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PLV: Plenarvorträge

PLV VI

PLV VI: Plenary Talk

Wednesday, March 20, 2024, 14:00–14:45, H 0105

Decoding and Steering Monitored Quantum Dynamics — •Matthew Fisher — UC Santa Barbara, Santa Barbara, CA USA

When a many-body quantum system is continuously ``monitored" (measured) by an observer, the resulting quantum trajectories constitute an ensemble of pure states, which can exhibit measurement-induced phase transitions (MIPT), for example between volume law and area law entanglement, or between phases with or without symmetry breaking and/or topological order. Here, measurements are being employed to induce a new non-unitary dynamics, rather than just measuring a property of a given state. While the MIPT occurs generically in a number of different models, its verification can be challenging even on an error-corrected quantum computer, due to the so-called *post-selection problem*. I will briefly describe a recent proposal to access experimentally measurement-induced phase transitions (MIPT) employing ``decoding", (computing so-called quantum-classical observables), specifically the linear cross-entropy, without requiring any post- selection of quantum trajectories. With active feed forward, the ensemble of (measured) quantum trajectories can be ``steered" towards a unique pure state, which does not suffer from post selection. I will illustrate this with a simple model wherein the resulting pure state spontaneously breaks a continuous symmetry for a 1d chain of qubits.

Keywords: quantum, measurements, many-body