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
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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