Observation of Energy Levels Quantization in Underdamped Josephson Junctions above the Classical-Quantum Regime Crossover Temperature

We present a clear observation of the presence of energy levels quantization in high quality Nb-AlOx -Nb underdamped Josephson junctions at temperatures above the quantum crossover tempera- ture. This has been possible by extending the measurements of the escape rate out of the zero-voltage state at higher sweeping frequency (dI/dt up to 25 A/sec) in order to induce nonstationary conditions in the energy potential describing the junction dynamics.