Quantum and classical resonant escapes of a strongly-driven Josephson junction

TL;DR

This study investigates phase escape in a strongly-driven Josephson junction at very low temperatures, revealing quantum and classical resonant escape phenomena influenced by microwave power and frequency, with implications for understanding quantum-to-classical crossover.

Contribution

It demonstrates the coexistence of quantum and classical resonant escape mechanisms in a Josephson junction under strong ac driving at low temperatures, supported by experimental and theoretical analysis.

Findings

Resonant peaks shift with microwave power and frequency.

Quantum resonant phase escape involves single and two-photon processes.

Classical bifurcation leads to large oscillation amplitude escape.

Abstract

The properties of phase escape in a dc SQUID at 25 mK, which is well below quantum-to-classical crossover temperature $T_{cr}$, in the presence of strong resonant ac driving have been investigated. The SQUID contains two Nb/Al-AlO$_{x} $/Nb tunnel junctions with Josephson inductance much larger than the loop inductance so it can be viewed as a single junction having adjustable critical current. We find that with increasing microwave power $W$ and at certain frequencies $\nu $ and $\nu $/2, the single primary peak in the switching current distribution, \textrm{which is the result of macroscopic quantum tunneling of the phase across the junction}, first shifts toward lower bias current $I$ and then a resonant peak develops. These results are explained by quantum resonant phase escape involving single and two photons with microwave-suppressed potential barrier. As $W$ further increases, the primary peak gradually disappears and the resonant peak grows into a single one while shifting further to lower $I$. At certain $W$, a second resonant peak appears, which can locate at very low $I$ depending on the value of $\nu $. Analysis based on the classical equation of motion shows that such resonant peak can arise from the resonant escape of the phase particle with extremely large oscillation amplitude resulting from bifurcation of the nonlinear system. Our experimental result and theoretical analysis demonstrate that at $T\ll T_{cr}$, escape of the phase particle could be dominated by classical process, such as dynamical bifurcation of nonlinear systems under strong ac driving.