Date post: | 22-Dec-2015 |
Category: |
Documents |
View: | 217 times |
Download: | 0 times |
http://qt.tn.tudelft.nl/research/fluxqubit/qubit_rabi.jpghttp://qt.tn.tudelft.nl/research/fluxqubit/qubit_rabi.jpg
http://www-drecam.cea.fr/http://www-drecam.cea.fr/
www.physics.ku.edu www.physics.ku.edu
Superconducting qubits – a timelineSuperconducting qubits – a timeline
1911
Heik
e K
am
erl
ingh O
nnes
Sup
erc
ondu
ctiv
ity in
Hg
1933
Walt
er
Meis
sner
“Meis
sner
eff
ect
” 1957
Sch
nir
man
et
al. –
th
eore
tica
lpro
posa
l fo
r JJ q
ubit
s
1962Superc
urr
ent
thro
ugh a
non-
superc
onduct
ing
gap
1997
Bard
een,
Cooper,
Sch
rieff
er
Th
eory
of
Sup
erc
ondu
ctiv
ity
1998
Devore
t gro
up (
Sacl
ay)
firs
t C
ooper
Pair
Box q
ubit
2000
Luke
ns,
Han (
SU
NY S
B)
Flux q
ub
it
2002
Mart
inis
(N
IST)
phase
qubit
1999
Naka
mu
ra,
Tsa
i (N
EC
)R
abi osc
illati
ons
in C
PB
2006
Mart
inis
(U
CSB
)tw
o-q
ubit
gate
(87%
fid
elit
y)
““Scalable physical system Scalable physical system with well-characterized qubits”with well-characterized qubits”
The system is physical – it is amicrofabricated device withwires, capacitors and such
The system is in principlequite scalable. Multiplecopies of a qubit can beeasily fabricated using thesame lithography, etc.
But: the qubits can never be madeperfectly identical (unlike atoms). Each qubit will have slightly differentenergy levels; qubits must be characterized individually.
““ability to initialize qubit state”ability to initialize qubit state”
Qubits are initialized by cooling to low temperatures (mK)in a dilution refrigerator. This is how:
Energy splittings between qubit states are of the order off = 1 - 10 GHz (which corresponds to T = hf/kB = 50 - 500 mK)
If the system is cooled down to T0 = 10 mK, the ground stateoccupancy is, according to Boltzmann distribution:
P|0> = exp(-hf/kBT0) = 0.82 – 0.98
Lower temperature dilution refrigerators mean better qubitinitialization!
““(relative) long coherence times”(relative) long coherence times”
Coherence times from a fraction of a nanosecond (charge qubits)to tens of nanoseconds (flux) to microseconds (“quantronium”).Correspond to about 10 – 1000 operations before decoherence.Many sources of noise (it’s solid state!)
““universal set of quantum gates”universal set of quantum gates”
Single qubit gates: applying microwaves (1 – 10 GHz) for a prescribed period of time.
Two-qubit gates: via capacitive or inductive coupling of qubits.
Science 313, 1432 (2006) – entanglement of two phase qubits (Martinis’ group – UCSB)
““qubit-specific measurement”qubit-specific measurement”
Measurement depends on the type of qubit.
Charge qubit readout: bifurcation amplifier with bimodal response corresponding to the state of the qubit.
Flux and phase qubits readout: built-in DC-SQUID that detects the change of flux.
Martinis’ qubit: a Martinis’ qubit: a largelarge JJ phase qubit JJ phase qubitUCSB
~1.3 mm
• 95% readout fidelity• 67% Rabi oscillation contrast• 87% entangled state (corrected) fidelity
Superconducting qubits - pros and consSuperconducting qubits - pros and cons
• Cleanest of all solid state qubits.• Fabrication fairly straightforward,uses standard microfab techniques• Gate times of the order of ns(doable!)• Scaling seems straightforward
• Need dilution refrigerators(and not just for noise reduction)• Initialization will always be limitedby Boltzmann factor• No simple way to couple to flying qubits (RF photons not good)• Longer coherence needed, may beimpossible
Superconducting qubits – what can weSuperconducting qubits – what can weexpect in near term?expect in near term?
• More research aimed at identifying and quantifying the major source(s) of decoherence.
• Improved control of the electromagnetic environment – sources, wires, capacitors, amplifiers.
• Entanglement demonstrations in other types of SC qubits.
• Integration of the qubit manipulation electronics (on thesame chip as the qubits themselves).