Stochastic action for the entanglement of a noisy monitored two-qubit system

We study the effect of local unitary noise on the entanglement evolution of a two-qubit system subject to local monitoring and inter-qubit coupling. We construct a stochastic Hamiltonian by incorporating the noise into the Chantasri-Dressel-Jordan path integral and use it to identify the optimal entanglement dynamics and to develop a diagrammatic method for a closed-form approximation of the average entanglement dynamics with an analytical dependence on the noise and measurement intensity. We find that both the optimal trajectory and diagrammatic expansion capture the oscillations of entanglement at short times. Numerical investigation of long-time steady-state entanglement reveals a non-monotonic relationship between concurrence and noise strength.