By combining superconducting structures, which are inherently free of dissipation, with Josephson tunnel junctions, it is possible to microfabricate a device whose state is characterized by a quantum mechanical wavefunction. Such a device makes it possible to move from studies of the dynamics of two-state systems at the atomic level to explorations of the strangeness of the quantum world at the scale of circuits on a chip, including quantum coherent superpositions of different macroscopic states, as first proposed about 20 years ago [Leggett, Garg, Phys. Rev. Lett., 54, 857 (1985)].
| Inversion resonance of ammonia molecule, a classic example of an atomic-scale two-state system. Nitrogen atom (blue) can be in two different positions relative to plane of hydrogen atoms (red). For a good review of this system, see Chapter 9, Vol. III of the Feynman Lectures on Physics (Addison-Wesley). | |
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Spin-1/2 particle in a magnetic field. The two states correspond to the spin oriented parallel and anti-parallel to the magnetic field. Radiation tuned to the energy difference between the two states drives transitions. This is the basis for modern spin-resonance techniques, such as nuclear magnetic resonance (NMR). |
Macroscopic here is relative. These circuits, roughly 1 - 100 microns in spatial extent, are small on the scale of, say, cats, yet they are quite large when compared to individual atoms or molecules, systems we are used to describing with quantum mechanics. In fact, in the past few years, several groups around the world, including my former team at Berkeley, have observed such superpositions of different macroscopic states of a superconducting device. Thus, in a sense, these entire circuits are able to do two different things at once!
| Basic superconducting flux qubit, consisting of superconducting loop (green) interrupted by Josephson junction(s) (represented by "X"). When external flux of (n+1/2)(h/2e), n integer, is applied to the loop, screening current J can flow clockwise, subtracting from the applied flux, or J can flow counterclockwise, adding to the applied flux. For appropriate conditions, it is possible for the device to form a coherent superposition of these two macroscopic states. |  |
Quantum coherent superconducting devices also lend themselves naturally to be the elements of a quantum computer, or "qubits" [there are many excellent introductions to the field of quantum computation and quantum information; one of these is "Quantum Computation and Quantum Information", by Nielsen and Chuang (Cambridge University Press)]. A quantum computer, in which each qubit can be in any arbitrary superposition of states and qubits can be entangled with one another, would be capable of solving many problems which are intractable on even the most powerful classical computer, such as the factorization of large numbers. Once we can fabricate a few of these superconducting devices on a chip, demonstrate quantum coherence, and implement qubit-qubit coupling interactions, it should, in principle, be possible to scale up to many such qubits on a chip using standard microfab techniques [the 2004 roadmap for superconducting approaches for quantum information processing is here (pdf file)].
There has been enormous progress in this field in recent years by several groups around the world; nonetheless, decoherence is still substantial in these systems and is the major hurdle which must be overcome. Sources of decoherence must be understood and reduced to implement simple quantum computing algorithms on a multi-qubit layout and to realize fundamental physics milestones, such as the detection of entanglement between two superconducting qubits through violations of Bell's inequality.
Between 2000-2004, BLTP was a postdoc in the research group of John Clarke at UC Berkeley working with an excellent team of people to perform experiments in the field of superconducting qubits. This team included Tim Robertson, Paul Reichardt, Travis Hime, Sven Linzen, and Cheng-En Wu. Below are some highlights of our recent results at Berkeley. The <publications> page contains references to our publications in this area. The research at Berkeley was funded by the Air Force Office of Scientific Research under Grant F49-620-02-1-0295, the Army Research Office under Grant DAAD-19-02-1-0187, and the National Science Foundation under Grant EIA-020-5641.
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Left: SEM image of chip containing 2 flux qubits surrounded by measurement Superconducting QUantum Interference Device (SQUID). A bias current pulse driven through the SQUID, using the leads attached to the top and bottom, causes the SQUID to produce a voltage pulse depending on the flux state of the qubits. Currents applied to the traces to the left and right of the SQUID couple flux to the SQUID and qubits. Right: closeup SEM image of one of the Josephson junctions in the circuit. |
Spectroscopy of ground-to-first excited state transition for one of the two qubits. Coupling microwave radiation to the qubit produces a peak or dip in the measurement of the SQUID when the microwave energy (vertical axis) is resonant with the splitting for a particular qubit flux bias (horizontal axis). Dispersion of resonance follows hyperbola, consistent with coherent tunneling between the two circulating current states in the qubit. Insets show higher-resolution spectrscopy in the vicinity of some of the spurious splittings. These may be due to resonant coupling between the qubit and defect states. |
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Rabi oscillations of qubit state. Measurement performed by fixing the qubit flux bias, then applying resonant microwave pulses of fixed amplitude, but variable width. This drives the qubit continuously between the ground and excited state in an oscillatory fashion. |
| Ramsey fringe measurement of dephasing. Measurement performed by applying a pi/2 pulse to produce a superposition of the two qubit states, followed by a second pi/2 pulse and a readout. Due to dephasing processes, the readout signal decays as the separation between the two pi/2 pulses (vertical axis) is increased. Detuning the microwaves from resonance (horizontal axis) results in fringe oscillations, with the fringe frequency equal to the detuning from resonance. |  |
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Controllable qubit-qubit coupling scheme based on circulating current in measurement SQUID. (a) Basic layout, with red SQUID and blue qubits. (b) varying bias current through SQUID changes inverse dynamic inductance of SQUID, thus altering the interaction strength between the qubits, as mediated by the SQUID circulating current. Schematic below shows a possible scalable architecture for many flux qubits involving this coupling scheme. |
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At Syracuse, we plan to investigate the various sources of decoherence in flux qubits, including dissipation from the circuit which reads out the qubit state. Based on our measurements at Berkeley and the work of other groups, the development of a non-dissipative readout technique for flux qubits should provide a substantial step towards extending the coherence times. In addition, we will work on techniques for controllable coupling to generate entanglement between qubits.