Phase-dependent microwave response of a graphene Josephson junction

Abstract

Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in-situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing an rf SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we deduce a lifetime of ∼ 17 ps for non-equilibrium populations.

A gate-tunable graphene Josephson parametric amplifier

“…In a graphene based Josephson junction, the gate voltage modifies the critical current of the junction, hence the full current phase relation, while keeping the phase across the junction at zero. This will give more flexibility, with the possibility of an additional magnetic control of the circuit [6] to tune independently the phase across the junction and its kinetic inductance, with interesting prospects regarding the mitigation of phase dependent dissipation processes [31][32][33].…”

“…This is a signature of nonlinear losses that are not observed in usual tunnel Josephson junctions. Nonlinear losses have been shown to exist in graphene Josephson junctions and have been attributed to the presence of subgap states inside the induced superconducting gap [30,33], which are absent in tunnel junctions. The position and density of these subgap states is influenced by the exact geometry of the junction [10] and its optimization should allow to mitigate their effect in the microwave range of interest.…”