(a) Conductance at VT = − 2.11 V with no microwave (MW) signal applied. Red arrows indicate sample-specific features present without irradiation. (b)–(e) Conductance at VT = − 2.11 V for several irradiation frequencies as a function of MW source power P and VSD. The onset of splitting in conductance features is indicated by blue arrows. Blue dotted lines indicate the fitted power dependence of low-bias conductance replicas. Blue dashed lines indicate an identical coupling strength shifted to high bias, which exactly matches high-bias conductance replicas. Zoom-ins close to zero bias highlight remnant supercurrent (bottom). Periodic replication of conductance features is indicated by green dashed lines.

Microwave-induced conductance replicas in hybrid Josephson junctions without Floquet—Andreev states

Light–matter coupling allows control and engineering of complex quantum states. Here we investigate a hybrid superconducting–semiconducting Josephson junction subject to microwave irradiation by means of tunnelling spectroscopy of the Andreev bound state spectrum and measurements of the current–phase relation. For increasing microwave power, discrete levels in the tunnelling conductance develop into a series of equally spaced replicas, while the current–phase relation changes amplitude and skewness, and develops dips. Quantitative analysis of our results indicates that conductance replicas originate from photon assisted tunnelling of quasiparticles into Andreev bound states through the tunnelling barrier. Despite strong qualitative similarities with proposed signatures of Floquet–Andreev states, our study rules out this scenario. The distortion of the current–phase relation is explained by the interaction of Andreev bound states with microwave photons, including a non-equilibrium Andreev bound state occupation. The techniques outlined here establish a baseline to study light–matter coupling in hybrid nanostructures and distinguish photon assisted tunnelling from Floquet–Andreev states in mesoscopic devices.

D. Z. Haxell, M. Coraiola, D. Sabonis, M. Hinderling, S. C. ten Kate, E. Cheah, F. Krizek, R. Schott, W. Wegscheider, W. Belzig, J. C. Cuevas, and F. Nichele
Nat. Commun. 14, 6798 (2023)