Here’s a measurement showing the response of a Josephson Junction as it switches from the zero voltage (superconducting) state to the voltage state, corresponding to escape of the phase from a potential well in the junction’s ‘washboard’ (energy) potential. The switching is a probabilistic process, so many measurements are compiled into a histogram at each temperature to get an average, stochastic response. The width of this histogram is then monitored as a function of temperature:
Classically, the phase gets excited out of the metastable minimum due to thermal fluctuations (the state gets a ‘kick’ out of the well from the thermal energy available in the system). As the temperature is decreased, this is less likely to happen, and so the state stays in the well for longer, and the histogram gets narrower. If the junction is small enough it may be possible to see escape due to quantum tunneling of the phase, a competing escape mechanism. This is a temperature independent process and the width should saturate at low temperatures if quantum tunneling occurs. Unfortunately, the prescence of external interference gives a very similar effect. So here we measure a large junction, which should behave classically to a very low temperature. Any saturation of the width would demonstrate noise limitation.
This is a textbook response: The straight line demonstrates that the system follows the thermal activation theory, and furthermore is not noise limited. So in future measurements of junctions, any saturation observed must be either due to quantum effects, or noise sources intrinsic to the junction itself.
So I can now believe that I’m seeing real quantum processes in the junctions.
This makes me happy.