Giant critical response in a driven-dissipative quantum gas

TL;DR

This study demonstrates that driven-dissipative quantum gases exhibit a critical response similar to equilibrium systems, with measurable slowing and amplification of fluctuations near the phase transition.

Contribution

It provides experimental evidence that fluctuation-response correspondence persists in non-equilibrium quantum systems, linking critical slowing and susceptibility in a photon Bose-Einstein condensate.

Findings

Critical slowing of intensity fluctuations peaks at condensate threshold.

Weak pump perturbations are amplified maximally at the critical point.

A single photon-reservoir mode governs both fluctuation slowing and response amplification.

Abstract

Systems close to a phase transition turn weak perturbations into large responses. At equilibrium, this amplification is closely linked to criticality: fluctuations grow, dynamics slow, and a common soft mode controls the response. Whether this correspondence survives in driven-dissipative quantum systems, sustained by continuous pumping and loss away from thermal equilibrium, remains an open question. Here we show experimentally that it does. In a room-temperature semiconductor photon Bose-Einstein condensate, the critical slowing of spontaneous intensity fluctuations and the amplification of weak pump perturbations are measured independently. Both peak at the same condensate population, $\bar{n}_c = 1250$, where the dimensionless slowing factor and susceptibility reach the same value, $\bar{n}_c/2 = 625$. A single weakly damped collective photon-reservoir mode governs both effects. This fluctuation-response correspondence in a finite open quantum gas establishes critical susceptibility as a measurable dynamical signature of condensation, with peak gain set by system size.