Josephson Junction As A Probe Of Classical And Quantum Decay Of Metastable States: An Introduction

Andrei D. Zaikin, Sergey Panyukov

Anedio Ranfagni, Pasquale Fabeni, Daniela Mugnai +5 more

The macroscopic quantum tunneling rate out of the metastable state of a Josephson junction has been measured in a range of temperature where the quantum contribution is comparable with the thermal escape. From such measurements the traversal time of the barrier has been deduced and compared with the theoretical predictions of a semiclassical model.

A. J. Berkley, H. Xu, M. A. Gubrud +4 more

The zero-voltage state of a Josephson junction biased with constant current consists of a set of metastable quantum energy levels. We probe the spacings of these levels by using microwaves to induce transitions and thereby enhance the escape rate to the voltage state. The widths of the resonances give a measurement of the spectroscopic coherence time of the two metastable states involved in the transitions. We observe a decoherence time shorter than that expected from dissipation alone in resonantly isolated $20\ifmmode\times\else\texttimes\fi{}5\ensuremath{\mu}{\mathrm{m}}^{2}$ area $\mathrm{Al}/\mathrm{AlO}x/\mathrm{Al}$ junctions at 60 mK. The data are well fit by a model that includes the dephasing effects of both low-frequency current noise and the escape rate to the voltage state. We discuss implications for quantum computation using current-biased Josephson-junction qubits, including limits on the minimum number of levels needed in the well.

Alwyn C. Scott

Vittorio Giulio Palmieri

Andrei D. Zaikin, Dmitry S. Golubev

This chapter considers one important physical example of that kind, quantum dissipative decay of a metastable state. It demonstrates how to evaluate the quantum decay rate of a metastable state for a quantum particle in the presence of dissipation. In the presence of phase fluctuations, however, such current states become unstable and decay simply because the “particle” can escape from the local potential minimum either by thermal activation or by quantum tunneling. The chapter also considers sufficiently low temperatures in which case the decay is due to quantum tunnelling. Classical process of thermally activated decay of supercurrent in Josephson junctions was first demonstrated in 1974, and the results turned out to be in a good agreement with theoretical predictions. Later experiments allowed to quantitatively verify a number of important theoretical predictions, for example, ones concerning the effect of temperature and damping on the quantum decay rate.