Proposed in 1954 by Robert H. Dicke, the Dicke `spin-boson’ model is among the most successful paradigms to describe the interaction of light and matter. Modeling the coupling of an ensemble of spins or atoms to a harmonic oscillator, it is thus sometimes referred to as the “standard model of quantum optics.”
As discovered in the 1970s, for strong enough coupling the Dicke model exhibits a phase transition between a dark phase, where the atoms do not emit any light, and a bright phase where the system strongly radiates, the so-called “superradiance.”

Now a team of researches from Harvard and ETH Zurich have found a new way how a system described by the Dicke model can enter yet a different phase.
The additional “breakdown phase transition” emerges from the presence of loss processes, so-called “dissipation,” in the system.

The new phase constitutes the fundamental di erence dissipation can make to a system. In this system, where decay of atoms from their excited state is taken into account, the interplay of excitation and dissipation results in a breakdown of the known phases. Cooperative behavior of the atoms results in a runaway heating process of the harmonic oscillator out of thermal equilibrium.
The new effect may be observable in setups on trapped ions or cold atoms. The sensitive nature of highly-excited oscillator states holds promise for an application in quantum sensing.

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