• Experimental observation confirms the existence of a diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate.
• The Nambu-Goldstone mode is characterized by spectral narrowing in the condensate's response to a probe laser.
• Symmetry breaking induces a gap in the spectrum, validating theoretical models of non-equilibrium phase transitions and polariton condensation.

Researchers have successfully characterized a diffusive Nambu-Goldstone (NG) mode in a non-equilibrium Bose-Einstein condensate composed of microcavity polaritons. The experiment involved observing the spectroscopic response of the condensate to a probe laser, noting the emergence of spectral narrowing. Furthermore, the researchers demonstrated that breaking the symmetry within the system leads to a gap in the imaginary part of the spectrum and the disappearance of the NG mode. This work supports existing theoretical frameworks concerning NG modes in non-equilibrium phase transitions and provides insights into the symmetry-breaking mechanisms underlying 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, and Alberto Bramati.
• When: Published 02 May 2025, Accepted 02 April 2025, Received 08 February 2024
• 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

The experimental observation of the diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate has significant implications for understanding phase transitions and symmetry-breaking mechanisms in complex physical systems. The results confirm theoretical predictions and provide a deeper understanding of polariton condensation. This research bridges the gap between theoretical models and experimental observations in the field of condensed matter physics.

The research successfully observed and characterized a diffusive Nambu-Goldstone mode in a non-equilibrium Bose-Einstein condensate, thereby validating theoretical predictions. Further research may focus on exploring the properties of this mode under varying conditions and in different systems to expand our understanding of non-equilibrium phase transitions.