Quantum Decoherence and Coupling-to-Environment Effects on Fusion Tunneling: Open-Quantum-System Perspectives on Barrier Penetration

Abstract

Heavy-ion fusion below the Coulomb barrier is a quantum tunnelling process in which the relative motion of the two nuclei is coupled to a large number of intrinsic and continuum degrees of freedom. When the relative motion is regarded as an open quantum system and the intrinsic excitations as its environment, the coupling generates both a coherent enhancement of the tunnelling probability and, through the irreversible loss of phase information, quantum decoherence of the tunnelling amplitude. This article develops an open-quantum-system perspective on barrier penetration, in which the reduced density matrix of the relative motion obeys a master equation of Lindblad form and the influence of the environment is encoded in a spectral density. The benchmark reaction ^6 Li+^208 Pb, whose weakly bound projectile provides a strongly coupled breakup environment, is analysed. It is shown that coherent coupling enhances sub-barrier fusion, whereas decoherence progressively erases this enhancement and smooths the fusion barrier distribution, and that the breakup environment suppresses complete fusion above the barrier. The decoherence time of the relative motion is found to be comparable to the tunnelling time, so that the transition from coherent to incoherent barrier penetration is realized within the interaction region. The open-quantum-system framework thus unifies the coherent coupled-channels enhancement and the dissipative suppression of tunnelling within a single description, and exposes the methodological requirements — beyond the Born–Markov and Ohmic approximations, and beyond the high-temperature Caldeira–Leggett closure — that a quantitatively reliable nuclear-physics implementation must ultimately meet...