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Hongwei Xiong

Publications and source records attributed to Hongwei Xiong.

At least 37 records · Page 2Linked to original sources
arXiv2017

Matter wave interference of dilute Bose gases in the critical regime

Ultra-cold atomic gases provide new chance to study the universal critical behavior of phase transition. We study theoretically the matter wave interference for ultra-cold Bose gases in the critical regime. We demonstrate that the interference in the momentum distribution can be used to extract the correlation in the Bose gas. A simple relation between the interference visibility and the correlation length is found and used to interpret the pioneering experiment about the critical behavior of dilute Bose gases [Science {\bf 315}, 1556 (2007)]. Our theory paves the way to experimentally study various types of ultra-cold atomic gases with the means of matter wave interference.

cond-mat.quant-gas↗
arXiv2016

Observation of quantum dynamical oscillations of ultracold atoms in the F and D bands of an optical lattice

We report the observation of quantum dynamical oscillations of ultracold atomic gases in the F and D bands of a single-well optical lattice. We are able to control the Bragg reflections at the Brillouin zone edge up to the third order. As a result, we can switch the quantum dynamics from oscillations across both the F and D bands to oscillations only within the F-band. Our capability to observe these remarkable oscillations comes from the innovative non-adiabatic technique which allows us to load ultracold atoms efficiently to the G-band of an optical lattice.

cond-mat.quant-gas↗
arXiv2015

Interference effect of critical ultra-cold atomic Bose gases

For ultra-cold atomic gases close to the critical temperature, there is a divergent correlation behavior within the critical regime. This divergent correlation behavior is the cornerstone of the universal behavior within the critical regime, e.g. the universal critical exponent for the same class with very different physical systems. It is still quite challenging to observe this divergent correlation behavior in experiments with ultra-cold atomic gases. Here we consider theoretically the interference effect of the critical atomic Bose gas by a Kapitza-Dirac scattering. We find that the Kapitza-Dirac scattering has the merit of enhancing the interference effect in the observation of the correlation behavior. This provides a potential method to study the critical behavior of ultra-cold Bose gases. A simple rule is found by numerical simulations to get the critical exponent and correlation amplitude ratio from the interference fringes after the Kapitza-Dirac scattering.

cond-mat.quant-gas↗
arXiv2015

Long-time nonlinear dynamical evolution for P-band ultracold atoms in an optical lattice

We report the long-time nonlinear dynamical evolution of ultracold atomic gases in the P-band of an optical lattice. A Bose-Einstein condensate (BEC) is fast and efficiently loaded into the Pband at zero quasi-momentum with a non-adiabatic shortcut method. For the first one and half milliseconds, these momentum states undergo oscillations due to coherent superposition of different bands, which are followed by oscillations up to 60ms of a much longer period. Our analysis shows the dephasing from the nonlinear interaction is very conducive to the long-period oscillations induced by the variable force due to the harmonic confinement.

cond-mat.quant-gas↗
arXiv2013

The observation of diffraction phases in matter wave scattering

We study the diffraction phase of different orders via the Dyson expansion series, for ultracold atomic gases scattered by a standing-wave pulse. As these diffraction phases are not observable in a single pulse scattering process, a temporal Talbot-Lau interferometer consisting of two standing-wave pulses is demonstrated experimentally with a Bose-Einstein condensate to explore this physical effect. The role of the diffraction phases is clearly shown by the second standing-wave pulse in the relative population of different momentum states. Our experiments demonstrate obvious effects beyond the Raman-Nath method, while agree well with our theory by including the diffraction phases. In particular, the observed asymmetry in the dependence of the relative population on the interval between two standing-wave pulses reflects the diffraction phase differences. The role of interatomic interaction in the Talbot-Lau interferometer is also discussed.

cond-mat.quant-gas↗
arXiv2013

Critical correlations in an ultracold Bose gas revealed by means of a temporal Talbot-Lau interferometer

We study experimentally the critical correlation in an ultra-cold Bose gas with a temporal Talbot-Lau (TL) interferometer. Near the critical temperature, we observe a bi-modal density distribution in an ultra-cold Bose gas after the application of the TL interferometer. The measured fraction of the narrower peak in the density distribution displays a clear peak within the critical regime. The peak position agrees with the critical temperature calculated with the finite-size and interaction corrections. The critical exponents are extracted from the peak and they agree with the critical exponents for the correlation length.

cond-mat.quant-gas↗
arXiv2013

Stability of $p$-orbital Bose-Einstein condensates in optical checkerboard and square lattices

We investigate $p$-orbital Bose-Einstein condensates in both the square and checkerboard lattice by numerically solving the Gross-Pitaevskii equation. The periodic potential for the latter lattice is taken exactly from the recent experiment [Nature Phys. 7, 147 (2011)]. It is confirmed that the staggered orbital-current state is the lowest-energy state in the $p$ band. Our numerical calculation further reveals that for both lattices the staggered $p$-orbital state suffers Landau instability but the situation is remarkably different for dynamical instability. A dynamically stable parameter region is found for the checkerboard lattice, but not for the square.

cond-mat.quant-gas↗
arXiv2012

Measurement of phase fluctuations of Bose-Einstein condensates in an optical lattice

Even at zero temperature, there exist phase fluctuations associated with an array of Bose-Einstein condensates confined in a one-dimensional optical lattice. We demonstrate a method to measure the phase fluctuations based on the Fourier spectrum of the atomic density for a condensate released from the optical lattice. The phase variance is extracted from the relative intensities of different peaks in the Fourier spectrum. This method works even for high lattice strength where interference peaks disappear in the atomic density distribution.

cond-mat.quant-gas↗
arXiv2011

Observation of universal behaviour of ultracold quantum critical gases

Quantum critical matter has already been studied in many systems, including cold atomic gases. We report the observation of a universal behaviour of ultracold quantum critical Bose gases in a one-dimensional optical lattice. In the quantum critical region above the Berezinskii-Kosterlitz-Thouless transition, the relative phase fluctuations between neighboring subcondensates and spatial phase fluctuations of quasi-2D subcondensates coexist. We study the density probability distribution function when both these two phase fluctuations are considered. A universal exponential density probability distribution is demonstrated experimentally, which agrees well with a simple theoretical model by considering these two phase fluctuations.

cond-mat.quant-gas↗
arXiv2010

Phase transition to Bose-Einstein condensation for a Bosonic gas confined in a combined trap

We present a study of phase transition to macroscopic superfluidity for an ultracold bosonic gas confined in a combined trap formed by a one-dimensional optical lattice and a harmonic potential, focusing on the critical temperature of this system and the interference patterns of the Bose gas released from the combined trap. Based on a semiclassical energy spectrum, we develop an analytic approximation for the critical temperature $T_{c}$, and compare the analytic results with that obtained by numerical computations. For finite temperatures below $T_{c}$, we calculate the interference patterns for both the normal gas and the superfluid gas. The total interference pattern shows a feature of ``peak-on-a-peak". As a comparison, we also present the experimentally observed interference patterns of $^{87}$Rb atoms released from a one-dimensional optical lattice system in accord with our theoretical model. Our observations are consistent with the theoretical results.

cond-mat.quant-gas↗
arXiv2010

Hidden vortices in a Bose-Einstein condensate in a rotating double-well potential

We study vortex formation in a Bose-Einstein condensate in a rotating double-well potential. Besides the ordinary quantized vortices and elusive ghost vortices, "hidden" vortices are found distributing along the central barrier. These hidden vortices are invisible like ghost vortex but carry angular momentum. Moreover, their core size is not given by the healing length, but is strongly influenced by the external potential. We find that the Feynman's rule can be well satisfied only after including the hidden vortices. There is no critical rotating frequency for the formation of hidden vortex while there is one for the formation of ordinary visible vortices. Hidden vortices can be revealed in the free expansion of the Bose-Einstein condensates. In addition, the hidden vortices in a Bose-Einstein condensate can appear in other external potentials, such as a rotating anisotropic toroidal trap.

cond-mat.quant-gas↗
arXiv2010

Dynamical Universal Behavior in Quantum Chaotic Systems

We discover numerically that a moving wave packet in a quantum chaotic billiard will always evolve into a quantum state, whose density probability distribution is exponential. This exponential distribution is found to be universal for quantum chaotic systems with rigorous proof. In contrast, for the corresponding classical system, the distribution is Gaussian. We find that the quantum exponential distribution can smoothly change to the classical Gaussian distribution with coarse graining.

cond-mat.quant-gas↗
arXiv2009

Imaginary Potential Induced Quantum Coherence for Bose-Einstein Condensates

The role of complex potentials in single-body Schrődinger equation has been studied intensively. We study the quantum coherence for degenerate Bose gases in complex potentials, when the exchange symmetry of identical bosons is considered. For initially independent Bose-Einstein condensates, it is shown that even very weak imaginary potential can induce perfect quantum coherence between different condensates. The scheme to observe imaginary potential induced quantum coherence is discussed.

cond-mat.quant-gas↗
arXiv2009

Atomic Quantum Corrals for Bose-Einstein Condensates

We consider the dynamics of Bose-Einstein condensates in a corral-like potential. Compared to the electronic quantum corrals, the atomic quantum corrals have the advantage of allowing direct and convenient observation of the wave dynamics. Our numerical study shows that this advantage not only allows people to explore the rich dynamical structures in the density distribution but also makes the corrals useful in many other aspects. In particular, the corrals for atoms can be arranged into a stadium shape for the experimental visualization of quantum chaos, which has been elusive with the electronic quantum corrals.

cond-mat.other↗
arXiv2009

Strong parity effect of particle number in the interference fringes of Bose-Einstein condensates released from a double-well potential

We study the parity effect of the particle number in the interference fringes of a Bose-Einstein condensate released from a double-well potential. For a coherently splitting condensate in the double-well potential, with a decoupled two-mode Bose-Hubbard model, there is well-known phase diffusion because of interatomic interactions. After a specific holding time of the double-well potential, the phase diffusion will make the interference patterns in the density distribution depend strongly on the parity of the total particle number by further overlapping two condensates. This parity effect originates from the quantized relative phase about the total particle number. The experimental scheme to observe this "even-odd" effect of the particle number is discussed.

cond-mat.quant-gas↗
arXiv2008

Density-density correlation and interference mechanism for two initially independent Bose-Einstein condensates

In an exciting experiment by MIT's group (Science 275, 637 (1997)), clear interference fringes were observed for two initially independent Bose condensates in dilute gas. Presently, there are two different theories (measurement-induced interference theory and interaction-induced interference theory) which can both explain MIT's experimental results. Based on our interaction-induced interference theory, we consider the evolution of the density-density correlation after the releasing of a double-well potential trapping two independent Bose condensates. Based on the interaction-induced interference theory, we find that the interference fringes in the density-density correlation exhibit a behavior of emergence and disappearance with the development of time. We find essential difference for the density-density correlation based on interaction-induced interference theory and measurement-induced interference theory, and thus we suggest the density-density correlation to experimentally reveal further the interference mechanism for two initially independent Bose condensates.

cond-mat.stat-mech↗
arXiv2007

Anomalous fermion bunching in density-density correlation

We consider theoretically density-density correlation of identical Fermi system by including the finite resolution of a detector and delta-function term omitted in the ordinary method. We find an anomalous fermion bunching effect, which is a quantum effect having no classical analogue. This anomalous fermion bunching is studied for ultracold Fermi gases released from a three-dimensional optical lattices. It is found that this anomalous fermion bunching is supported by a recent experiment (T. Rom et al Nature 444, 733 (2006)).

cond-mat.other↗