Feedback Induced Magnetic Phases in Binary Bose-Einstein Condensates

TL;DR

This paper develops a theoretical framework for using weak measurements and feedback control to engineer and explore novel magnetic phases in multicomponent Bose-Einstein condensates, enabling tunable interactions and phase transitions.

Contribution

It introduces a new quantum feedback control toolbox that creates effective interactions in BECs without altering atomic scattering parameters, leading to controllable magnetic phases.

Findings

Feedback induces a phase transition between ferromagnetic and paramagnetic phases.

Effective interactions can be tuned to mimic Feshbach resonances.

Feedback cooling effectively suppresses measurement backaction heating.

Abstract

Weak measurement in tandem with real-time feedback control is a new route toward engineering novel non-equilibrium quantum matter. Here we develop a theoretical toolbox for quantum feedback control of multicomponent Bose-Einstein condensates (BECs) using backaction-limited weak measurements in conjunction with spatially resolved feedback. Feedback in the form of a single-particle potential can introduce effective interactions that enter into the stochastic equation governing system dynamics. The effective interactions are tunable and can be made analogous to Feshbach resonances -- spin-independent and spin-dependent -- but without changing atomic scattering parameters. Feedback cooling prevents runaway heating due to measurement backaction and we present an analytical model to explain its effectiveness. We showcase our toolbox by studying a two-component BEC using a stochastic mean-field theory, where feedback induces a phase transition between easy-axis ferromagnet and spin-disordered paramagnet phases. We present the steady-state phase diagram as a function of intrinsic and effective spin-dependent interaction strengths. Our result demonstrates that closed-loop quantum control of Bose-Einstein condensates is a powerful new tool for quantum engineering in cold-atom systems.