Observation of a non-Hermitian phase transition in an optical quantum gas

• Published in: Science (American Association for the Advancement of Science) Vol. 372; no. 6537; pp. 88 - 91
• Main Authors: Öztürk, Fahri Emre, Lappe, Tim, Hellmann, Göran, Schmitt, Julian, Klaers, Jan, Vewinger, Frank, Kroha, Johann, Weitz, Martin
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 02.04.2021
• Subjects: Bose-Einstein condensates; Gases; Lasing; Light; Microcavities; Phase transitions; Photons; Polaritons; Trapping
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.abe9869

Our textbook understanding of quantum systems tends to come from modeling these systems isolated from the environment. However, an emerging focus is understanding how many-body quantum systems behave when interacting with their surroundings and how they subsequently become dissipative, or non-Hermitian, systems. Öztürk et al. formed a quantum condensate of light by trapping photons in an optical cavity, a system that is naturally dissipative. By altering the trapping conditions, they demonstrated that the system provides a powerful platform with which to explore the complex dynamics and phase transitions occurring in dissipative quantum systems. Science , this issue p. 88 A photon condensate is used to model the behavior of dissipative quantum systems. Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This characteristic makes them a tool to study dissipative phases, an emerging subject in quantum many-body physics. We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative phase characterized by a biexponential decay of the condensate’s second-order coherence. The phase transition occurs because of the emergence of an exceptional point in the quantum gas. Although Bose-Einstein condensation is usually connected to lasing by a smooth crossover, the observed phase transition separates the biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our approach can be used to study a wide class of dissipative quantum phases in topological or lattice systems.