Single-Shot Quantum Measurements

Summary

We are exploring fundamental limitations on single-shot measurements of spin-like (two-level) quantum systems, using as an example a balanced Josephson-junction comparator measuring the state of a superconductor flux qubit. The reason of this choice it that the experimentally measured properties of the comparator show that its quantum dynamics and statistics may be accurately described by a simple but representative Hamiltonian including coupling to dissipative environment [1]. Our recent calculations have shown [2] that, somewhat unexpectedly, the apparently adequate action-dimensional measure of the detector resolution may be well below the Planck’s constant. This paradox has been (to some extent) resolved by the calculation of the dephasing back-action of the comparator on the signal source (qubit).

Publications

1. T. J. Walls, T. V. Filippov, and K. K. Likharev, "Quantum Fluctuations in Josephson Junction Comparators", Phys. Rev. Lett., vol. 89, art. 217004, Nov. 2002..

2. T. J. Walls, D. V. Averin, and K. K. Likharev, "Josephson Junction Comparator as a Quantum-Limited Detector for Flux Qubit Readout", IEEE Trans. on Appl. Supercon., vol. 17, pp. 136-141, June 2007.

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