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“Ion Traps for Tomorrow’s Applications”
COST-IOTAEnrico Fermi Summer School, Varenna 2013
David Lucas
University of Oxford, U.K.
Ion Trap Quantum Computing groupwww.physics.ox.ac.uk/users/iontrap
Microfabricated Ion Traps
Lecture 2 outline
1. Motivations for microfabricated traps
2. 3D and 2D microtraps
3. Anomalous heating in traps
4. Near-field microwave techniques
1. Motivations
022
r
eV
m dω =
Ω
1. Tight traps
• easier laser cooling to ground state
• faster quantum logic gates
• easier separation of ions
• interfacing with solid state qubits?
Radial secular frequency:
d: ion-electrode distance scale
Blain et al. 2004
2. Scaling to large arrays of traps
3. Sensing applications
Quantum computing
quantum register
“accumulator”
segmented electrodes
“quantum CCD” architecture – Wineland et al. (1998)
Some microfabricated trap milestones
1990s First wafer traps at NIST
2001 NIST dual-zone trap
2005 Michigan chip trap (semiconductor process)
2006 Michigan T-junction trap
2006 Sandia chip trap (MEMS process)
2006 NIST surface-electrode trap
2008 MIT cryogenic surface traps
2012 NPL 3D silicon trap
2012 Oxford surface trap with integrated microwave elements
ion-electrode distance = 1.2 mmmotional frequencies ~ 1 MHz
UHV < 1x10-11 mbar
10µm
7 mm
10mm
Linear ion trap (“retro” version)
2D and 3D micro traps
Amini et al. 2008 (in “Atom Chips” ed. Vuletic & Reichel)
Univ.Ulm (Schmidt-Kaler group)
Material: evaporated gold on laser-machined alumina waferion-electrode distance 250µmRF drive 25MHz, 140Vtrap depth 76meVradial frequency 1.3MHzheating rate 2.1(3) quanta/ms
Example 3D microfab trap: Ulm
2D (planar) traps
Taken from J Britton’s thesis
Taken from J Britton’s thesis
2D (planar) traps
Material: electroplated gold on quartz (Ti and Ag seed layers)ion-surface distance 150µmRF drive 35MHz, 200Vtrap depth 82meVradial,axial frequencies 3.5MHz, 1.0MHz
filter capacitors
Example 2D microfab trap: Oxford
Trap fabrication process
SEM image of electrode layout
Vacuum System
Calcium ovenmounted atabove chip. Thermal beamis parallel totrap surface.
Imaging throughlarge viewport(conductively coated) Lasers enter and
exitthrough side ports
25-way D-subfor dc electrodes
Trap inUHV-compatibleplastic socket. Vacuum
pumps
RF feedthroughVacuum system
389nm
423nm
continuum
Ca4s2
4s4p
Photo-ionization trap loading
• high absolute efficiency • negligible charging effects
Operating Parameters
RF Amplitude =225 VRF Frequency =25.4 MHZRF Stability Parameter, q = 0.45
Trap Depth = 0.2 eVRadial Secular Frequencies = 4 MHz
-0.24 -3.15 -3.15 -3.15 -0.24
-0.24 -3.15 -3.15 -3.15 -0.24
-1.04
-1.04
rf
rf
0.95 0.95 0.95 0.95 0.95
0
0
rf
rf
1.12 1.12 1.12 1.12 1.12
-0.90
-0.95
rf
rf
5.0 1.9 1.9 1.9 5.0
0.9
rf
rf
-1.03 -1.03 -1.03 -1.03 -1.03
-0.95 -0.95 -0.95 -0.95 -0.95 5.0 1.9 1.9 1.9 5.0
‘Endcap’ Voltagesto produce a500kHz axialsecular frequency
‘Tilt’ voltages to rotate radial normal modes for optimal cooling
x-axis (up-down) micromotioncompensationvoltages (mV per V/m)
y-axis (out of plane) micromotioncompensationvoltages (mV per V/m)
0.9
DC control voltage sets
Micromotion compensation
866nm laser detuning
Trap charging by laser light
- this data for Ca+ at 397nm
- no charging for IR beams (866nm)
- could be worse for UV ions? (e.g. Be+, Mg+)
Junctions
NIST 2008
Michigan 2006
3. “Anomalous” heating
Anomalous heating
elec
tric
fiel
d no
ise
WARNING: do not attempt to reproduce these results at home !!!
Anomalous heating
In situ cleaning 1: pulsed laser cleaning
Allcock et al. NJP 2011
355nm Nd:YAG5ns pulsed100-200 mJ/cm2
In situ cleaning 1: pulsed laser cleaning
Allcock et al. NJP 2011
cleaned zone
control zone before and after cleaning
In situ cleaning 2: Ar+ ion bombardment
Hite et al. PRL 2012
In situ cleaning 2: Ar+ ion bombardment
Hite et al. PRL 2012
elec
tric
fiel
d no
ise
Microwave near-field techniques
Quantum logic with near-field microwaves
C. Ospelkaus et al. Theory: PRL (2008), Experiment: Nature (2011)
static B0
Microwave trap design
50
xy
z
HFSS Simulation
Microwave Testing
Microwave trap design
500um Sapphire substrate for heat dissipation
HFSS simulation of currents
and B-field in trap region
Ion is 75um
off surface
at B-field null
43Ca+ Intermediate Field Hyperfine Qubit
43Ca+ S1/2 Ground State at 146 Gauss
3.2GHz
43Ca+ Intermediate Field Hyperfine Qubit
Use stretch transition
to servo B-field
static B-field (gauss offset from 146G)
c.f. B. Keitch et al. (2007) T2 = 1.2 sec (single 43Ca+ ion, low-field clock state)
C. Langer et al. (2005) T2 = 15 sec (single Be+ ion, intermediate-field clock state)
J. Bollinger et al. (1992) T2 ~ 600sec (~1000 ions, high-field clock state)
43Ca+ qubit: coherence time measurements
with CPMG
sequence
0.93 at 16sec
T2 = 48(10)sec
Randomized benchmarking of single-qubit gates
Recipe (Knill et al. 2007):
Apply random Clifford gates (π/2 pulses) from set x=σX,x=σ-X,y=σY,y=σ-Y
x x y y x x y x y x x y x y y x …
Then randomize again by inserting Pauli gates (pi pulses) randomly chosen
from the set +I,-I,+X,-X,+Y,-Y,+Z,-Z:
x Z x I y X y Z x Y x I y Z x Z y Y x Z x X y X x X y Z y I x …
Finish by rotating the qubit into the measurement (Z) basis:
x Z x I y X y Z x Y x I y Z x Z y Y x Z x X y X x X y Z y I x … y [measure]
Similar implementation to K.Brown et al. (2011)
Randomized benchmarking of single-qubit gates
Prep./readout
error 7x10-4
T 2=50se
c
~160ms
Mean error per gate
= 0.9(3) parts-per-million
gate time (pi/2) = 12µs
Paul trap evolution
~
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