm-thick crystals [2] and even in nanocrystals [3].

We will finally discuss our recent results about a single-mirror antenna (see fig. 1) operating at

cryogenic temperature, in terms of single photon generation. We demonstrate a photon flux in

the Fourier-limited line higher than 1MHz at detectors, compatible with the metrological

requests for the new definition of the quantum candela [4].

A: schematic (top) and k-vector distribution (bottom) for reference sample, simplified (single

mirror) and complete (double mirror) antenna design;

B: radiated (blue) and emitted (losses included, red) power for double mirror configuration.

[1] Checcucci et al., LS&A (2017); [2]Trebbia et al., PRA (2010);

[3] Pazzagli et al., ACS Nano (2018); [4] https://www.siqust.eu/


Implementing and characterizing resonant two photon excitation in

quantum dot nanowires

Morgan Mastrovich1*, Arash Ahmadi1, Klaus D. Jöns2, Sara Hosseini3, Simon J. Daley4, Jeff

Z. Salvail1, Kevin J. Resch1, Michael E. Reimer4

1. Institute for Quantum Computing and Department of Physics & Astronomy,

University of Waterloo, Waterloo ON Canada N2L 3G1

2. Department of Applied Physics, Royal Institute of Technology (KTH),

Alba Nova University Center, SE, Stockholm Sweden 106 91

3. Institute for Quantum Computing, University of Waterloo,

Waterloo ON Canada N2L 3G1

4. Institute for Quantum Computing and Department of Electrical & Computer

Engineering, University of Waterloo, Waterloo ON Canada N2L 3G1

*[email protected]

Bright sources of single photons with high purity and indistinguishability are desirable for

diverse quantum technologies, including quantum cryptographic protocols and photonic

quantum information processing. Spontaneous parametric downconversion (SPDC) sources

are fundamentally limited in their brightness, as they must be pumped at low power to

preserve the quality of the emitted photons. Quantum dots are a promising alternative

quantum emitter, with the potential to greatly exceed the brightness of SPDC sources without

compromising the quality of the emitted photons.

Embedding the quantum dot within a tapered nanowire waveguide greatly increases the

photon extraction efficiency, while utilizing resonant two-photon excitation decreases the

emission time jitter, thus improving both the single photon purity and indistinguishability.

However, these two techniques have not yet been implemented together. Here, we seek to

combine these two strategies in an effort to improve the quality of emitted photons while

preserving the advantages of the nanowire architecture. We will discuss our progress and

current challenges, both in implementation and measurement.

6th


Towards Scalable Quantum Dot Quantum Technologies: Spatially and

Spectrally Deterministic Technologies

Magdalena Moczała-Dusanowska1, Ł. Dusanowski

1, S. Gerhardt

1, M. Reindl

2, A. Rastelli

2,

R. Trotta2,3

, C. Schneider1, and S. Höfling

1,4

1 Technische Physik and Wilhelm Conrad Röntgen Research Center for Complex Material

Systems, Physikalisches Institut, Würzburg University, Am Hubland, Würzburg, Germany

2 Institute of Semiconductor and Solid State Physics, Johannes Kepler University,

Altenbergerstr. 69, 4040 Linz, Austria

3 Department of Physics, Sapienza University of Rome, Piazzale Aldo Moro 5, Rome, Italy

4 SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, UK


Bright sources of indistinguishable single photons are key components for advanced quantum

optics applications such as quantum communication and linear optical quantum computing.

Among different kinds of quantum emitters epitaxially grown self-assembled quantum dots

(QDs) have been shown to be one of the prime candidates for efficient single photon

generation. In order to maximize number of photons extracted from the device, QDs are

frequently embedded into photonic structures such as miropillar or photonic crystal cavities,

nanowires, waveguides, ring resonators, gratings and micro-lenses. For this purpose, different

methods of deterministic fabrication of micropillar cavities with centered bright quantum dots

have been studied. By combining a low-temperature micro-photoluminescence with a

widefield sample illumination a full position resolved emission spectral maps have been

recorded. It allowed for registering positions of quantum dots in respect to the alignment

marks, making feasible deterministic fabrication of the different kinds of photonic structures

including micropillar cavities by utilizing the high-yield e-beam lithography.

Once the QD is positioned inside an optical cavity, the Purcell effect ensures that light is

emitted predominantly into the cavity mode. However, the exploitation of the Purcell effect

requires a careful spectral alignment of the QD emission and the cavity mode lines. This can

be done by deterministic fabrication of micropillar cavities but still post-fabrication reversible

spectral fine-tuning is required to achieve e.g. for detailed studies of the coupling interaction

or spectral resonance of multiple single-photon source.

In this contribution we demonstrate results of emission tuning of InAs/GaAs QDs embedded

in micropillar cavites. The application of an external stress produces roughly linear shifts of

QDs emission. QD emission can be easily shifted through micropillar cavity.


6th


Quantum-Dot Single-Photon Sources

for Entanglement Enhanced Interferometry

M. Müller1,

*,†, H. Vural

1, C. Schneider

2, A. Rastelli

3, O. G. Schmidt

4,

S. Höfling2,5

, and P. Michler1,

*

1. Institut für Halbleiteroptik und Funktionelle Grenzflächen, Universität Stuttgart, DE

2. Technische Physik, Physikalisches Institut, Universität Würzburg, DE

3. Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, AT

4. Institute for Integrative Nanosciences, IFW Dresden, DE

5. UPA, School of Physics and Astronomy, University of St. Andrews, Scotland, UK

† Currently: Joint Quantum Institute, NIST and University of Maryland, College Park, US

* Electronic address: [email protected], [email protected]

Multiphoton entangled states such as “N00N states” have attracted a lot of attention because

of their possible application in high-precision, quantum enhanced phase determination. So far,

N00N states have been generated in spontaneous parametric down-conversion processes and

by mixing quantum and classical light on a beam splitter. Here, in contrast, we demonstrate

superresolving phase measurements based on two-photon N00N states generated by quantum

dot single-photon sources making use of the Hong-Ou-Mandel effect on a beam splitter [1].

By means of pulsed resonance fluorescence of a charged exciton state, we achieve, in

postselection, a quantum enhanced improvement of the precision in phase uncertainty, higher

than prescribed by the standard quantum limit. An analytical description of the measurement

scheme is provided, reflecting requirements, capability, and restraints of single-photon

emitters in optical quantum metrology. Our results point toward the realization of a real-world

quantum sensor in the near future.

Figure 1: Phase-dependent single-photon count rates (top row) and biphotonic coincidence

rates (bottom row) for exciton (left), biexciton (middle), and trion (right) states [1].

[1] M. Müller et al., Phys. Rev. Lett. 118, 257402 (2017)




Hectometer Revivals of Quantum Interference

M. Rambach1,*, W.Y.S. Lau1, S. Laibacher2, V. Tamma3,2, A.G. White1, and T.J. Weinhold1

1. ARC Centre for Engineered Quantum Systems, School of Mathematics and Physics,

University of Queensland, Brisbane, Queensland 4067, Australia

2. Institut für Quantenphysik and Center for Integrated Quantum Science and Technology

(IQST), Universität Ulm, Ulm, Baden-Württemberg 89069, Germany

3. Faculty of Science, SEES and Institute of Cosmology & Gravitation, University of

Portsmouth, Portsmouth, Hampshire PO1 2UP, United Kingdom


The Hong-Ou-Mandel (HOM) effect is the most famous signature of nonclassical interference.

This inherently quantum effect has a vast amount of applications including photonic entangling

gates, measurement processes and femtosecond spectroscopy and its sensitivity to

distinguishability can be exploited as a precise sensor for phase shifts or temporal and spatial

displacements. Traditional single photon sources exhibit a single dip with a width on the order

of millimetres to centimetres, however, the possibility to significantly extend this distance

opens up new applications in both metrology and quantum networks. In our research we have

created single photons with a linewidth of 429kHz at the Rubidium D1 transition, the narrowest

photons from down-conversion to date, using a novel cavity-enhanced type-II SPDC

source[1,2]. We introduce a new technique, the ‘half-wave plate trick’, cancelling out

birefringence to achieve a triply resonant cavity, spectrally narrowing our photons and

simplifying the stabilisation scheme. The exceptional linewidth combined with the modelocked

nature of the source allows us to observe HOM dip revivals between photons delayed by more

than 100 meters (1/3 of a microsecond), equating to the 84th revival[3], see Fig. 1. Additionally,

the source creates two-photon NOON states, deterministically selectable via their timing

information, which show first-order interference with sub-femtosecond precision. The source

can serve as a novel metrological tool experiencing phase-sensitive NOON-state

superresolution, allowing enhanced precision and the establishment of optical perimeters on a

sub-wavelength scale in a quantum secure way: the HOM will vanish if either photon is altered.

Furthermore, the photons can naturally be exploited in quantum networks based on time

resolved measurements[4] and the entangled frequency combs emitted by the source are a

promising resource for frequency-multiplexed quantum information processing[5].

Figure 1: Coincidence probability of the photon pairs in HOM experiments with delays close

to multiples of the effective cavity round trip (T = 8.28ns, L » 2.5m). The results are presented

for: (a) no delay, (b) half a round-trip, (c) one round trip, and (d) 42 round-trips, the 84th

revival of the interference effect.

[1] M. Rambach, A. Nikolova, T. J. Weinhold, and A. G. White, APL Photonics 1, 096101 (2016). [2] M.

Rambach, A. Nikolova, T. J. Weinhold, and A. G. White, APL Photonics 2, 119901 (2017). [3] M. Rambach, W.

Y. S. Lau, S. Laibacher, V. Tamma, A. G. White, and T. J. Weinhold, Phys. Rev. Lett. 121, 093603 (2018). [4]

V. Tamma and S. Laibacher, Phys. Rev. Lett. 114, 243601 (2015). [5] H.-H. Lu, J. M. Lukens, N. A. Peters, O.

D. Odele, D. E. Leaird, A. M. Weiner, and P. Lougovski, Phys. Rev. Lett. 120, 030502 (2018).


Bright Single InAsP Quantum Dots at Telecom Wavelengths in

Position-Controlled InP Nanowires

S. Haffouz1, K.D. Zeuner2, D. Dalacu1, P.J. Poole1, J. Lapointe1, D. Poitras1, K. Mnaymneh1,

X. Wu1, M. Couillard1, M. Korkusinski1, E. Schöll2*, K.D. Jöns2, V. Zwiller2, and

R.L. Williams1

1National Research Council of Canada, Ottawa, Ontario, Canada, K1A 0R6 2KTH Royal Institute of Technology, Stockholm, 100 44 Sweden


Future developments in photonic quantum information technologies require bright non-

classical light sources, with high collection efficiency, that deterministically emit single

photons. For the implementation of fiber-based quantum networks, sources emitting at telecom

wavelengths around 1310 and 1550 nm are needed. Single semiconductor quantum dots (QDs)

in nanowires are promising candidates to fulfil these requirements.

Site-selected InAsP/InP nanowire QD sources, have previously shown excellent optical and

quantum optical properties for wavelengths around 950 nm such as bright, directional emission

with a Gaussian beam profile1, ultra-low multi-photon emission probabilities (g2(0)<0.005)2

and emission of entangled photon pairs with high fidelity (F>80 %)3.

In this contribution4, we are tailoring the emission properties of the QDs towards telecom

wavelengths. In a first step the emission wavelength of the QD is tuned by changing the As

content in the QD. This results in a drastic decrease in intensity, since the nanowire and shell

dimension do not fit the emission wavelength anymore. Subsequently, the intensity is restored

by adjusting the cladding diameter.

InAsP/InP nanowire QDs with carefully tailored nanowire claddings show bright emission

tunable from 880 nm to 1550 nm with a very low multi-photon emission probability of

g2(0)=0.02. Our results show a promising quantum emitter platform for quantum information

processing applications in the telecom wavelength range.

Figure 1 a) InAsP/InP nanowire QD with tapered InP cladding. b) PL spectra of InAsP/InP

nanowire QDs with tailored cladding diameters.

[1] G. Bulgarini et al., Nano Lett., 14 (7), 4102–4106 (2014).

[2] D. Dalacu et al., Nano Lett. 12, 5919-5923 (2012).

[3] K. D. Jöns et al., Scientific Reports 7, 1700 (2017).

[4] S. Haffouz et al., Nano Lett., 18, 3047–3052 (2018).

a) b)



Coherent Control of On-demand Single Photons from a Quantum Dot in a

Hybrid Quantum Network

L. Schweickert‡*, K. D. Jöns‡, M. Namazi§, G. Cui§, T. Lettner‡, K. D. Zeuner‡, L. Scavuzzo

Montaña‡, S. F. Covre da Silva^, M. Reindl^, H. Huang^, R. Trotta^†, A. Rastelli^, V. Zwiller‡,

and E. Figueroa§

‡Dep. of Applied Physics, Royal Institute of Technology, Albanova University Centre,

Roslagstullsbacken 21, 106 91 Stockholm, Sweden §Dep. of Physics & Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA

^Institute of Semiconductor and Solid State Physics, JKU Linz, 4040, Austria †Dip. di Fisica, Sapienza Università di Roma, Piazzale A. Moro 1, I-00185 Roma, Italy

*Electronic Address: [email protected]

The realization of a long range quantum network, consisting of nodes and links, is currently

pursued intensely with varying technologies. Photons lend themselves to be used as quantum

information carriers, linking network nodes, due to their small interaction cross-section. Since

classical amplification of quantum information to overcome transmission losses cannot be

applied, quantum repeaters are needed along the communication channel. We are developing

a hybrid repeater architecture, interfacing solid state and atomic systems, to combine the

strengths of both. A deeper understanding of the underlying mechanisms governing light

matter interaction is necessary to reliably connect different quantum systems.

Here, we show coherent manipulation of near Fourier-limited single photons, generated on-

demand with a GaAs/AlGaAs quantum dot, using a room temperature 87Rubidium quantum

memory. We show excellent spectral alignment of pure single photons [1] to the F=1 F’=1

transition using precise strain tuning and high resolution spectroscopy. We investigate the

temporal shape of the photons after passive interaction with the atomic vapor at different

temperatures. Active manipulation of the light matter interaction with a control field resonant

with the F=2 F’=1 transition results in an increase in transmission of quantum dot photons.

Our theoretical model confirms the experimental findings with an increase in transmission by

27% when we turn on the control field. [2]

Figure 1: A hybrid interface between a solid state quantum dot source of single photons

(right) and a 87Rubidium vapor cell quantum memory (left).

[1] L. Schweickert et al., On-demand generation of background-free single photons from a

solid-state source, Appl. Phys. Lett. 112, 093106 (2018).

[2] L. Schweickert et al., Electromagnetically Induced Transparency of On-demand Single

Photons in a Hybrid Quantum Network, arXiv:1808.05921 (2018).

6th International Workshop on “Engineering of Quantum Emitter Properties” – Rome, Italy

Hong-Ou-Mandel interference between two weak coherent pulses retrievedfrom room-temperature quantum memories

A. Scriminich1, M. Namazi2, M. Flament2, S. Gera2, S. Sagona-Stophel2, G. Vallone1, P.Villoresi1, and E. Figueroa2.

1. Department of Information Engineering, University of Padova, via Gradenigo 6b, 35131Padova, Italy.

2. Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794,USA.


The possibility to use quantum memories to reversibly transfer quantum states between lightand matter, and to store them and retrieve them on-demand, significantly benefits the field oflong-distance quantum communication. Quantum memories would enable the development oflarge photonic quantum networks, by providing the capability to coherently manipulate,buffer, and re-time photonic signals. They are thus an essential building block for QuantumRepeaters, which have the potential to enable the distribution of entanglement beyond thelimits imposed by the quantum channel [1].Here we present data showing Hong-Ou-Mandel interference between the outputs retrievedfrom two separate room-temperature quantum memories, a milestone in the development of aquantum repeater.Our quantum memories operate through the utilization of Electromagnetically-InducedTransparency (EIT) in a vapor of 87Rb atoms heated at 60°C. In EIT, the interaction of a weaksignal pulse with the atomic medium is mediated via a strong control pulse. Polarization qubitstorage is achieved with a dual-rail configuration. We previously demonstrated ultra-low-noise storage of single-photon-level polarization qubits with a fidelity above 90% and storagetime up to 40μs [2].In this work, we generate temporally identical memory outputs by carefully matching all therelevant parameters in the quantum memories such as two-photon detunings, storage time,filtering transmission, and EIT linewidths. Two 400ns-long weak coherent pulses are stored intwo separate quantum memories and retrieved after a storage time of 1μs. The coincidencerate was measured versus the relative polarization of the output pulses, obtaining aninterference visibility of V = (46.8 ± 3.4)% – close to the theoretical maximum of 50% [4].This result opens the way to the implementation of our quantum memories in a cryptographicnetwork, exploiting for instance Memory-Assisted Measurement-Device-IndependentQuantum Key Distribution (MA-MDI-QKD).__________________________________________________________________________________

[1] N. Lo Piparo, N. Sinclair, and M. Razavi, “Memory-assisted quantum key distributionresilient against multiple-excitation effects,” Quantum Sci. Tech. 3, 014009 (2018).

[2] M. Namazi et al., “Free space quantum communication with a portable quantummemory”, Phys. Rev. Appl., 8, 64013 (2017).

[3] M. Namazi et al., “Ultralow-Noise Room-Temperature Quantum Memory for PolarizationQubits”, Phys. Rev. Appl., 8, 034023 (2017).

[4] M. Namazi, M. Flament, A.Scriminich et al. “A multi-node room-temperature quantumnetwork”, arXiv [1808.07015].

https://doi.org/10.1088/2058-9565/aa9cfb


All-electric on-demand single/entangled photon source with high emission

rate

N. Sherlekar*, B. Buonacorsi, F. Sfigakis, S. Hosseini, J. Baugh and M. E. Reimer

Institute for Quantum Computing, University of Waterloo, Waterloo, Canada N2L 3G1 *

*[email protected]

The development of an on-demand, electrically controlled source of indistinguishable and

entangled photon pairs with a high emission rate is key to the practical realization of novel

quantum technologies such as secure quantum communication [1], quantum metrology [2] and

quantum illumination [3]. Current state-of-the-art such as spontaneous parametric down-

conversion and four-wave mixing sources emit photon pairs with a high level of

indistinguishability and entanglement [4] but do so probabilistically. Ongoing efforts to build

an on-demand source include semiconductor quantum dots [5, 6, 7]. While significant

improvements are being made to these sources, their brightness, collection and extraction

efficiency need further enhancement. Our proposed device aims to solve these problems.

The two major components of this device are (i) a quantized source of electrons that drive the

emission of photons via (ii) a planar p-n junction. By changing the number of electrons pumped

across the p-n interface, both single and entangled photon pair generation can be achieved. This

architecture results in an intrinsically on-demand and deterministic source of photons. It will

also possess very high photon emission rates, in the GHz regime, with the efficiency of

collection improved using Distributed Bragg Reflector (DBR) stacks, thus meeting the

requirements for future quantum technologies. References are allowed, numbering them in

order of appearances [1].

[1] Scarani, V. et al. Rev. Mod. Phys. 81, 1301–1350 (2009).

[2] Cheung, J. Y. et al., J. Mod. Opt. 54, 373–396 (2007).

[3] Lloyd, S., Science 321, 1463–1465 (2008).

[4] Kwiat, P. G. et al., Phys. Rev. Lett. 75, 4337–4341 (1995).

[5] Fognini, A. et al., https://arxiv.org/abs/1710.10815 (2017).

[6] Chen, Y., Zopf, M., Keil, R., Ding, F. & Schmidt, O. G., Nature Communications 9,

(2018).

[7] Huber, D. et al., Physical Review Letters 121, (2018).


Droplet Epitaxy GaAs/AlGaAs QD nucleation regimes on vicinal

GaAs(111)A substrates

Artur Tuktamysheva, Alexey Fedorovb, Sergio Biettia, Shiro Tsukamotoa and Stefano

Sanguinettia

(a) L-NESS and Department of Material Science, University of Milano-Bicocca, 20125

Milano, Italy

(b) L-NESS and CNR-IFN Institute, 22100 Como, Italy

Nanostructure formation is an extremely interesting field of research as it blends fundamental

physics and device application aspects. Among nanostructures, self-assembled epitaxial

quantum dots (QDs) have found application in photonic devices such as QD lasers,

photodetectors, single, and entangled photon emitters. Our interest is a creation of entangled

photon sources, which are necessary building blocks for the long-distance fully secured

quantum key distribution. However, QDs grown on (001) surface is well-studied, but for

entangled photon application is not suitable due to C2v symmetry of the surface and

corresponding fine-structure splitting (FSS). In the case of growth on (111)-oriented surfaces

it is possible to use C3v symmetry and decrease FSS to insignificant values.

Common self-assembly QD growth technique relies on strain-driven Stranski-Krastanov (SK)

growth mode. In SK, 3D islands of the deposited material are formed on the surface of substrate

with different lattice constant. Thereby, in order to fabricate self-assembly GaAs QDs on

Ga(Al)As surfaces, it is necessary to use droplet epitaxy (DE), which does not rely on strain

relaxation for QD fabrication. In DE liquid droplets of a group III element are first formed and

then crystallized by subsequent annealing in group V atmosphere. DE technique allows to

control the size, the density and the shape by growth temperature and fluxes of group III and

V elements.

The fabrication of QDs on singular GaAs(111)A and (111)B, and QD formation on vicinal

GaAs(111)B wafers with a 1° miscut towards (211) has been recently demonstrated. In this

study, we investigated the Ga droplet nucleation and formation of GaAs QDs on vicinal

GaAs(111)A substrates with 2° miscut towards (-1-12) by DE. Two different regimes of Ga

droplet nucleation have been observed. At high deposition temperatures more than 400°C, a

first regime takes place. The critical cluster size i equals 1.5±0.5, meaning that two Ga atoms

are sufficient form stable cluster at high deposition temperature. For this regime the diffusion

energy of Ga adatoms have been calculated and it equals 1.65±0.10 eV. At lower deposition

temperatures we have observed second regime: the density of GaAs QDs depends on Ga flux

almost linearly, thus implying that stable cluster formation may b dominated by the direct

impingement of Ga adatoms with atoms from the flux. The activation energy for 2nd nucleation

regime equals 1.47±0.10 eV. The energy is slightly higher than dissociation energy for Ga-Ga

bond, thus possibly implying that at low deposition temperature stable nuclei may consist more

than two Ga atoms. Capture zone distribution (CZD) method was used to determine the critical

cluster size of Ga droplets in the whole temperature range. At the deposition temperature of

450°C the critical cluster size i, calculated via CZD, equals the value, obtained via flux density

dependence, and at this temperature we have observed Ga droplets formation by diffusion-

limited aggregation. At low deposition temperatures we have obtained that critical Ga cluster

size equals 3±1 atoms. From AFM measurements, at low deposition temperature, the average

capture zone shape has a symmetrical hexagonal shape, which confirms the absence of the step

influence to determine the droplet density distribution. At high deposition temperature the

average capture area becomes asymmetrical and elongated in the direction along steps. Thus

showing the dominance of step controlled Ga diffusion on Ga droplet formation.


Pyramidal Quantum Dots: from artificial atoms to systems

S. Varo1* S. T. Moroni1, G. Juska1, T. H. Chung1, A. Gocalinska1 and E. Pelucchi1

1Tyndall National Institute, University College Cork, Dyke Parade, Cork, Ireland


Quantum dots have emerged as one of the most promising candidates as sources of flying

qubits and entangled photons for quantum information and communication1. However,

despite the tremendous effort on the part of the community2, the inherently random nucleation

process of self-assembled quantum dots has hindered the realization of the truly compact and

scalable devices that should be the hallmark of the second quantum revolution.

On the other hand, the epitaxial growth of site-controlled Pyramidal Quantum Dots3, being

more similar to a conventional semiconductor device fabrication workflow, opens up a series

of possibilities for the engineering of the QD’s immediate surroundings.

Recent endeavours undertaken by our group have shown that with a selectively etched

sacrificial layer, single pyramidal quantum dots can be deterministically transferred to

different substrates, such as optical fibers, allowing in principle a great reduction of the

technological challenges associated to the implementation of linear optical quantum

computation architectures. Recent results and future directions will be discussed.

Figure 1: Pyramidal Quantum Dots transferred on a single mode optical fiber

[1] Benson, O., Santori, C., Pelton, M. & Yamamoto, Y. “Regulated and Entangled Photons

from a Single Quantum Dot”, Phys. Rev. Lett. 84, (2000).

[2] Lodahl, P., Mahmoodian, S. & Stobbe, S. “Interfacing single photons and single quantum

dots with photonic nanostructures”, Rev. Mod. Phys. 87 (2015).

[3] Pelucchi, E., et al., “Semiconductor nanostructures engineering: Pyramidal quantum dots”,

Current Opinion in Solid State and Materials Science, Volume 16, Issue 2 (2012)

https://www.sciencedirect.com/science/journal/13590286

https://www.sciencedirect.com/science/journal/13590286/16/2


Reconfigurable Modulation of a Quantum Light Source in the C-Band

S. Gyger1, K. D. Zeuner1,*, K. D. Jöns1, A. W. Elshaari1, M. Paul1, C. Reuterskiöld Hedlund2,

M. Hammar2, O. Ozolins3, V. Zwiller1

1. KTH Royal Institute of Technology, Department of Applied Physics, Albanova University

Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

2. KTH Royal Institute of Technology, Department of Electronics, Electrum 229, 164 40

Kista, Sweden

3. NETLAB, RISE AB, Isafjordsgatan 22, 164 40 Kista, Sweden


Large scale quantum networks require reconfigurable sources of single photons in order to

transmit bits of information between distant nodes. Given their ability to cover large distances

as flying qubits combined with the possibility to take advantage of the available global fiber

network, photons, particularly in the telecommunication C-band (1.55 µm) are considered the

ideal candidates to exchange quantum information and distribute entanglement between

network nodes.

Semiconductor quantum dots (QDs) emitting at 1.55 µm using a metamorphic growth buffer

have established themselves as sources of single [1] and entangled photons [2] as well as

integrable and tunable [3] sources of single photons and cascaded photon pairs. In order to

establish multi-node quantum networks, multiple single photon sources are required which need

to be tuned in resonance to one another or multiplexed within the same physical channel.

We experimentally demonstrate the reconfigurable integration of our QD-based single photon

source with a commercially available phase modulator. This allows us to create single photon

sidebands with up to 26.5 GHz separation while bridging data-center distances of 1.6 km. We

show that the modulated photons in the sidebands follow the same photon statistics as the

photons directly emitted by the QD.

Figure 1: Setup for single photon sideband generation and generated spectra.

[1] M. Paul, et al., Appl. Phys. Lett. 111, 033102 (2017).

[2] F. Olbrich, et al., Appl. Phys. Lett. 111, 133106 (2017).

[3] K. D. Zeuner, et al., Appl. Phys. Lett. 112, 173102 (2018).

Documents

EQEP-2018 Main Topics and Organizerseqep-workshop.eu/wp-content/uploads/2018/11/Book... · in quantum dot spatial positions after growth, location techniques that enable the creation