Observation of the photon-blockade breakdown phase transition

TL;DR

This paper reports the experimental observation of a first-order dissipative quantum phase transition in a driven circuit QED system, characterized by photon blockade breakdown and bimodal phase space distribution, with implications for quantum measurement and many-body physics.

Contribution

It demonstrates the first experimental observation of a first-order quantum phase transition in a driven circuit QED system, highlighting photon blockade breakdown and stable pointer states.

Findings

Photon blockade is broken by increasing drive power.

Bimodal phase space distribution observed.

Enhanced stabilization of classical attractors with more atoms.

Abstract

Non-equilibrium phase transitions exist in damped-driven open quantum systems, when the continuous tuning of an external parameter leads to a transition between two robust steady states. In second-order transitions this change is abrupt at a critical point, whereas in first-order transitions the two phases can co-exist in a critical hysteresis domain. Here we report the observation of a first-order dissipative quantum phase transition in a driven circuit quantum electrodynamics (QED) system. It takes place when the photon blockade of the driven cavity-atom system is broken by increasing the drive power. The observed experimental signature is a bimodal phase space distribution with varying weights controlled by the drive strength. Our measurements show an improved stabilization of the classical attractors up to the milli-second range when the size of the quantum system is increased from one to three artificial atoms. The formation of such robust pointer states could be used for new quantum measurement schemes or to investigate multi-photon quantum many-body phases.