Silicon-based Quantum Computation

Cheuk Chi Lo Kinyip Phoa

Dept. of EECS, UC Berkeley

C191 Final Project PresentationNov 30, 2005

Silicon-based Quantum Computation: Presentation Outline

I. IntroductionII. Proposals for Silicon Quantum

ComputersIII. Physical Realization: Technology and

ChallengesIV. Summary and Conclusions

Introduction: Why Silicon?

We know silicon from years of building classical computers

Donor nuclear spins are well-isolated from environment low error rates and long decoherence time

Integration of quantum computer with conventional electronics

Scalability advantages?

Introduction: DiVincenzo’s Criteria

1. Well-defined qubits

2. Ability to initialize the qubits

3. Long decoherence time

4. Manipulation of qubit states

5. Read-out of qubit states

6. Scalability (~105 qubits)

II. Overview of Silicon Quantum Computation Architectures

Silicon Quantum Computer Proposals

Shallow Donor Qubits Deep Donor Qubits Silicon-29 Qubits

Exchange Coupling

Magnetic Dipolar Coupling

Electron Shuttling

Silicon Shallow Donor Qubits: Qubit Definition and State Manipulation

barrierSilicon-28

Control gate

A-Gate(Hyperfine Interaction)

J-Gate(Exchange Coupling)

magnetic dipolar coupling

S-Gates(Electron shuttling)

BDC

BAC

BE Kane, Nature, 393 14 (1998)AJ Skinner et al, PRL, 90 8 (2003)R de Sousa et al, Phys Rev A, 70 052304 (2004)

Spin Resonance

Qubit

Summary of Silicon Shallow Donor Qubits

Qubit: donor nuclear spin or hydrogenic qubit (nucleus + electron spins)

Initialization: Recycling of nuclear state read-out + nuclear spin-state flip via interaction with donor electron

Decoherence time: e.g. at 1.5K nucleus spin T1 > 10 hours electron spin T1 > 0.3hours

Qubit Manipulation Single Qubit Manipulation: hyperfine interaction + spin

resonance Multi-qubit Interaction: Exchange coupling, Magnetic

dipolar coupling or Electron shuttling Read-out: Transfer of nucleus spin state to donor electron

via hyperfine interaction, then read-out of electron spin state

Physical Realization of a Si QC

Some common features that must be realized in a shallow donor Si QC are:Array of single, activated 31P atoms:Single-spin state read-out: Integrated control gatesProcess Variations

Formation of Ordered Donor Arrays

JL O’Brien et al, Smart Mater. Struct., 11 741 (2002)

“Top-down” single ion implantation

“Bottom up” STM based Hydrogen Lithography

T Schenkel et al, APR, 94(11) 7017 (2003)

Spin-State Read-out with SET’s & Fabrication of Control Gates

Read-out Challenges: i. SET’s are susceptible to 1/f and telegraphic

noises (from the random charging and discharging of defect/trap states in the silicon host)

ii. alignment and thermal budget of SET’s with the donor atom sites also present as a fabrication challenge.

Read-out: Spin state Charge state (e.g. measurement by SET)

Control Gate Challenges: Qubit-qubit spacing requirements for different coupling

mechanisms: Exchange Coupling: 10-20nm Magnetic Dipolar Coupling: 30nm Electron Shuttling: >1m

State-of the art electron beam lithography: can do ~10nm, but not dense patterns Qubit interaction control gates extremely challenging!

(L Chang, PhD Thesis, EECS)

(UNSW)

Process Variations

(IBM)

Process Variations may arise from:i. substrate temperature gradient,

ii. uneven reagent use during fabrication,

iii. differences in material thermal expansion

iv. strain induced by the patterning of the substrate (leads to uncertainty in ground state donor electron wavefunction, due to incomplete mixing of states)

Consequences:i. Need careful tuning and initialization of

qubits

ii. Limit of scalability?

iii. Introduce strain in silicon intentionally?

• lifts degeneracy of electronic state less vulnerable to process variations

Silicon Deep Donors Proposal

Excited State

Ground State

Optical Excitation

Bi Er Bi

Bi Er Bi

Bi Er Bi

AM Stoneham et al, J. Phys.: Condens. Matter, 15 (2003), L447

Initialization, Manipulationand Readout?

Initialization by polarized light or injection of polarized electron both are not very possible under room temperature

Manipulation with microwave pulses like the work by Charnock et. al. on N-V centers in diamond

Readout optically detection of photons emitted potentially require detection of single photon

Disorderness of donor ion Irreproducibility and difficult to address qubits

Decoherence Time andThermal Ionization

Summary of Silicon Deep Donor Qubits

Qubit: deep donor (e.g. Bismuth) nuclear spin, proposed to work at room temperature.

Initialization: Optical pumping or injection of polarized electron, questionable in feasibility.

Decoherence time: fraction of nanosecond at room temperature

Qubit Manipulation: via applying intense microwave pulse, like N-V centers in diamond

Read-out: optical readout of photon emitted from transition between two states

Silicon-29 Quantum Computer Overview

NMR-type quantum computer

Initialize with circularly polarized light

Manipulating qubits by Dysprosium (Dy) magnet

Readout using MRFM CAI

TD Ladd et. al. , PRL, 89(1) 017901, 2002

Decoherence Times

Long decoherence time (T1 and T2)Reported T1 as large as 200 hours,

measured in darkExperimentally find T2 as long as 25

secondsT2 is reduced by the presence of 1/f

noise due to the traps at lattice defects and impurities

Summary of Silicon NMR quantum computer

Qubit: Chains of silicon-29 isotope for ensemble measurement

Initialization: Optical pumping with circularly polarized light Decoherence time: measured as long as 200 hours in dark

at 77K for T1 but only 25 seconds for T2 Qubit Manipulation: combination of static magnetic field

and RF magnetic field Read-out: with cantilever, performing MRFM CAI

Problem:RF Coil, Dy Magnet & MRFM

The deposition method of Dy magnet is not outlined! It won’t be trivial to incorporate

The cantilever tip for MRFM is not included in the schematic. How to insert it?

TD Ladd et. al. , PRL, 89(1) 017901, 2002

Summary and Conclusions

Several proposals for implementing quantum computer in silicon Shallow donor (phosphorus) qubit Deep donor (bismuth) qubit Silicon-29 NMR quantum computer

Difficulties faced in each proposals Arguments on the feasibility Most experimental efforts are on shallow donor qubits Convergence with conventional electronics processing

requirements: Currently: 90nm technology node (~45nm features) 22nm technology node in 2016! Strained-silicon: hot topic of research in semiconductor industry Narrower transistor performance window with ordered dopants Single-electron transistors and other nanoelectronics

(http://www.ITRS.net)

Thank You

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