• A diffusive Nambu-Goldstone mode was experimentally observed in a non-equilibrium Bose-Einstein condensate of microcavity polaritons.
• The observation confirms theoretical predictions for Nambu-Goldstone modes in non-equilibrium phase transitions.
• The study reveals insights into the symmetry-breaking mechanism underlying polariton condensation.

The study presents the experimental characterization of a diffusive Nambu–Goldstone mode in a non-equilibrium Bose–Einstein condensate of microcavity polaritons. The research team observed the mode through the spectroscopic response of the condensate to an extra probe laser, noting spectral narrowing and the emergence of a tilted frequency plateau. Further experimentation involved breaking the symmetry with a phase-fixing beam, which caused a gap to open in the imaginary part of the spectrum and the disappearance of the Nambu–Goldstone mode. These observations support theoretical predictions regarding the Nambu-Goldstone mode of non-equilibrium phase transitions and enhance understanding of the symmetry-breaking mechanism that underpins polariton condensation.

• What: Experimental characterization of a diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate of microcavity polaritons.
• Who: Ferdinand Claude, Maxime J. Jacquet, Quentin Glorieux, Michiel Wouters, Elisabeth Giacobino, Iacopo Carusotto, Alberto Bramati.
• When: Received: 08 February 2024, Accepted: 02 April 2025, Published: 02 May 2025
• Where: Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-Université PSL, Collège de France, Paris, France; TQC, Universiteit Antwerpen, Antwerp, Belgium; Pitaevskii BEC Center, INO-CNR and Dipartimento di Fisica, Università di Trento, Trento, Italy

This research provides experimental validation for the theoretical existence of diffusive Nambu-Goldstone modes in non-equilibrium phase transitions. The study's detailed observation of spectral changes in response to a probe laser and symmetry breaking offers crucial insights into the behavior of Bose-Einstein condensates in non-equilibrium conditions. This work contributes to a deeper understanding of polariton condensation and symmetry-breaking mechanisms in quantum systems. The identification of the diffusive mode through spectroscopic analysis marks a significant step forward in the field.

The experimental observation of the diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate provides substantial evidence supporting theoretical predictions. The study offers insights into the symmetry-breaking mechanisms underlying polariton condensation. Further research can build on these findings to explore the properties and applications of non-equilibrium quantum systems.