Jaewook Ahn

Neutral Atom Quantum Computing
Jaewook Ahn
Department of Physics, KAIST
February 20, 2013 at KIAS
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Quantum Computing
Quantum computers (aka, quantum supercomputer) are computation devices
that make direct use of quantum‐mechanical phenomena, such as superposition
and entanglement, to perform operations on data.
quantum computation
bit = 0 or 1
Bit measurements are
independent.
Measurement collapses qubit states.
Universal quantum computation requires :
qubit rotations (at least about 2 axes) + controlled NOT gate (C‐NOT)
Prototype Quantum Computers
Requires: (1) qubit coupling to other qubits and control field must be strong
but (2) decoherence must be minimal.
Ion traps (1D ion array in a Paul‐trap)
 Up to 14 trapped ions
 hyperfine state qubits, entangled by vibration
 1D architecture requires T‐junctions.  chip‐based ion‐traps.
Josephson junction array (superconducting qubits)
 Commercial D‐Wave One (128 qubits)
 Quantum annealing process for quantum simulation
NMR quantum computer (liquids, solids)
Solid‐state qubits (quantum dots, diamond NV‐centers, ions in solids, etc.)
Linear optical quantum computer (KLM scheme, integrated photonic circits)
Neutral atom quantum computer (optical lattice, Rydberg‐atom, etc.)
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Neutral Atomic Quantum Gates (1)
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Neutral Atomic Quantum Gates (2)
C‐Phase gate
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Rydberg‐Atom Fast Quantum Gates
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Rydberg atom dipole blockade
Blockade radius |r>
X
|0>
Atom 1 Atom 2
Rydberg blockade effect Properties
(Rubidium)
Orbital radius (m)
Radiative lifetime(s)
Ryd‐Ryd int. (GHz m)
Blockade radius (m)
Speed limit (GHz)
scaling
53d
62d
82d
90d
0.19
0.26
0.45
0.54
126
202
468
618
73
409
8870
25000
0.4~0.8 1.1~1.7 2.1~3.0
44
27
11
10.5
8.3
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Computation Clock Speed
Computational power can be enhanced via the clock speed improvement.
Operation Number = Coherence Time X Clock Speed
• Clock speeds of current quantum computer technologies : MHz‐level • Femtosecond laser technology can increase the Rabi frequency to THz‐level.
CW‐Laser
Ultrafast Laser (This proposal)
Recent experiment [Lim] achieved single‐qubit operations at a THz clock speed,
so we intend to further extend this capability to N‐qubit operations.
Lim et al., “Terahertz‐rate qubit operations by ultrafast laser pulses," (2013).
Ultrafast Optical
Quantum Optics
: Quantum computing at THz clock
speeds
Jaewook Ahn
KAIST – physics
(at QUC spring)
Ultrafast optical control of atomic qubit
(1) THz-rate Rabi oscillation
(2) fs-Ramsey interferometry
(3) Towards N qubits
Contributors
Jongseok Lim, Han-gyeol Lee, Hyosub Kim
(KAIST)
Sangkyung Lee, Changyong Park (KRISS)
Funded by
Rabi and Ramsey Measurements
We have developed femtosecond-laser version of
Rabi and Ramsey measurement methods
Rabi measurement
ultrashort pulse
x-rotation
(1) Apply a resonant pulse for different Rabi-oscillation.
(2) Measure the upper-state population.
(3) Often used for energy relaxation time(T1).
photo-ionization
Ramsey measurement
phase shift (fs-resolution) y-rotation
(1) Apply a -pulse (R1).
(2) After a time delay, apply another -pulse (R2).
(3) Measure the upper-state population for dephase time(T2).
(4) In the middle, apply a -pulse to recover the dephase (spin-echo).
(5) Z-rotation (z) shifts the Ramsey oscillation.
pulse-composite
dispersive pulse
(working)
z-rotation
error-correction
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Ultrafast Single‐Qubit Gates
Universal quantum computation requires all 1‐qubit gates and the 2‐qubit C‐NOT.
All 1‐qubit SU(2) gates were implemented in our recent experiment.
Control parameters : phase, detuning, and pulse area.
Cascaded operations
• Rabi rotations and Hadamard gates are performed with tailored fs‐laser pulses.
• Cascaded quantum operations are performed at a THz clock speed.
Lim et al., “Terahertz‐rate qubit operations by ultrafast laser pulses," (2013).
Example: Rabi osc. in a 3-level system
Two-photon transition 5S-5P-5D (Rb)
laser oscillator
100 MHz
0.1
0
8-pass
amplifier





(a)
beam shaping apparatus
ND filter
(b)
Rb cell
100
120
60
0
-100
sit
io
n
0
0
( 100
m
)
0
−100
100
50
0
J. Lim, H. Lee et al, Opt. Lett. 37, 3378 (2012).
180
po
pixel #
interference
filter (420 nm)
200
100
position
(c)
(m)
intensity
6P3/2
420 nm
780 nm

3.5 Rabi cycles within 35 fs duration!
PMT
KAIST - Physics

pulse area total
5P3/2
5S1/2

compressor
Rb atom
776 nm
5D
pump laser
Pockels cell
1 kHz
Region 3
0.2
intensity (arb.)
stretcher
Region 2
0.3
1 2 2
 1  2
2
population
total
Region 1
0
50
100
150
200
intensity (arb. unit)
−400
0
position (m)
400
Atoms at the focus of laser beam
(a)
Laser oscillator
100 MHz
Pump laser
ND filters
cm/s
BS
BS
(b)
BS
BS
Compressor
PBS
Rubidium D2 line 
Optical pumping
1mm
B
B
O
mechanical
shutter
85
Rb
MOT
delay
stage
MCP
Bias plate
KAIST - Physics
8-pass
amplifier
Pockel cell
1 kHz
Stretcher
(c) Rb atom
ionization
cont.
Trapping laser
Repumping laser
Magneto-optical trap
spectrum
795 nm
6-axis optical cooling
+ anti-Helmholtz coil
397 nm
5P1/2
795 nm
5S1/2
MCP
Spatial average effect
Cold atoms trapped in Magneto‐Optical trap (MOT)
Interaction region
Laser beam
MOT profile
x, position
Trapped atom
Rabi measurement
pulse
pulse
Spatial average
effect
Sixteen – pulses (fidelity check)
Spatial average effect
16 pulses
Single-operation
Fidelity > 0.95
Pulse shaper
In the interaction picture, phase is induced by
carrier-envelope phase (phase-control)
without changing the inter-pulse delay.
CEP control
phase shift = subcycle pulse delay
/2
pulses
Ionization pulse sweep
Sequential quantum gates (– control)
Ultrafast Ramsey interferometry
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Repeat (2 Hz)
~0.7 ps
MOT off
/2
pulse
0~2 ps
X-rotation
Pulse ()
time
10 ps
/2
pulse ()
Ionization MOT on
pulse
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Qubit‐State Measurement: fs‐Ramsey
The read‐out of qubit states requires both population and phase measurements.
We devised femtosecond Ramsey interferometer to measure the phase.
The population is measured through photo‐ionization of the qubit 1‐state.



Lim et al., “Terahertz‐rate qubit operations by ultrafast laser pulses," (2013).
Ultrafast Optical
Quantum Optics
: Quantum computing at THz clock
speeds
Jaewook Ahn
KAIST – physics
(at QUC spring)
Ultrafast optical control of atomic qubit
(1) THz-rate Rabi oscillation
(2) fs-Ramsey interferometry
(3) Towards N qubits
Contributors
Jongseok Lim, Han-gyeol Lee, Hyosub Kim
(KAIST)
Sangkyung Lee, Changyong Park (KRISS)
Funded by