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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
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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
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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
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(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
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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
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Superconducting qubits: experimental achievements
Margareta Wallquist, Innsbruck
2004
2000
2007
2002
1999
2003
2003
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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+
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Ions and molecules: experimental achievements
Margareta Wallquist, Innsbruck
2003
2004
1995
2004
2001
2004
2003
2005
2004
2005
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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?
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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
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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
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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
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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
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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
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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
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Thanks for your attention!
Margareta Wallquist, Innsbruck
Greetings from Tyrol