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Quantum control and computation in circuit quantum electrodynamics.

Gerard Milburn, The University of Queensland

(Session 1 : Thursday from 4:30-5:15)

Abstract. The new field of circuit quantum electrodynamics (circuit QED for short) has developed in less than a decade driven by technological improvements in the ability to fabricate small circuits from superconducting metals. Much of this development has been motivated by the possibility of implementing quantum computing in such systems, but they are of much wider interest. In this talk I will discuss the feasibility of a number of schemes for quantum feedback control enabled by the new technology. Lehnert has recently demonstrated quantum limited interferometry with high readout efficiency, equivalent to that of a photo-detector reading out an ideal interferometor with efficiency=0.27. This opens up the possibility of doing some important quantum feedback control experiments that are very difficult to do in an atomic or quantum optical setting but very much more feasible in circuit QED. In a quantum optical setting, quantum limited feedback requires that we use all the light leaving the cavity in the measurement process. This is difficult to do in an optical setting but in principle easier in a circuit QED setting. Unlike in an optical setting, all the measured fields are voltages and currents at GHz frequencies on a superconducting wire and thus there is no need to convert from an optical frequency down to a fast electronic signal. Finally the time scales are slower in a circuit to what they are in an all-optical setting and thus fast feedback is more feasible, even with some in-line signal processing. On the other hand, circuit QED presents a difficulty that is not found in optics: we need to make quantum limited homodyne measurements on the cavity output. Lehnert's scheme uses a Josephson parametric amplifier (JPA) which is a phase sensitive amplifier. JPAs have long been used in superconducting electronics, but a key difference in the new devices is the presence of a significant Kerr nonlinearity. I will discuss the quantum noise performance of such devices in circuit QED.