Interfacing quantum optics and solid-state devices

Hybrid solutions for quantum computing

Margareta WallquistInstitute for Theoretical Physics

University of Innsbruck Institut for Quantum Optics and Quantum Information

Austrian Academy of Sciences

Outline

Scalable quantum information processing – why hybrids?

Superconducting circuits – useful QC hardware?

Ionic and molecular qubits

Hybrid devices – not trivial, not boring

A detail: molecule cooling via superconducting cavity

Conclusion and outlook

Margareta Wallquist, Innsbruck

Hardware requirements for scalable quantum computing

• Ability to initialize [in quantum ground state]

• Universal set of gates feasible with hardware

• Hardware specific measurement for read-out

• Scalable hardware with well characterized qubits

• Coherence time much longer than gate operation time

• Ability to interconvert stationary and flying qubits (photons)

Margareta Wallquist, Innsbruck

Innsbruck

(optical) quantumcommunication

fast quantumoperations

transmission lines(electric)

scalability

long-lived quantum memories

Vision of a hybrid quantum information processor

Thanks P. Rabl for figures

Margareta Wallquist, Innsbruck

Superconducting circuits – macroscopic quantum two-level systems

• Designed and fabricated for specific tasks

• Scalable construction

• Basic element: Josephson junction

– (μm)^2

– capacitance, fF

– tunneling: critical current, nA

• Charge/Charge-Phase qubits Flux qubits Phase qubits

• Straightforward control: bias current, voltage, flux

• Fast gate operations, ~ ns

• Too short coherence time - ~ ns – μs

– major problem to be solved

Margareta Wallquist, Innsbruck

φ

EJ

CSaclay

Delft

Superconducting qubits: experimental achievements

Margareta Wallquist, Innsbruck

2004

2000

2007

2002

1999

2003

2003

Atomic quantum computers

• High-precision control of single / few ions

– developed for e.g. atomic clocks

• Qubit encoded in electronic states, laser controlled

• Scalable construction

– multi-zone traps, surface electrode traps,...

• Weak interaction with its environment long coherence time, > 10 ms

• Gate operations are relatively slow, ~10 μs

Margareta Wallquist, Innsbruck

Michigan T-trapInnsbruckquantumcomputer

S1/2

P1/2D5/2

qubit

Ca+

Ions and molecules: experimental achievements

Margareta Wallquist, Innsbruck

2003

2004

1995

2004

2001

2004

2003

2005

2004

2005

Cooper Pair Box(quantum processor)

superconducting microwavestripline cavity(photon bus)

polar molecular ensemble(quantum memory)

Thanks to P. Rabl

Hybrid quantum information processor- an example

Margareta Wallquist, Innsbruck

A.Andre et al, Nature Physics 2, 636 (2006)CPB+stripline cavity: A. Wallraff et al, Nature 431, 162 (2004)

• Ion - ion via wire

• Ion – superc. circuit

• Nanoresonator -superc. circuit

• Rydberg atom –superc. striplinecavity?

A technical detail: how to cool the molecule motionM. Wallquist, P. Rabl and P. Zoller

• Polar molecules in harmonic electrical trap

• CaF, SrO, CaCl, OH,..

• Electronic and vibrational d.o.f. in the ground state

• 1 MHz 50 μK

• Anharmonic rotor spectrum

– choose two levels

– mw transition frequency

• Rotational states are longlived

– ground state cooling not easy

Margareta Wallquist, Innsbruck

A technical detail: how to cool the molecule motion

• Polar molecules

• Anharmonic rotation spectrum

– choose two levels

– microwave (GHz) transition

• Hybrid device: coupling to superconducting stripline cavity

• Microwave cavity photons (Ghz): resonance

• Cavity photon decay

– transfers energy out of the system

Margareta Wallquist, Innsbruck

Microwave cooling the molecule motion

• bad cavity limit: κ large

• cavity d.o.f is eliminated

• .γ effective decay rateof rotational excitation

• analogue of laser cooling for ions.

• mw field drives red sideband transitions:

Margareta Wallquist, Innsbruck

Sideband resolved limit

Doppler limit

Here: g (x) = gx ~ a + a+

cavity

Microwave cooling the molecule motion

• analogue of laser cooling for ions

• .γ effective decay rate of rotational excitation

• N: thermal occupation in cavity,limits cooling

Margareta Wallquist, Innsbruck

Sideband resolved limit, weak drive

Doppler limit, weak drive

Gradient-g cooling the molecule motion

• Use gradient of cavity field g(x)(on scale of trap)

• Couples rotation and trap motion to one cavity photon

• Interference effects if simultaneously using mw-driven cooling and gradient-g-cooling

Margareta Wallquist, Innsbruck

Doppler limit, strong drive

Here: Ω (x) = Ω

g (x)

trap

Conclusions and outlook

• Quantum information processing imposes strict constraints on thehardware

• Solid-state devices, for example superconducting qubits

– Flexible design, straightforward control techniques, fast operations. Short coherence times. Exp: 2-qubit gate (NEC, Delft).

• Ionic and molecular qubits

– Stable against the environment. Robust quantum memory. Exp: 8 entangled qubits (Innsbruck).

• Let systems complement each other in hybrid devices

– Example: both superconducting charge qubits and polar molecules coupled to the same superconducting stripline cavity.

• Hybrid devices interesting as such

– provide new physical insights, unknown hightech applications...

Margareta Wallquist, Innsbruck

Thanks for your attention!

Margareta Wallquist, Innsbruck

Greetings from Tyrol