Quantum behaviour of Josephson junctions

Switching currents distributions out of the zero-voltage state of josephson junctions has been measured in non-stationary conditions. This has been possible by extending the measurements at higher sweeping frequency of the current bias (dJ/dt up to 25 A/sec). In this way a clear evidence of the presence of the levels quantization has been observed and the relative effects on the escape rate has been analysed. In our case the escape rate is an oscillating functions of the bias current, being the oscillation related to the spacing of the energy levels. The oscillation amplitude decreases as sweeping frequency decreases. Any quantum effect disappears when quasi-stationary conditions are reached, namely at sweeping frequency below 1 KHz.