Bonn 2025 – wissenschaftliches Programm

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AGPhil: Arbeitsgruppe Philosophie der Physik

AGPhil 6: History and Philosophy of Physics

AGPhil 6.2: Vortrag

Mittwoch, 12. März 2025, 17:15–17:30, HS XVII

Simulating spin measurement as unitary time evolution — •Thomas Dittrich, Oscar Rodríguez, and Carlos Viviescas — Departamento de Física, Universidad Nacional de Colombia, Bogotá D.C., Colombia

Quantum measurement is studied as a unitary time evolution of the measurement object, coupled to an environment representing the meter and the apparatus. Modelling the environment as a heat bath comprising a large but finite number of boson modes, it can be fully included in the time evolution of the total system. As a prototype of quantum measurement, we perform numerical simulations of projective measurements of the polarization of spin-1/2 particles. Their spin is prepared in an unpolarized pure state, the environment as a product of coherent states with a thermal distribution of centroids. Initially, the spin gets entangled with the heat bath and loses coherence, reproducing the collapse of the wave packet. For most of the initial states of the environment, we see a definite outcome of the measurement as the spin returning asymptotically to a pure state, either spin up or spin down with equal probability. Unitarity allows us to run the simulations backwards, undoing the measurement and recovering the initial state of the apparatus that led to the specific final spin state, relating it to the respective initial conditions of the heat bath, i.e., the observed randomness to quantum and thermal noise of the macroscopic environment. Extending our approach to a complete EPR setup with two arms remains as a challenge for future work.

Keywords: quantum measurement; quantum randomness; spin polarization; finite heat bath; projective measurement