Quantization of the superconducting energy gap in an intense microwave field

TL;DR

This paper investigates how intense microwave fields influence the superconducting energy gap in Nb/AlOx/Nb Josephson junctions, revealing quantized gap values due to nonequilibrium effects and mutual locking phenomena.

Contribution

It provides a quantitative calibration of microwave fields and demonstrates the quantization of the superconducting gap induced by microwave photon interactions.

Findings

Superconducting gap assumes quantized values as multiples of quarter photon energy.

Photon assisted tunneling steps vary non-monotonously with microwave power and current.

Mutual locking of step series causes staircase-like voltage variations.

Abstract

We study photon assisted tunneling in Nb/AlOx/Nb Josephson junctions. A quantitative calibration of the microwave field in the junction allowed direct verification of the quantum efficiency of microwave photon detection by the junctions. We observe that voltages of photon assisted tunneling steps vary both with the microwave power and the tunneling current. However, this variation is not monotonous, but staircase-like. The phenomenon is caused by mutual locking of positive and negative step series. A similar locking is observed with Shapiro steps. As a result the superconducting gap assumes quantized values equal to multiples of the quarter of the photon energy. The quantization is a manifestation of nonequilibrium tuning (suppression or enhancement) of superconductivity by the microwave field.