ASSESSMENT OF CURRENT RESULTS AND OUTLOOK ON FUTURE EFFORTS
SUPERCONDUCTING CIRCUITSA. Physical approach and perspective
Quantum computation with superconducting circuits exploits the intrinsic coherence of the superconducting state, into which all electrons are condensed. Quantum information is stored in the number of superconducting electrons (charge qubit), in the direction of a current (flux qubit) or in oscillatory states (phase qubit). Systems are fabricated with thin film technology and operated at temperatures below 100 mK. Measurements are performed with integrated on-chip instruments. Coupling between qubits can be made strong. In principle the system is scalable to large numbers. The US QIST roadmap gives more detailed information and references, though not quite up to date . A general background is provided in .
Approximately 30 groups work on superconducting quantum bits in Europe, Japan, China and the USA. European groups in experiments are: D. Esteve and D. Vion (Saclay, FR), J. Rooij and H. Harmans (Delft, NL), P. Delsing (Chalmers, SE), A. Zorin (PTB, DE), E. Ilichev (Jena, DE), A. Ustinov (Erlangen, DE), F. Hekking, O. Buisson (Grenoble, FR), J. Pekola (Helsinki, FI), S. Paroanu Jyväskyla, FI), D. Haviland (Stockholm, SE), C. Cosmelli and M. G. Castellano (Rome, IT), and others. In theory: G. Schön, (Karlsruhe, DE), R. Fazio (Pisa, IT) A. Wilhelm (München, DE), G. Wendin (Chalmers, SE), M. Grifoni (Regensburg, DE), G. Falci (Catania, IT), K. Bruder (Basel, CH), and others.B. State of the art
- Qubits can be readily fabricated with suitable parameters. Small variation of qubit parameters can be achieved.
- Initialization proceeds by relaxation into the ground state before quantum operations start.
- Single qubit operations are performed with microwave pulses or DC pulses.
- 1-pulse Rabi oscillations and 2-3 pulse Ramsey or spin-echo signals have been realized.
- Decoherence times of several microseconds have been observed, shortest time needed for a basic quantum operation is several nanoseconds.
- a) With charge 2-qubit systems a controlled-not gate has been realized with DC pulses. b) The presence of coupling has been demonstrated in flux qubits with spectroscopy.
- Strong coupling, allowing exchange of a single photon, has been achieved between a harmonic oscillator and a qubit in two different types of qubit.
- Rabi oscillation between two Josephson junction qubits has been achieved, and simultaneous single-shot readout has been performed to detect the anticorrelations in a Bell state.
All of these results have been achieved within Europe, apart from 6.a) (Japan) and 8 (USA).C. Strengths and weaknesses
- High potential for scalable integrated technology.
- Strong coupling between qubits possible.
- Flexible opportunities with different qubit types.
- Mature background technology, 20 years of experience.
- Driver of applications in solid-state quantum engineering.
- Long history of pushing the limits of measurement towards quantum limits.
- Low-temperature or superconducting technologies necessary for integration with solid state microtraps for hybrid systems.
- Coherence limited by defects in tunnel barriers.
- Slight variation in qubit parameters associated with fabrication.
- Realize reliable two-qubit gates in all types of qubits.
- Realize non-destructive single shot readout of individual qubits in multi-qubit circuits
- Improve fidelity of operation and readout.
- Investigate and eliminate main sources of decoherence.
- Develop junctions with lower 1/f noise.
- Realize fully controllable three-qubit clusters within a generally scalable architecture.
- Develop switchable coupling between qubits.
- Realize systems of multiple qubits coupled through common harmonic oscillator buses – solid-state cavity QED.
- Demonstrate teleportation and rudimentary quantum error correction.
- Make first experimental tests of quantum algorithms with 3-5 qubits.
All of the above goals are among the priorities and can be achieved by European labs.E. Long-term goals (2010 and beyond) (cf. also )
- Develop multi-qubit circuits (5-10 or more).
- Improve fidelity to the level needed for large-scale application.
- Develop interfaces to microwave and optical transmission lines.
- Develop interfaces for hybrid solutions to long term storage and communication.
- All of the above goals are among the priorities and can be achieved by European labs.
 T.P. Orlando, ‘‘Superconducting approaches to Quantum Information Processing and Quantum Computing’’, in ‘A Quantum Information Science and Technology Roadmap, Part 1: Quantum Computation’, Version 2.0, section 6.7 and references therein; available from http://qist.lanl.gov.
 D. Esteve, ‘‘Superconducting qubits’’, in ‘Proceedings of the Les Houches 2003 Summer School on Quantum Entanglement and Information Processing’, (D. Esteve and J.-M. Raimond, editors), Elsevier (2004).