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arXiv:2601.20787

Semiclassical effective description of a quantum particle on a sphere with non-central potential

Guillermo Chacon-Acosta, H. Hernandez-Hernandez, J. Ruvalcaba-Rascon

quant-phSubmitted Jan 28, 2026• Announced Jan 29, 2026• 0 citations
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Abstract

We develop a semiclassical framework for studying quantum particles constrained to curved surfaces using the momentous quantum mechanics formalism, which extends classical phase-space to include quantum fluctuation variables (moments). In a spherical geometry, we derive quantum-corrected Hamiltonians and trajectories that incorporate quantum back-reaction effects absent in classical descriptions. For the free particle, quantum fluctuations induce measurable phase shifts in azimuthal precession of approximately 8-12%, with uncertainty growth rates proportional to initial moment correlations. When a non-central Makarov potential is introduced, quantum corrections dramatically amplify its asymmetry. For strong coupling ($γ$ = -1.9), the quantum-corrected force drives trajectories preferentially toward the southern hemisphere on timescales 40% shorter than classical predictions, with trajectory densities exhibiting up to 3-fold enhancement in the preferred region. Throughout evolution, the solutions rigorously satisfy Heisenberg uncertainty relations, validating the truncation scheme. These results demonstrate that quantum effects fundamentally alter semiclassical dynamics in curved constrained systems, with direct implications for charge transport in carbon nanostructures, exciton dynamics in curved quantum wells, and reaction pathways in cyclic molecules.

Comments
15 pages, 5 figures, 2 tables, 1 appendix

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We study the back-reaction of quantum systems onto classical ones. Taking the starting point that semi-classical physics should be described at all times by a point in classical phase space and a quantum state in Hilbert space, we consider an unravelling approach, describing the system in terms of a classical-quantum trajectory. We derive the general form of the dynamics under the assumptions that the classical trajectories are continuous and the evolution is autonomous, and the requirement that the dynamics is linear and completely positive in the combined classical-quantum state. This requirement is necessary in order to consistently describe probabilities, and forces the dynamics to be stochastic when the back-reaction is non-zero. The resulting equations of motion are natural generalisations of the standard semi-classical equations of motion, but since the resulting dynamics is linear in the combined classical-quantum state, it does not lead to the pathologies which usually follow from evolution laws based on expectation values. In particular, the evolution laws we present account for correlations between the classical and quantum system, which resolves issues associated with other semi-classical approaches. In addition, despite a breakdown of predictability in the classical degrees of freedom, the quantum state evolves deterministically conditioned on the classical trajectory, provided a trade-off between decoherence and diffusion is saturated. As a result, the quantum state remains pure when conditioned on the classical trajectory. To illustrate these points, we numerically simulate a number of semi-classical toy models, including one of vacuum fluctuations as a source driving the expansion of the universe. Finally, we discuss the application of these results to semi-classical gravity, and the black-hole information problem.
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