Quantum and phase diffusion crossovers in small Al Josephson junctions

Switching current distributions (SCDs), describing switching statistics from a superconducting to a resistive state, are measured for a set of aluminum-made Josephson junctions (JJ) in a temperature range from 15 mK to 1 K. The measured data are compared with existing theories and computer simulation in the frame of the second order pendulum model, with account of noise and temporal driving. Generalizing the obtained data, it is shown that the crossover temperature between the running state and the phase diffusion regime scales exponentially versus 𝑘B𝑇 ∕ℏ𝜔 for JJ critical currents from 70 to 1000 nA. Besides, the quantum crossover temperature decreases with decreasing critical current. Also, the quantum floor (the SCD width) below the quantum crossover temperature is not constant, but has a finite tilt, proportional to temperature as ℏ𝜔 + 𝑘B𝑇 , due to residual thermal activation switches.