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Acta Physica Sinica

Dynamical phase changes of the self-trapping of Bose-Einstein condensates and its characteristic of entanglement

Ma Yun, Fu Li-Bin, Yang Zhi-An, Liu Jie, (1)北京应用物理与计算数学研究所,北京 100088; (2)济南大学理学院,济南 250022

Published Jan 1, 2006• 33 citations
Publisher

Abstract

In this paper, we exploit two-mode model to investigate the influence of quantum fluctuation on the self-trapping of the Bose-Einstein Condensate(BEC). We found that, even for the finite number of particles, the transition to self-trapping is still observed.In particular, because of the quantum fluctuation due to the finite number of particles, the critical phenomenon becomes fuzzily: the smaller the number of particles is, the more obviously fuzzily the critical phenomenon becomes. Quantum entanglement entropy is then introduced to depict the critical behavior near the transition point.

DOI
10.7498/aps.55.5623

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In this paper, we exploit two-mode model to investigate the influence of quantum fluctuation on the self-trapping of the Bose-Einstein Condensate(BEC). We found that, even for the finite number of particles, the transition to self-trapping is still observed.In particular, because of the quantum fluctuation due to the finite number of particles, the critical phenomenon becomes fuzzily: the smaller the number of particles is, the more obviously fuzzily the critical phenomenon becomes. Quantum entanglement entropy is then introduced to depict the critical behavior near the transition point.
We investigate the nonlinear self-trapping phenomenon of a Bose-Einstein condensate (BEC) in a symmetric double well, emphasizing its underlying dynamical phase transition. As the nonlinear parameter characterizing the interaction between the degenerate atoms increases, the BEC becomes self-trapped, manifesting an asymmetric distribution of the atomic density profile. The essence of this phenomenon is revealed to be a continuous phase transition and the underlying critical behavior is studied analytically and found to follow a logarithmic scaling law. We then go beyond the mean-field treatment and extend our treatment to discuss the effect of many-body quantum fluctuations on the transition. It is found that the transition point is shifted and the scaling law is broken. In particular, the quantum phase transition is accompanied by a change of the entanglement entropy, which is found to reach a maximum at the transition point. The underlying physics is revealed.
Self-trapping of Bose-Einstein condensates (BEC) in double-well trap is investigated. Two kinds of self-trapping are discussed through phase space analysis in the mean-field approximation: 1) The number of atoms oscillates near an equilibrium point in the phase space, while relative phase increases monotonously with time (running-phase); 2) Both the number of particles and the relative phase oscillate near an equilibrium point in the phase space. In particular, we investigate how an external periodic filed influence the self-trapping. It is found that the external periodic field may dramatically modulate the critical points at which the transition to self-trapping occurs. With this, we can observe self-trapping phenomenon in a dilute Bose-Einstein condensate with a very weak interaction as well. Finally, the effect of many-body quantum fluctuation on self-trapping is also studied. We also discuss how to observe the self-trapping phenomenon with present experimental techniques.
The ground-state entanglement of the two-mode Bose-Einstein condensate is investigated through a quantum-phase-transition approach. The entanglement measure is taken as the order parameter and this is a nonlocal order parameter, which is different from the conventional order parameter of the Mott-insulator--superfluid-phase transitions. For this nonlocal order parameter, the scaling behavior corresponding to a continuous phase transition is obtained and a power-law divergence near the critical region follows it. This scaling behavior of quantum entanglement is analyzed by the finite-size scaling and the critical exponents are obtained as $\ensuremath{\nu}=1.01$ and $\ensuremath{\gamma}=0.86$. A close connection between quantum fluctuations and the phase transition of entanglement is also obtained.
In this paper we study self-trapping of Bose-Einstein condensates in a symmetric double-well potential with uniform noise or Gaussian noise existing respectively. We find that both uniform noise and Gaussian noise destroy the critical behavior of self-trapping with the interaction increasing and create a transition zone between Josephson oscillation and self-trapping, between which exists a critical point originally. Furthermore, the stronger the noise becomes, the wider the transition zone is. Meanwhile, we find that the phase space falls into complete confusion when we increase noise intensity to a certain extent with interaction fixed, and the trajectories reappear after we increase the interaction without increasing the noise intensity. In the full-quantum situation, when noise exists there is a transition zone instead of a critical value, and stronger noise creates wider transition zone, which is the same as in mean field treatment. What is different is that in full-quantum situation noise creates self-trapping, and the stronger the noise, the more obvious the self-trapping becomes.
Dynamical phase transition of Bose-Einstein condensates (BEC) in double-well trap with external periodic modulation applied to the interaction term is investigated in this paper. Particularly, phase transition to chaos is studied. It is found that, when resonance between the modulation frequency and the natural frequency of the system happens, the system turns to instability (chaos) at certain values of the parameters. In the chaotic region, distribution of particles is random, and the average of population difference fluctuates around zero. Especially, chaos can be manifested by the entanglement entropy. In the chaotic zone, both of the average entanglement entropies approach to their maxima.
The effect of decoherence on the phase transition of a Bose—Einstein condensate in a symmetric double-well potential is determined by the mean atom number difference. It still has two phases, the tunneling phase and the self-trapping phase, even under decoherence. The density matrix and the operator fidelity also show very different behaviors in the two phases. This suggests that operator fidelity can be used to characterize the phase transition of this Bose—Einstein condensate model, even under decoherence.