Solitary waves explore the quantum-to-classical transition

¹ Department of Physics, Indian Institute of Science Education and Research (IISER) Bhopal, Madhya Pradesh 462066, India
² New Zealand Institute for Advanced Study and Centre for Theoretical Chemistry and Physics, Massey University Auckland 0632, New Zealand
³ Department of Physics, Imperial College - SW7 2AZ, London, UK
⁴ Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg - Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
⁵ SAP Deep Learning Center of Excellence and Machine Learning Research SAP SE - Dietmar-Hopp-Allee 16, D-69190 Walldorf, Germany

^(a) E-mail: aparna16@iiserb.ac.in (corresponding author)

Received: 7 March 2022
Accepted: 25 October 2022

Abstract

How the classical appearance of our environment arises from the underlying quantum many-body theory is an open fundamental question. We propose that phenomena involved in the quantum-to-classical transition can be probed in collisions of bright solitary waves in Bose-Einstein condensates, where thousands of atoms form a large compound object at ultra cold temperatures. For the experimentally most relevant quasi-1D regime, where integrability is broken through effective three-body interactions, we find that ensembles of solitary waves exhibit complex interplay between phase coherence and entanglement generation in beyond mean-field simulations using the truncated Wigner method: An initial state of two solitons with a well-defined relative phase looses that phase coherence in the ensemble, with its single-particle two-mode density matrix exhibiting similar dynamics as a decohering two-mode superposition. This apparent decoherence is a prerequisite for the formation of entangled superpositions of different atom numbers in a subsequent soliton collision. The necessity for the solitons to first decohere is explained based on the underlying phase-space of the quintic mean-field equation. We show elsewhere that superpositions of different atom numbers later further evolve into spatially entangled solitons. Loss of ensemble phase coherence followed by system internal entanglement generation appear in an unusual order in this closed system, compared to a typical open quantum system.