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arXiv · 2002.12035

Mean square displacement of a free quantum particle in a thermal state

Roberto Marquardt
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Abstract

The mean square displacement $\langle\left(x(t)-x(0)\right)^2\rangle$ of the position $x$ of a free particle of mass $m$ at thermal equilibrium is evaluated quantum mechanically. An analytical expression is obtained which shows an initial quadratic increase of the mean square displacement with time and later on a linear growth, with the slope $\hbar/m$, quite at variance with the result from classical statistical mechanics. Results are discussed in relation to observables from helium scattering or spin-echo experiments, and their possible interpretation in terms of the classical and quantum mechanical expression for the mean square displacement of an essentially free particle.

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Roberto Marquardt. 2020-02-27. Mean square displacement of a free quantum particle in a thermal state. https://doi.org/10.1080/00268976.2021.1971315

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