Dynamical phases and quantum correlations in an emitter-waveguide system with feedback

TL;DR

This paper explores how feedback in an emitter-waveguide system can induce complex dynamical phases like time-crystals and generate quantum correlations such as spin squeezing, with potential applications in quantum metrology.

Contribution

It introduces a minimal model demonstrating feedback-controlled dynamical phases and quantum correlations, including the emergence of a time-crystal phase and controllable spin squeezing.

Findings

Feedback strength controls the transition to a time-crystal phase.

Spin squeezing is achieved and controlled via feedback.

Critical scaling of squeezing near the phase transition is demonstrated.

Abstract

We investigate the creation and control of emergent collective behavior and quantum correlations using feedback in an emitter-waveguide system using a minimal model. Employing homodyne detection of photons emitted from a laser-driven emitter ensemble into the modes of a waveguide allows to generate intricate dynamical phases. In particular, we show the emergence of a time-crystal phase, the transition to which is controlled by the feedback strength. Feedback enables furthermore the control of many-body quantum correlations, which become manifest in spin squeezing in the emitter ensemble. Developing a theory for the dynamics of fluctuation operators we discuss how the feedback strength controls the squeezing and investigate its temporal dynamics and dependence on system size. The largely analytical results allow to quantify spin squeezing and fluctuations in the limit of large number of emitters, revealing critical scaling of the squeezing close to the transition to the time-crystal. Our study corroborates the potential of integrated emitter-waveguide systems -- which feature highly controllable photon emission channels -- for the exploration of collective quantum phenomena and the generation of resources, such as squeezed states, for quantum enhanced metrology.