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 $\mathrm{Nb}-{\mathrm{AlO}}_x-\mathrm{Nb}$ underdamped Josephson junctions at temperatures above the quantum crossover temperature. 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.