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arXiv:2103.05392 · Physical Review A

Loading ultracold atoms onto nonlinear Bloch states and soliton states in bichromatic lattices

Tomotake Yamakoshi, Shinichi Watanabe

Published Jul 9, 2021• 3 citations
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Abstract

We simulate and analyze an experimental method of loading interacting ultracold atoms onto nontrivial quantum states such as nonlinear Bloch wave and soliton solutions in a one-dimensional bichromatic lattice. Of standard bands, inverted bands, and bands with Dirac-like points permitted by a bichromatic lattice, we consider the case of an inverted band and examine the loading process in terms of nonlinear Bloch waves formed by an aggregate of ultracold atoms described by the mean-field model. Specifically, we solved the Gross-Pitaevskii equation numerically and found an appropriate standing wave-pulse sequence for the inverted band, which sequence proved to be a suitable protocol for producing soliton solutions. In addition, we examined the effect of an external potential and dynamical instabilities for the postloading process. We also provide an appropriate data set for future experimental realization of our findings.

DOI
10.1103/physreva.104.013309

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We simulate and analyze an experimental method of loading interacting ultracold atoms onto nontrivial quantum states such as nonlinear Bloch wave and soliton solutions in a one-dimensional bichromatic lattice. Of standard bands, inverted bands, and bands with Dirac-like points permitted by a bichromatic lattice, we consider the case of an inverted band and examine the loading process in terms of nonlinear Bloch waves formed by an aggregate of ultracold atoms described by the mean-field model. Specifically, we solved the Gross-Pitaevskii equation numerically and found an appropriate standing wave-pulse sequence for the inverted band, which sequence proved to be a suitable protocol for producing soliton solutions. In addition, we examined the effect of an external potential and dynamical instabilities for the postloading process. We also provide an appropriate data set for future experimental realization of our findings.
We study the dynamics of ultracold atoms in tailored bichromatic optical lattices. By tuning the lattice parameters, one can readily engineer the band structure and realize a Dirac point, i.e., a true crossing of two Bloch bands. The dynamics in the vicinity of such a crossing is described by the one-dimensional Dirac equation, which is rigorously shown beyond the tight-binding approximation. Within this framework we analyze the effects of an external potential and demonstrate numerically that it is possible to demonstrate Klein tunneling with current experimental setups.
An experimental group in Beijing [Yueyang Zhai et al., Phys. Rev. A 87, 063638 (2013)] introduced the method of the standing-wave pulse sequence for efficiently preparing ultracold bosonic atoms into a specific excited band in a one-dimensional optical lattice. Here, we report on a theoretical extension of their work to the problem of one-dimensional bichromatic superlattices in order to understand a wave packet shaping process in higher bands. Varying the lattice parameters leads to the so-called Dirac point where a pair of excited bands crosses. This paper thus discusses simultaneously efficient excitation of the wave packet to the proximity of the Dirac point and its subsequent dynamics in the force field of a parabolic trap. We numerically explore optimal pulse-sequence parameters and find an optimized sequence which excites more than 99% of the atoms in an ideal situation to the first and second excited bands within 100 $\ensuremath{\mu}\mathrm{s}$. Our main finding is that the system permitting the Dirac point possesses a region of parameters where the excited energy bands become nearly parabolic, conducive to a robust wave packet with high coherence and isochronicity. We also provide an appropriate data set for future experimentation, including effects of the atom-atom interaction by way of the mean-field nonlinear term.
We analyze the existence and stability of spatially extended (Bloch-type) and localized states of a Bose-Einstein condensate loaded into an optical lattice. In the framework of the Gross-Pitaevskii equation with a periodic potential, we study the band-gap structure of the matter-wave spectrum in both the linear and nonlinear regimes. We demonstrate the existence of families of spatially localized matter-wave gap solitons, and analyze their stability in different band gaps, for both repulsive and attractive atomic interactions.
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We present the results of experimental work on the dynamics of a Bose-Einstein condensate (BEC) loaded in a 1D moving optical lattice. The system is studied both in the regime of low atomic density (expanding BEC) and high atomic density (trapped BEC). In the low-density regime, the stable dynamics of the system is well-described in terms of Bloch waves. In the presence of nonlinearities, as in a high-density sample, the Bloch waves are not stable solutions of the Gross-Pitaevskii equation (GPE) and dynamical instability takes place.