Johannes Schachenmayer
Laboratory of Computational Quantum Many-body Theory, CESQ/ISIS, CNRS and Université de Strasbourg
website
Classical ways of unraveling open quantum dynamics
Quantum computers could become powerful tools, as simulating large-scale quantum dynamics on classical computers is considered a hard problem. However, quantum systems are never perfectly isolated; interactions with their environment introduce decoherence. Since this process renders dynamics more classical, open-system quantum dynamics should, in principle, be more amenable to classical simulation.
Yet, large-scale open-system dynamics is still often considered difficult to simulate, as it requires transitioning from a state vector to a density matrix description. It remains poorly understood how noise can be optimally exploited to enable classical simulations, and the critical noise rates that make systems simulable are not well characterized. I this talk, I present two recent advances on this topic:
The dynamics of open-system master equations can be unraveled into stochastic evolutions of pure states using standard quantum trajectory techniques. For large quantum systems, such unravelings are highly non-unique. I introduce a numerical algorithm that adaptively reduces entanglement in quantum trajectories (NUMU [1]). Additionally, I discuss how, in pure Dicke superradiant decay, trajectories can be chosen to be product states (coherent spin states), implying that entanglement is absent in this process [2]. In both cases, reduced entanglement enables a classical state representation via matrix product states.
[1] SciPost Phys. 18, 048 (2025); https://dx.doi.org/10.21468/SciPostPhys.18.2.048
[2] Phys. Rev. Lett. 135, 133602 (2025); https://doi.org/10.1103/xcxr-sm9c
