Uploaded December 2025 | Updated September 2026, 2 weeks ago
Speaker: Graham Kells (Maynooth University)
Abstract: I will discuss measurement-induced dynamics and the phenomenon of entanglement transitions – the critical boundaries separating quantum state behaviours characterised by either boundary or bulk scaling of entanglement. These transitions arise from two competing processes in quantum dynamical systems: the natural build-up of entanglement through interactions between neighbouring subsystems, and its suppression through measurement operations that decouple local elements from their environment. The nature of these transitions emerges when studying average entanglement across many realisations of stochastic quantum evolution. However, quantum trajectory approaches face a fundamental challenge in experimental implementation: the postselection problem. In random dynamics with many qubits, the probability of any particular state recurring becomes vanishingly small, making it impossible to accumulate sufficient statistics about individual trajectories. I will give an overview of an alternative approach using replica density matrices—a framework that offers a deterministic, non-linear method for revealing entanglement structures without requiring individual trajectory realisations. I will focus in particular on replica cut-offs, the necessary approximations for implementing this scheme on numerical or quantum hardware. I will then discuss how to faithfully construct these replica cut-offs and demonstrate their effectiveness on small-scale systems.
Speaker: Graham Kells (Maynooth University)
Abstract: I will discuss measurement-induced dynamics and the phenomenon of entanglement transitions – the critical boundaries separating quantum state behaviours characterised by either boundary or bulk scaling of entanglement. These transitions arise from two competing processes in quantum dynamical systems: the natural build-up of entanglement through interactions between neighbouring subsystems, and its suppression through measurement operations that decouple local elements from their environment. The nature of these transitions emerges when studying average entanglement across many realisations of stochastic quantum evolution. However, quantum trajectory approaches face a fundamental challenge in experimental implementation: the postselection problem. In random dynamics with many qubits, the probability of any particular state recurring becomes vanishingly small, making it impossible to accumulate sufficient statistics about individual trajectories. I will give an overview of an alternative approach using replica density matrices—a framework that offers a deterministic, non-linear method for revealing entanglement structures without requiring individual trajectory realisations. I will focus in particular on replica cut-offs, the necessary approximations for implementing this scheme on numerical or quantum hardware. I will then discuss how to faithfully construct these replica cut-offs and demonstrate their effectiveness on small-scale systems.
