Quantum sensing close to a dissipative phase transition: Symmetry breaking and criticality as metrological resources

We study the performance of a single qubit laser as a quantum sensor to measure the amplitude and phase of a driving field. By using parameter estimation theory we show that certain suitable field quadratures are optimal observables in the lasing phase. The quantum Fisher information scales linearly with the number of bosons and thus the precision can be enhanced by increasing the incoherent pumping acting on the qubit. If we restrict ourselves to measurements of the boson number observable, then the optimal operating point is the critical point of the lasing phase transition. Our results point to an intimate connection between symmetry breaking, dissipative phase transitions, and efficient parameter estimation.