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Superconducting QubitsAndreas Wallraff
Department of Applied Physics, Yale University
NIST
ChalmersNEC
TU Delft
with supporting material from: M. Devoret, D. Esteve, S. Girvin, J. Mooij, R. Schoelkopf, L. Vandersypen
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Motivation
M. Nielsen, I. Chuang, Quantum Computation and Quantum Information (Cambridge, 2000).
• build scalable macroscopic quantum circuits
• control open quantum systems
• investigate quantum measurement process
• learn about decoherence in solid state systems
• build a quantum computer
• solve computationally hard problems
current goals for solid state implementations:
long term goals:
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Schematic of a Generic Quantum Processor
M. Nielsen, I. Chuang, Quantum Computation and Quantum Information (Cambridge, 2000).
01
01
01
2 level systems:qubits
2 qubit gates:controlled
interactions
U1U1single qubit
gates
0
readout
1?
with excellent gate, readout, … accuracy for Q.C.
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Outline
• how to make qubits from superconducting circuits
• realizations of superconducting qubits
• controlling qubits
• coherence/decoherence
• qubit readouts and measurements
• coupled qubits
• conclusions
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A Generic Qubit
D. P. DiVincenzo, arXiv:quant-ph/0002077 (2000)
• existence of quantum two level system (a qubit)
• qubit initialization (reset)
• qubit coherence (no dissipation, no dephasing)
• qubit control (gate operations)
• qubit readout
DiVincenzo criteria:
two-level quantum system (a spin ½)
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Building Qubits with Integrated Circuits
requirements for quantum circuits:
• low dissipation
• non-linear (non-dissipative elements)
• low (thermal) noise
a solution:
• use superconductors
• use Josephson tunnel junctions
• operate at low temperaturesU(t) voltage source
inductor
capacitor
resistor
voltmeters
nonlinear element
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LC Oscillator as a Quantum Circuit
+qφ
-q
φ
E
[ ], iqφ = h
1 GHz ~ 50 mK
problem I: equally spaced energy levels (linearity)
low temperature required:hamiltonian
momentumposition
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Dissipation in an LC Oscillator
impedance
quality factor
internal losses:conductor, dielectric
external losses:radiation, coupling
total losses
excited state decay rate
problem II: avoid internal and external dissipation
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A Superconducting Nonlinear Element
M. Tinkham, Introduction to Superconductivity (Krieger, Malabar, 1985).
Josephson junction
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The Josephson Junction
a nonlinear inductor without dissipation
nonlinear current flux relation:
gauge inv. phase difference:
nonlinear Josephson inductance:
voltage:
Josephson energy:
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Building Blocks for Qubits
Eartificial
atom
~ 0.5 K (10 GHz)
all ingredients available: macroscopic artificial atoms:
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Superconducting Qubits
NISTChalmersNEC
TU Delft
flux phasecharge
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Charge Qubits
Cooper pair box
electrostatic energy Josephson energy
charging energy
Josepshon energy
gate charge
V. Bouchiat, D. Vion, P. Joyez, D. Esteve, and M. H. Devoret, Physica Scripta T76, 165 (1998).
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Tunable Charge Qubits
split Cooper pair box
SQUID modulation of Josephson energy
J. Clarke, Proc. IEEE 77, 1208 (1989)
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Cooper Pair Box Energy Levels
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Two-State Approximation
K. Lehnert et al. PRL. 90, 027002 (2003).
Nakamura, Pashkin, Tsai et al. Nature 398, 421, 425 (1999,2003, 2003)
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Control of Charge Qubit
effective hamiltonian
energy splitting
control parameter
gate charge
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Control of Charge Qubit
effective hamiltonian
energy splitting
control parameter
gate charge
X
Z
Y
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Single Qubit Control
Z
1
0
X
Y
t
x,y rotations by microwave pulses
1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0
- 5 0
0
5 0
1 6 G H zπ p u ls e
Mic
row
ave
outp
ut v
olta
ge (m
V)
t im e ( n s )
z rotations by adiabatic pulses
Bloch sphere representation of single qubit manipulation
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Flux Qubits
A. Barone and G. Paterno, (Wiley, New York, 1992)
charging energy
inductive energy
Josephson energy
kinetic energy potential energy
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RF-SQUID Potential
parabolic potentialwith cosine corrugation
energy level splitting at bias flux dependence
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Control of Flux Qubits
effective hamiltonian
splitting energy
control parameter
flux frustration
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Variation of the Flux Qubit
J. E. Mooij, T. P. Orlando, ... , C. H. van der Wal and S. Lloyd, Science 285, 1036 (1999)C. H. van der Wal, A. C. J. ter Haar, ... , S. Loyd and J. E. Mooij, Science 290, 773 (2000).
persistent-current quantum bit:flux qubit with three junctions, small geometric loop inductance
H = hσz + tσxwith h=(Φ/Φo-0.5) ΦoIp
0
-1
0
1
0.5
↑Icirc
↑E
Φ/Φo→
2t
+Ip
-Ip
0
Φk
i
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Phase Qubits
J. M. Martinis, M. H. Devoret and J. Clarke, Phys. Rev. B 35, 4682 (1987)
current biased junction
kinetic energy potential energy
charging energy
Josephson energy
bias current
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Potential of Current Biased Junction
particle in a washboard potential
potential
J. M. Martinis, M. H. Devoret and J. Clarke, Phys. Rev. B 35, 4682 (1987)
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Energy level quantization
cubic potential near I = I0
oscillation frequency
barrier height
J. M. Martinis, S. Nam, J. Aumentado, and C. Urbina, Phys. Rev. Lett. 89, 117901 (2002)
use eigenstates as basis states of qubit
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Control of Phase Qubits
M. Steffen, J. Martinis and I. L. Chuang, Phys. Rev. B 68, 224518 (2003).
effective hamiltonian
‘splitting’ energy
control parameter
bias current
operations:
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relaxation: transverse fluctuations at qubit transition frequency
Decoherence: Relaxation and Dephasing
dephasing: parallel fluctuations (in qubit level sep.) at low frequencies
1
life time coherence time
A. Abragam, Principles of Nuclear Magnetic Resonance (Oxford University Press, Oxford, 1985)
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Measuring Relaxation
x
y
z
π/2pulse
πpulse
0
1tw
wait measure
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0 1 2 4 5 6 730
35
40
45
50
switc
hing
pro
babi
lity
(%)
time tw (µs)
Relaxation Measurement
Exponential fit:T1 = 1.84 µs
Q1 ~ 90 000
tw
D. Vion, A. Aassime, A. Cottet, ... , D. Esteve, and M.H. Devoret, Science 296, 286 (2002).
~ ex
cite
d st
ate
occu
patio
n pr
obab
ility
(%)
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time
prepa-ration
π/2pulse
freeevolution
π/2pulse
measure-ment
Ramsey fringe experiment
Measuring Quantum Coherence (I)
determine coherence time T2
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time
prepa-ration
π/2pulse
freeevolution
π/2pulse
measure-ment
Ramsey fringe experiment
Measuring Quantum Coherence (II)
determine coherence time T2
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Measurement of Ramsey Fringes
0.0 0.1 0.2 0.3 0.4 0.5 0.6
30
35
40
45
sw
itchi
ng p
roba
bilit
y (%
)
time between pulses ∆t (µs)
0.0 0.1 0.2 0.3 0.4 0.5 0.6
30
35
40
45
sw
itchi
ng p
roba
bilit
y (%
)
time between pulses ∆t (µs)
νRF = 16409.50 MHz
0.0 0.1 0.2 0.3 0.4 0.5 0.6
30
35
40
45
sw
itchi
ng p
roba
bilit
y (%
)
time between pulses ∆t (µs)
Qϕ ~ 25000
∆t
f-f01=20.6 MHz
D. Vion, A. Aassime, A. Cottet, ... , D. Esteve, and M.H. Devoret, Science 296, 286 (2002).
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Qubit Readouts
- negligible coupling between readout and qubit in OFF state
- no dephasing, no relaxation
- strong coupling in ON state
- minimal relaxation (QND)
- high fidelity
01 0
readout
1?
back action
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Readouts for Superconducting Qubits
NIST
ChalmersNEC
TU Delft
Schoelkopf et al, Yale
fluxphase
dispersive charge
charge-phase
charge
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Phase Qubit Direct Tunneling Readout
|0>|1>
pump&probe: ω21 microwave pulse current pulse (lower barrier)
<V> = 0 <V> = 1 mV
phase
pote
ntia
l
|2>
tunneling rates
advantages: on-chip built-in amplification
disadvantages: - on-chip dissipation- quasi particle generation- decoherence
state measurement : |0> : zero voltage|1> : voltage
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Phase Qubit: Rabi Oscillations
J. M. Martinis, S. Nam, J. Aumentado, and C. Urbina, Phys. Rev. Lett. 89, 117901 (2002)
ω10 ω31
tr
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Phase Qubit SQUID Readout
R. W. Simmonds, K. M. Lang, ... and J. M. Martinis, Phys. Rev. Lett. 93, 077003 (2004)
tunneling readout with on-chip DC-SQUID amplifier
U(δ)
~50
00 s
tate
s
“0”
“1”
… fastdecay
1 Φ0
SQUID flux
Sw
itchi
ngcu
rren
t10 µΑ
0- sample and hold readout- no quasi particles
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Phase Qubit: Rabi oscillations
R. W. Simmonds, K. M. Lang, ... and J. M. Martinis, Phys. Rev. Lett. 93, 077003 (2004)
Is
Iφ
time
qubit cycle
Qubit Op Meas Amp Reset flux
Measure p1
large visibility
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Cooper Pair Box Readout: Quantronium
D. Vion, A. Aassime, A. Cottet, ... , D. Esteve, and M.H. Devoret, Science 296, 286 (2002).
- high impedance capacitively coupled write and control port- low impedance inductive readout
I0 Ib
V=0or
V≠0
U
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Ramsey oscillations in the Quantronium
D. Vion, A. Aassime, A. Cottet, ... , D. Esteve, and M.H. Devoret, Science 296, 286 (2002).
- operation at optimal point- long coherence time
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Flux Qubit with Bulit-In Readout
I. Chiorescu, Y. Nakamura, C. J. P. M. Harmans, and J. E. Mooij, Science 299, 1869 (2003).
- inductively coupled hysteretic DC-SQUID for readout- high impedance inductive write and control port
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Rabi Oscillations with Flux Qubit
I. Chiorescu, Y. Nakamura, C. J. P. M. Harmans, and J. E. Mooij, Science 299, 1869 (2003).
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Ramsey Fringes with Flux Qubit
I. Chiorescu, Y. Nakamura, C. J. P. M. Harmans, and J. E. Mooij, Science 299, 1869 (2003).
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Dispersive Read-Out of Charge Qubit
A. Wallraff, D. Schuster, A. Blais, ..., S. M. Girvin and R. J. Schoelkopf, Nature 431, 162 (2004).
- dispersive measurement of qubit susceptibility- no on-chip dissipation- quantum non-demolition measurement (QND)- measurement back-action understood
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The CPB: State Dependent Capacitance
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A Cooper Pair Box in a Cavity
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Ramsey Fringes with Circuit QED Readout
A. Wallraff, D. Schuster, A. Blais, ..., S. M. Girvin and R. J. Schoelkopf, unpublished (2004).
- long life time T1 ~ 5 µs- long coherence time
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Realizations of Coupled Qubits
J. B. Majer, F. G. Paauw, A. C. J. ter Haar, C. P. J. Harmans, J. E. Mooij, cond-mat/0308192.Pashkin Yu. A., … , Nakamura Y., Averin D. V., and Tsai J. S., Nature 421, 823-826 (2003).
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Coupled Qubits
Ising coupling
1
0 0
1
π
1 0
00
01
10
11
00
11
10
01
CNOT
control bit (2)target bit (1)
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2 Qubit Gates: Controlled-NOT
if spin B is ↑
YA90 XA
-90Delay1/2JAB
if spin B is ↓
Before AfterA B A B↑ ↑ ↑ ↑↑ ↓ ↓ ↓↓ ↑ ↓ ↑↓ ↓ ↑ ↓
” flip A if B ↓”
xy
z
xy
z
xy
z
xy
z
xy
z
xy
z
xy
z
xy
z
| 0 ⟩
| 1 ⟩
| 0 ⟩ + i | 1 ⟩√2
| 0 ⟩ + | 1 ⟩√2
time
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Realization of Controlled-NOT
T. Yamamoto, Yu. A. Pashkin, O. Astaflev, Y. Nakamura, J. S. Tsai Nature 425, 941 (2003)
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Conclusions
• superconducting qubit architectures have been realized
• different readout strategies have been tested
• qubit initialization, single qubit control has been demonstrated
• first two-qubit gates have been implemented
• realize high fidelity, single-shot qubit readout
• control decoherence (increase T1 and T2)
• understand limitations imposed by circuit materials and fabrication
• integrate multi-qubit circuits
challenges:
achievements:
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NIST
ChalmersNEC
TU Delft
many thanks to:
M. Devoret, D. Esteve, S. Girvin, J. Mooij, R. Schoelkopf, L. Vandersypen