Photon pairs, squeezed light and the quantum wave mixing effect in a cascaded qubit system

TL;DR

This paper presents a theoretical framework for quantum wave mixing in a cascaded superconducting qubit system, revealing how nonclassical photon correlations influence the spectrum and can be used to probe photon statistics.

Contribution

It introduces a master-equation approach to describe quantum wave mixing with nonclassical light and identifies spectral selection rules related to photon correlations.

Findings

Suppression of odd-photon sidebands in the QWM spectrum.

Numerical confirmation of correlated photon pairs in the process.

Theoretical link between photon statistics and spectral features.

Abstract

We develop a theoretical description of quantum wave mixing (QWM) in a cascaded waveguide-QED system of two superconducting qubits, where the probe is driven by an external coherent tone and by the resonance fluorescence of a strongly driven source qubit. Starting from the field correlation functions of the source emission, we derive an effective master-equation treatment for the probe and identify the regime in which the incident fluorescence is characterized by anomalous correlations. When the coherent Rayleigh component of the source spectrum is suppressed, the probe equations of motion become equivalent to those for a qubit driven by a coherent tone and broadband squeezed light. This equivalence implies a selection rule for the peaks of the QWM spectrum, with a strong suppression of sidebands associated with processes involving an odd number of photons taken from the source field. Numerical simulations of the full cascaded two-qubit model for different ratios of radiative decay rates unambiguously confirm the participation of correlated photon pairs in QWM processes. The current research illustrates that the analysis of peak amplitudes can be used to probe photon statistics in the incident nonclassical field.