  Stochastic switching-current distribution in a graphene-based Josephson
junction exhibits a crossover from the classical to quantum regime, revealing
the macroscopic quantum tunneling (MQT) of a Josephson phase particle at low
temperatures. Microwave spectroscopy measurements indicate a multi-photon
absorption process occurring via discrete energy levels in washboard potential
well. The crossover temperature for MQT and the quantized level spacing are
controlled with the gate voltage, implying its potential application to
gate-tunable superconducting quantum bits.
