Experimental observation of spontaneous symmetry breaking in a quantum phase transition

TL;DR

This paper reports an experimental observation of spontaneous symmetry breaking during a quantum phase transition in a superconducting circuit, highlighting the role of decoherence in the emergence of classical reality.

Contribution

First experimental demonstration of the process bridging symmetric quantum phases and classical ordered phases in a quantum Rabi model.

Findings

Observation of symmetry-breaking field components

Role of environment-induced decoherence in SSB

Transition from normal to superradiant phase

Abstract

Spontaneous symmetry breaking (SSB) plays a central role in understanding a large variety of phenomena associated with phase transitions, such as superfluid and superconductivity. So far, the transition from a symmetric vacuum to a macroscopically ordered phase has been substantially explored. The process bridging these two distinct phases is critical to understanding how a classical world emerges from a quantum phase transition, but so far remains unexplored in experiment. We here report an experimental demonstration of such a process with a quantum Rabi model engineered with a superconducting circuit. We move the system from the normal phase to the superradiant phase featuring two symmetry-breaking field components, one of which is observed to emerge as the classical reality. The results demonstrate that the environment-induced decoherence plays a critical role in the SSB.