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)

http://qist.lanl.gov/qcomp_map.shtmlhttp://qist.lanl.gov/qcomp_map.shtml

““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

Devoret’s qubit: the “quantronium”Devoret’s qubit: the “quantronium”Yale

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).