Direct detection of down-converted photons spontaneously produced at a single Josephson junction

TL;DR

This paper reports the direct observation of multi-photon down-conversion from a Josephson junction, demonstrating photon triplet emission and providing evidence of ultra-strong light-matter coupling in a superconducting quantum system.

Contribution

It presents the first direct detection of multi-mode photon fluorescence and triplet emission from a Josephson junction in a high impedance environment, advancing quantum optics research.

Findings

Observation of multi-mode fluorescence in a Josephson junction

Explicit demonstration of photon triplet emission

Evidence of ultra-strong light-matter coupling

Abstract

We study spontaneous photon decay into multiple photons triggered by strong non-linearities in a superconducting quantum simulator of the boundary sine-Gordon impurity model. Previously, spectroscopic signatures of photon-conversion were reported and evidenced as resonances in the many-body spectrum of these systems. Here, we report on the observation of multi-mode fluorescence of a small Josephson junction embedded in a high impedance superconducting transmission line. Measurement of the down-converted photons is achieved using state-of-the-art broadband parametric amplifiers. Photon triplet emission is explicitly demonstrated at a given frequency as the counterpart of inelastic photon decay at three-times the emission frequency. These results open exciting prospects for the burgeoning field of many-body quantum optics and offer a direct signature of the ultra-strong light-matter coupling.