Self-trapping dynamics in a 2D optical lattice

TL;DR

This paper investigates the dynamics of macroscopic quantum self-trapping in a 2D optical lattice of ultracold bosons, comparing mean-field and advanced quantum models to experimental results, and proposing a new quantum correlation-based trapping mechanism.

Contribution

It introduces a new self-trapping mechanism driven by quantum correlations, supported by t-DMRG calculations, and evaluates the limitations of mean-field and TWA models in describing experimental observations.

Findings

Mean-field models overestimate expansion rates.

TWA improves but does not fully match experiments.

Quantum correlations can suppress tunneling without density gradients.

Abstract

We describe theoretical models for the recent experimental observation of Macroscopic Quantum Self-Trapping (MQST) in the transverse dynamics of an ultracold bosonic gas in a 2D lattice. The pure mean-field model based on the solution of coupled nonlinear equations fails to reproduce the experimental observations. It greatly overestimates the initial expansion rates at short times and predicts a slower expansion rate of the cloud at longer times. It also predicts the formation of a hole surrounded by a steep square fort-like barrier which was not observed in the experiment. An improved theoretical description based on a simplified Truncated Wigner Approximation (TWA), which adds phase and number fluctuations in the initial conditions, pushes the theoretical results closer to the experimental observations but fails to quantitatively reproduce them. An explanation of the delayed expansion as a consequence of a new type of self-trapping mechanism, where quantum correlations suppress tunneling even when there are no density gradients, is discussed and supported by numerical time-dependent Density Matrix Renormalization Group (t-DMRG) calculations performed in a simplified two coupled tubes set-up.