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Cold atoms trapped in the volume of a high-finesse optical resonator [1] form a hybrid quantum system, where strong atom-light coupling leads to a bistability involving hyperfine ground states [2]. This transition can be exploited both in the preparation and in the detection of atomic quantum states. We experimentally demonstrate an optical bistability between two hyperfine ground states [3]. We interpret the phenomenon in terms of the recent paradigm of first-order, driven-dissipative phase transitions, where the transmitted and driving fields are understood as the order and control parameters. The key point is to quantify and understand fluctuations in the vicinity of the critical point. A unique feature of cavity systems is that the cavity field and its fluctuations can be monitored real time [4], that helps us to reveal the very nature of these phase transitions. Arranging the atoms in an incommensurate lattice with respect to the resonator mode, the scattering into the cavity is suppressed by destructive interference: resulting in a subradiant atomic array. We experimentally demonstrate strong collective coupling of the atoms to the cavity vacuum field by linear scattering from a transverse drive. We show, that strong collective coupling leads to a drastic modification of the excitation spectrum, as evidenced by well-resolved vacuum Rabi splitting in the intensity of the fluctuations [5].
[1] D. Varga, B. Gábor, B. Sárközi, K.V. Adwaith, D. Nagy, A. Dombi, T.W. Clark, F.I.B. Williams, P. Domokos, A. Vukics: Loading atoms from a large magnetic trap to a small intra-cavity optical lattice, Phys. Lett. A 505, 129444 (2024).
[2] B. Gábor, D. Nagy, A. Vukics, P. Domokos: Quantum bistability in the hyperfine ground state of atoms, Phys. Rev. Research 5, L042038 (2023).
[3] B. Gábor, D. Nagy, A. Dombi, T. W. Clark, F. I. B. Williams, K. V. Adwaith, A. Vukics, P. Domokos: Ground-state bistability of cold atoms in a cavity, Phys. Rev. A 107, 023713 (2023).
[4] T. W. Clark, A. Dombi, F. I. B. Williams, Á. Kurkó, J. Fortágh, D. Nagy, A. Vukics, P. Domokos: Time-resolved observation of a dynamical phase transition with atoms in a cavity, Phys. Rev. A, 105, 063712 (2022).
[5] B. Gábor, K. V. Adwaith, D. Varga, B. Sárközi, Á. Kurkó, A. Dombi, T. W. Clark, F. I. B. Williams, D. Nagy, A. Vukics, P. Domokos: Demonstration of strong coupling of a subradiant atom array to a cavity vacuum, EPJ Quantum Technology, 12:93 (2025).