Resonance fluorescence of an artificial atom with a time-delayed coherent feedback

TL;DR

This paper demonstrates theory and experiments on a superconducting artificial atom with time-delayed feedback, revealing non-Markovian effects and modified resonance fluorescence spectra, including the first observation of Mollow triplets in this regime.

Contribution

It provides the first experimental observation of Mollow triplets in a non-Markovian regime with time-delayed feedback in a superconducting circuit.

Findings

Pronounced non-Markovian effects observed in qubit dynamics due to feedback delay.

Resonance fluorescence spectrum significantly altered by feedback, showing quantum nonlinear phenomena.

First experimental report of Mollow triplets in a non-Markovian regime.

Abstract

The model of light-matter interaction in quantum electrodynamics typically relies on the Markovian approximation, which assumes that the system's future evolution depends solely on its current state, effectively treating it as a ``memoryless" process. However, this approximation is not valid in scenarios when retardation effects are significant. These memory and retardation effects have the potential to improve existing quantum technologies (e.g., large-scale quantum networks, quantum information processing) and unlock new phenomena for future applications. In this work, we show theory and experiments of a time-delayed coherent feedback system using a transmon artificial atom (treated as a qubit) embedded in a superconducting circuit waveguide, in both linear and nonlinear excitation regimes. By using a feedback loop with a delay time comparable to the qubit relaxation time, pronounced non-Markovian effects appear in the dynamics of the qubit evolution. We also show how the resonance fluorescence spectrum, including elastic and inelastic scattering (such as the well-known Mollow triplet), can be significantly modified through the interaction between the qubit and feedback loop to show genuine non-Markovian and quantum nonlinear phenomena that cannot be explained with instantaneous coupling parameters. This work presents the first experimental report of Mollow triplets in the non-Markovian regime.