Observation of the diffusive Nambu-Goldstone mode of a non-equilibrium phase transition

Year: 2025

Authors: Claude F., Jacquet M.J., Glorieux Q., Wouters M., Giacobino E., Carusotto I., Bramati A.

Autors Affiliation: Sorbonne Univ, ENS Univ PSL, Coll France, Lab Kastler Brossel,CNRS, Paris, France; Univ Antwerp, TQC, Antwerp, Belgium; Univ Trento, Pitaevskii BEC Ctr, CNR, INO, Trento, Italy; Univ Trento, Dipartimento Fis, Trento, Italy.

Abstract: Second-order phase transitions are governed by a spontaneous symmetry-breaking mechanism, which yields collective excitations with a gapless spectrum called Nambu-Goldstone modes. Although these modes propagate as sound waves in conservative systems, non-equilibrium phase transitions have been predicted to feature a diffusive Nambu-Goldstone mode. Here, we present the experimental characterization of such a mode in a non-equilibrium Bose-Einstein condensate of microcavity polaritons. The mode appears in the spectroscopic response of the condensate to an extra probe laser as spectral narrowing, along with the emergence of a tilted frequency plateau. Breaking the symmetry with another phase-fixing beam causes a gap to open in the imaginary part of the spectrum and the disappearance of the Nambu-Goldstone mode. These observations confirm theoretical predictions for the Nambu-Goldstone mode of non-equilibrium phase transitions and reveal the symmetry-breaking mechanism underlying polariton condensation.

Journal/Review: NATURE PHYSICS

More Information: We are thankful to J. H. Wilson for enlightening conversations on NG modes in various field theories. We acknowledge financial support from the H2020-FETFLAG-2018-2020 project PhoQuS (n.820392). I.C. acknowledges financial support from the Provincia Autonoma di Trento, from the Q@TN Initiative and from the National Quantum Science and Technology Institute through the PNRR MUR Project (Grant No. PE0000023-NQSTI), co-funded by the European Union – NextGeneration EU. M.J.J. and A.B. acknowledge financial support from the Sirteq DIM. Q.G. and A.B. are members of the Institut Universitaire de France.
KeyWords: Laser Threshold; Condensation; Microcavity; Analogy
DOI: 10.1038/s41567-025-02902-z