Quantum dynamics and entanglement in coherent transport of atomic population

Abstract

In this work we look at the quantum dynamics of the process known as either transport without transit (TWT), or coherent transfer of atomic population (CTAP), of a Bose-Einstein condensate from one well of a lattice potential to another, non-adjacent well, without macroscopic occupation of the well between the two. This process has previously been analysed and in this work we extend those analyses by considering the effects of quantum statistics on the dynamics and entanglement properties of the condensate modes in the two relevant wells. In order to do this, we go beyond the mean-field analysis of the Gross-Pitaevskii type approach and utilise the phase-space stochastic methods so well known in quantum optics. In particular, we use the exact positive-P representation where it is suitable, and the approximate truncated WIgner representation otherwise. We find strong agreement between the results of these two methods, with the mean-field dynamics not depending on the initial quantum states of the trapped condensate. We find that the entanglement properties do depend strongly on the initial quantum states, with quantitatively different results found for coherent and Fock states. Comparison of the two methods gives us confidence that the truncated Wigner representation delivers accurate results for this system and is thus a useful method as the collisional nonlinearity increases and the positive-P results fail to converge.