We put forward a phenomenological theory for entanglement dynamics in monitored quantum many-body systems with well-defined quasiparticles. Within this theory entanglement is carried by ballistically propagating non-Hermitian quasiparticles which are stochastically reset by the measurement protocol with a rate given by their finite inverse lifetime. We write down a renewal equation for the statistics of the entanglement entropy and show that, depending on the spectrum of quasiparticle decay rates, different entanglement scalings can arise and even sharp entanglement phase transitions. When applied to a quantum Ising chain where the transverse magnetization is measured by quantum jumps, our theory predicts a critical phase with logarithmic scaling of the entanglement, an area-law phase and a continuous phase transition between them, with an effective central charge vanishing as a square root at the transition point. We compare these predictions with exact numerical calculations on the same model and find an excellent agreement.

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• Entanglement entropy
• Open quantum systems & decoherence
• Quantum measurements
• Quantum simulation
• Quantum stochastic processes
• 1-dimensional spin chains
• Nonequilibrium systems
• Quantum spin chains