Switchable Superradiant Phase Transition with Kerr Magnons

TL;DR

This paper proposes a hybrid quantum system with Kerr magnons that can restore and control the superradiant phase transition in cavity QED, overcoming the no-go theorem caused by the ${f A}^2$ term.

Contribution

The work introduces a novel hybrid system combining CQED and Kerr magnons to enable and manipulate the superradiant phase transition against the intrinsic no-go theorem.

Findings

Kerr magnons effectively induce an additional tunable ${f A}^2$ term.

The critical coupling for SPT is significantly reduced with Kerr magnons.

The forbidden SPT can be recovered in the presence of intrinsic ${f A}^2$ term using Kerr magnons.

Abstract

The superradiant phase transition (SPT) has been widely studied in cavity quantum electrodynamics (CQED). However, this SPT is still subject of ongoing debates due to the no-go theorem induced by the so-called ${\bf A}^2$ term (AT). We propose a hybrid quantum system, consisting of a single-mode cavity simultaneously coupled to both a two-level system and yttrium-iron-garnet sphere supporting magnons with Kerr nonlinearity, to restore the SPT against the AT. The Kerr magnons here can effectively introduce an additional strong and tunable AT to counteract the intrinsic AT, via adiabatically eliminating the degrees of freedom of the magnons. We show that the Kerr magnons induced SPT can exist in both cases of ignoring and including the intrinsic AT. Without the intrinsic AT, the critical coupling strength can be dramatically reduced by introducing the Kerr magnons, which greatly relaxes the experimental conditions for observing the SPT. With the intrinsic AT, the forbidden SPT can be recovered with the Kerr magnons in a reversed way. Our work paves a potential way to manipulate the SPT against the AT in hybrid systems combining CQED and nonlinear magnonics.