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FRI: Friday Contributed Sessions

FRI 10: Foundational / Mathematical Aspects – Unconventional Approaches

FRI 10.2: Vortrag

Freitag, 12. September 2025, 11:00–11:15, ZHG103

Quantum randomness revisited: simulating quantum measurement as a unitary time evolution — •Thomas Dittrich, Oscar Rodríguez, and Carlos Viviescas — Departamento de Física, Universidad Nacional de Colombia, Bogotá D.C., Colombia

Quantum measurement is usually regarded as incompatible with unitary time evolution, since the collapse of the wave packet breaks time reversal invariance. We challenge this view, studying quantum measurement as a unitary time evolution of the measurement object coupled to an environment that represents the meter and the apparatus. Modelled as a heat bath comprising only a finite, if large, number of boson modes, it is fully included in the time evolution of the entire system. We perform unitary numerical simulations of projective measurements of σz in spin-1/2 particles. They are prepared in a neutral pure state, the environment in a product of coherent states with centroids chosen at random from a thermal distribution. Initially, the spin gets entangled with the heat bath and loses coherence, reproducing the collapse. For large times and most of the initial states of the environment, the spin returns to a pure state, either spin up or spin down with equal probability, as definite outcome of the measurement. Unitarity allows us to run the simulations backwards from final state to preparation, undoing the measurement and tracing its result back to those initial conditions of the heat bath that entailed this result. That reveals the observed randomness as amplified quantum and thermal noise of the macroscopic environment. Extending our approach to an EPR setup is sketched as work in progress.

Keywords: quantum measurement; quantum randomness; spin; finite heat bath; unitary time evolution