Identifying dissipative phase transitions from entropy measurements

Dissipative phase transitions (DPTs) occur when a small quantum system interacts with a bath of harmonic oscillators. They are expected to be accompanied by an entropy change, signaling the decoherence of the system. I will review recent experimental progress showing that charge sensors of quantum dot systems allow measurement of entropy through Maxwell relations, as well as theoretical predictions on exotic states that can be detected using this method. Typically, a weak coupling to the charge sensor is assumed. Here I will argue theoretically that more generally these experiments realize DPTs due to the decoherence of the system by the charge sensor. This may allow for a first observation of the equilibrium Kosterlitz-Thouless transition in the spin-boson model in a mesoscopic system.