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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
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Lecture 2 outline
1. Motivations for microfabricated traps
2. 3D and 2D microtraps
3. Anomalous heating in traps
4. Near-field microwave techniques
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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
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Quantum computing
quantum register
“accumulator”
segmented electrodes
“quantum CCD” architecture – Wineland et al. (1998)
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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
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ion-electrode distance = 1.2 mmmotional frequencies ~ 1 MHz
UHV < 1x10-11 mbar
10µm
7 mm
10mm
Linear ion trap (“retro” version)
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2D and 3D micro traps
Amini et al. 2008 (in “Atom Chips” ed. Vuletic & Reichel)
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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
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2D (planar) traps
Taken from J Britton’s thesis
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Taken from J Britton’s thesis
2D (planar) traps
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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
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Trap fabrication process
SEM image of electrode layout
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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
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389nm
423nm
continuum
Ca4s2
4s4p
Photo-ionization trap loading
• high absolute efficiency • negligible charging effects
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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
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Micromotion compensation
866nm laser detuning
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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+)
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Junctions
NIST 2008
Michigan 2006
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3. “Anomalous” heating
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Anomalous heating
elec
tric
fiel
d no
ise
WARNING: do not attempt to reproduce these results at home !!!
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Anomalous heating
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In situ cleaning 1: pulsed laser cleaning
Allcock et al. NJP 2011
355nm Nd:YAG5ns pulsed100-200 mJ/cm2
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In situ cleaning 1: pulsed laser cleaning
Allcock et al. NJP 2011
cleaned zone
control zone before and after cleaning
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In situ cleaning 2: Ar+ ion bombardment
Hite et al. PRL 2012
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In situ cleaning 2: Ar+ ion bombardment
Hite et al. PRL 2012
elec
tric
fiel
d no
ise
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Microwave near-field techniques
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Quantum logic with near-field microwaves
C. Ospelkaus et al. Theory: PRL (2008), Experiment: Nature (2011)
static B0
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Microwave trap design
50
xy
z
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HFSS Simulation
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Microwave Testing
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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
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43Ca+ Intermediate Field Hyperfine Qubit
43Ca+ S1/2 Ground State at 146 Gauss
3.2GHz
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43Ca+ Intermediate Field Hyperfine Qubit
Use stretch transition
to servo B-field
static B-field (gauss offset from 146G)
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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
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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)
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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
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Paul trap evolution
~
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