Quantum fluctuations and unusual critical exponents in a quantum Rabi triangle

The quantum fluctuations of a quantum Rabi triangle are investigated using an analytical method that goes beyond mean-field theory. An artificial magnetic field applied in three cavities leads to the breaking of timereversal symmetry, which is evident in the directional transfer of photons. In contrast to previous studies, we focus on the scaling exponents of quantum fluctuations of the local photon number and the position variance near the critical point. Specifically, the fluctuations of photons do not diverge as the coupling strength approaches the critical value from below for the chiral superradiant phase transition. Attributing to geometric frustration, two distinct scaling laws arise for the frustrated cavity and the remaining cavities. Specifically, in the frustrated cavity, the scaling exponent in the chiral superradiant phase differs from that without an artificial magnetic field for the frustrated antiferromagnetic superradiant phase. The unusual scaling exponents indicate distinct universality classes in contrast to the single-cavity Rabi one. We suggest that accurate critical exponents in few-body systems are useful for identifying exotic quantum phase transitions in light-matter coupling systems.