Kondo-Zeno crossover in the dynamics of a monitored quantum dot

Continuously monitoring a quantum system can strongly affect its properties and even suppress its coherent evolution via the Quantum Zeno effect. Well understood for few body quantum systems, the role of quantum measurements on entangled many-body states is still largely unexplored. Here we focus on one of the simplest entangled many-body state, arising via the Kondo effect in a strongly interacting quantum dot coupled to a metallic bath, and investigate the effect of continuous monitoring of the dot total charge. We show that the decay rate of an initially polarized spin displays a crossover from Kondo screening, with a decay rate controlled by interactions, to Quantum Zeno effect, with a decay rate which decreases with bare dissipation as the monitoring rate is increased. Remarkably we show that the long-lived Kondo state is robust to weak dissipation, as further confirmed by the dot spectral function which features a clear Kondo peak at finite dissipation, even in a regime where charge fluctuations and the associated Hubbard bands have been quenched by the monitoring protocol. We derive an effective model for the long-time dynamics which is described, at weak dissipation, by a non-Hermitian Kondo model with complex-valued spin exchange which is known to host exotic low-energy physics and a dissipative phase transition between Kondo and non-Kondo steady-state. Finally, as the dephasing is increased heating due to doublon production takes over and control the spin decay.