arXiv subjects

Wenhua Hai

Publications and source records attributed to Wenhua Hai.

27 records · Page 2Linked to original sources

arXiv2008

Exact Floquet states of a driven condensate and their stabilities

We investigate the Gross-Pitaevskii equation for a classically chaotic system, which describes an atomic Bose-Einstein condensate confined in an optical lattice and driven by a spatiotemporal periodic laser field. It is demonstrated that the exact Floquet states appear when the external time-dependent potential is balanced by the nonlinear mean-field interaction. The balance region of parameters is divided into a phase-continuing region and a phase-jumping one. In the latter region, the Floquet states are spatiotemporal vortices of nontrivial phase structures and zero-density cores. Due to the velocity singularities of vortex cores and the blowing-up of perturbed solutions, the spatiotemporal vortices are unstable periodic states embedded in chaos. The stability and instability of these Floquet states are numerically explored by the time evolution of fidelity between the exact and numerical solutions. It is numerically illustrated that the stable Floquet states could be prepared from the uniformly initial states by slow growth of the external potential.

quant-ph↗

arXiv2004

Exactly pairing two-dimensional charged particles using a magnetic field

It is demonstrated that a uniform magnetic field can exactly pair the two-dimensional (2D) charged particles only for some quantized magnetic intensity values. For the particle-pair consisting of two like charges the Landau level of the center-of-mass motion is multiple degenerate that implies the shell structure. However, any particle-pair has only two non-degenerate relative levels, which are associated with the diamagnetic and paramagnetic states respectively. There exist a upper critical magnetic strength and a lower critical magnetic length across which two like charges cannot be paired. The theoretical results agree with the experimental data on the sites and widths of the integral and fractional quantum Hall plateaus in a CaAs-Al$_x$Ga$_{1-x}$As heterojunction, that gives a new explanation for the quantum Hall effect.

cond-mat.mes-hall↗

arXiv2003

Quasi-spin Model for Macroscopic Quantum Tunnelling between Two Coupled Bose-Einstein Condensates

The macroscopic quantum tunneling between two coupled Bose-Einstein condensates (BEC) (radio-frequency coupled two-component BECs or two BECs confined in a double-well potential) is mapped onto the tunneling of an uniaxial spin with an applied magnetic field. The tunneling exponent is calculated with an imaginary-time path-integral method. In the limit of low barrier, the dependence of tunneling exponent on the system parameters is obtained, and the crossover temperature from thermal regime to quantum regime is estimated. The detailed information about the tunnelling will give help to control population conversion between coupled BECs and realize quantum computation with coupled BECs.

cond-mat.mes-hall↗

arXiv2003

Phase-Dependent Spontaneous Spin Polarization and Bifurcation Delay in Coupled Two-Component Bose-Einstein Condensates

The spontaneous spin polarization and bifurcation delay in two-component Bose-Einstein condensates coupled with laser or/and radio-frequency pulses are investigated. We find that the bifurcation and the spontaneous spin polarization are determined by both physical parameters and relative phase between two condensates. Through bifurcations, the system enters into the spontaneous spin polarization regime from the Rabi regime. We also find that bifurcation delay appears when the parameter is swept through a static bifurcation point. This bifurcation delay is responsible for metastability leading to hysteresis.

cond-mat.mes-hall↗

arXiv2003

Physical solution of 1D quantum N-boson system with zero-range pair interaction

The mathematically exact solution of a one-dimensional (1D) quantum N-identical-boson system with zero-range pair interaction has been well known. We find that this solution is non-physical, since there exists a paradox of its energy expectation value, leading a basic contradiction to the mean-field theory of the system. A new integral equation that is equivalent to the corresponding Schrodinger one is established and its formally physical solution is derived. Energy correction from the average potential and harmonic waves of different momentums are demonstrated, and scattering amplitude of the physical solution and mean-field theory of the system are discussed.

quant-ph↗

arXiv2003

Wave-packet trains of a time-dependent harmonic oscillator

By using a test-function method, we construct $n$ exact solutions of a quantum harmonic oscillator with a time-dependent "spring constant". Any $n$-th solution describes a wave-packet train consisting of $n+1$ packets. Its center oscillates like the classically harmonic oscillator with variable frequency, and width and highness of each packet change simultaneously. When the deformation is small, it behaves like a soliton train, and the large deformation is identified with collapse and revival of the wave-packet train.

quant-ph↗

arXiv2003

Propagation, breathing and transition of matter-wave packet trains

We find a set of new exact solutions of a quantum harmonic oscillator, which describes some wave-packet trains with average energy being proportional to both the quantum level and classical energy of the oscillator. Center of the wave-packet trains may oscillate like a classical harmonic oscillator of frequency $ω$. Width and highness of the trains may change simultaneously with frequency $2 ω$ as an array of breathers. Under some perturbations the wave-packet trains could transit between the states of different quantum numbers. We demonstrate analytically and numerically that the wave-packet trains can be strictly fitted to the matter-wave soliton trains observed by Strecher et al. and reported in Nature 417, 150(2002). When the wave-packets breathe with greater amplitudes, they show periodic collapse and revival of the matter-wave.

quant-ph↗

arXiv2002

Conditional Spontaneous Spin Polarization and Bifurcation Delay in Coupled Two-Component Bose-Einstein Condensates

The spontaneous spin polarization and bifurcation delay in two-component Bose-Einstein condensates coupled with Raman pulses are investigated. We find that the bifurcation and the spontaneous spin polarization depend not only on the system parameters, but also on the relative phase between two components. Through bifurcations, the system enters into the spontaneous spin polarization regime from the Rabi regime. We also find that bifurcation delay appears when the parameter is swept through the static bifurcation point. This bifurcation delay is responsible for metastability leading to hysteresis. The area enclosed in the hysteresis loop increases with the sweeping rate of the parameter.

cond-mat.mes-hall↗

arXiv2000

Chaotic atomic population oscillations between two coupled Bose-Einstein condensates with time-dependent asymmetric trap potential

We have investigated the chaotic atomic population oscillations between two coupled Bose-Einstein condensates (BEC) with time-dependent asymmetric trap potential. In the perturbative regime, the population oscillations can be described by the Duffing equation, and the chaotic oscillations near the separatrix solution are analyzed. The sufficient-necessary conditions for stable oscillations depend on the physical parameters and initial conditions sensitively. The first-order necessary condition indicates that the Melnikov function is equal to zero, so the stable oscillations are Melnikov chaotic. For the ordinary parameters and initial conditions, the chaotic dynamics is simulated with numerical calculation. If the damping is absent, with the increasing of the trap asymmetry, the regular oscillations become chaotic gradually, the corresponding stroboscopic Poincare sections (SPS) vary from a single island to more islands, and then the chaotic sea. For the completely chaotic oscillations, the long-term localization disappears and the short-term localization can be changed from one of the BECs to the other through the route of Rabi oscillation. When there exists damping, the stationary chaos disappears, the transient chaos is a common phenomenon before regular stable frequency locked oscillations. And proper damping can keep localization long-lived.

quant-ph↗