Macroscopic Quantum Phenomena in Underdamped Josephson Junctions

Abstract

We present an experimental study of the supercurrent decay in extremely underdamped Josephson junctions. In the thermal regime, we present data on the rate of escape from the supercurrent state as function of the external energy both in quasi-stationary and non-stationary conditions for the system. We evaluate the relevant junction parameters and the effective dissipation and we get experimental evidence of energy level quantization at temperatures well above the classical-quantum crossover one. In the quantum limit we show results on macroscopic quantum tunneling in different extremely underdamped Josephson junctions. All measurements employ low noise electronics, as well as different techniques to reduce the effective dissipation by filtering the lines wiring the junction. The extremely low value of dissipation obtained in the observation of incoherent effects in underdamped Josephson junctions, is encouraging in view of experiments of coherent tunneling between energy levels in an rf-squid system.