The characteristics of the ground states of Bose-Einstein condensates (BEC) in spin-dependent bilayer square optical lattices are investigated in this paper. The relative twist angle between the two lattices and the interlayer coupling strength are the main tunable parameters that affect the density distribution of the ultracold atoms. When the lowest band of the lattices exhibits a single-well dispersion, the localization of the ultracold atoms in the Moiré lattice can be determined from the twist angle, interlayer coupling strength, number of atoms, and lattice depth. When the lowest band of the lattices exhibits a double-well dispersion, the twist between the lattices leads to the twist of the two spin states. With an increase in interlayer coupling strength, the two twisted spin states will overlap. The results of this work will stimulate further exploration of novel quantum effect with ultracold atoms in twisted optical lattices.
The dynamics of quantum gases with time-varying interactions have attracted research interests owing to recent advances in experimental techniques such as optical Feshbach resonance. A range of novel dynamic behaviors including the Farady pattern and Bose fireworks have been observed in these systems. In this research, the dynamic problem of two harmonically trapped atoms with periodically modulating interaction strength is investigated. Because of the Hamiltonian time dependence, the system energy is an unconserved quantity. However, we may continue to utilize the Floquet theory for the time-periodic Hamiltonian and define its quasi-energy. The exact equations for the quasi-energies of the two-body problem are derived. Upon numerically solving these equations, we identify that the two-body quasi-energy spectrum exhibits various novel behaviors for different driven parameters or frequencies.
The Gross-Pitaevskii equation is widely applied in Bose-Einstein condensate research, yet is rarely analytically determined; thus, it is important to develop a numerical method with high precision to resolve this. Accordingly, a numerical method was developed in this work, considering the splitting step method, Crank-Nicolson algorithm, and Numerov algorithm with four-order accuracy. The corresponding test shows that compared with the finite difference method using five points, the proposed algorithm is more efficient and costs less memory.
In this study, based on the lossy SU(2) and SU(1,1) interferometer models, phase estimation in interferometers was investigated. The general expression for the overestimated quantum Fisher information (QFI), which exists when performing single-parameter phase estimation compared to two-parameter phase estimation, was theoretically studied. In addition, the variation in the overestimated QFI with the loss factor or beam splitting ratio was numerically analyzed with the input of coherent and squeezed vacuum states, and the disappearance and recovery of the overestimated QFI was related to the beam splitting ratio, gain factor, and squeeze amplitude. By adjusting the beam splitting ratio and loss factor, the best sensitivity was obtained, which is beneficial for quantum precision measurements in lossy environments.
In the study of high-order harmonic generation mechanisms, the main focus has been on interband polarization and intraband currents, as well as anomalous current mechanisms caused by Berry curvature. The long-neglected mixture term currents have been obtained by decomposing the strong laser-induced intact currents into contributions from different mechanisms. In this study, the peak amplitude and laser wavelength dependence of the high harmonics generated by different mechanisms were investigated by numerically solving the semiconductor Bloch equations (SBE). The interference between the current mechanisms was explored. It was found that the high-order harmonic spectra induced by the mixture term currents and the interband polarization currents have extremely similar variation patterns and extremely close harmonic intensities, both with the change in wavelength and the change in peak amplitude. Additionally, the anomalous harmonics were found to produce only even harmonics perpendicular to the polarization direction of the laser field. The anomalous harmonics are unique in that they have a minimum during wavelength and peak intensity variations. Analyzing the interference effects between the different mechanisms revealed that the interband polarization harmonics and intraband harmonics (including anomalous harmonics) interfere significantly with each other in the vertical polarization direction, while the interference of mixture term harmonics with intraband harmonics is negligible.
We propose an efficient grating coupler design scheme based on a lithium niobate guided mode structure and its optimized optical excitation configuration. The coupling effect of the grating coupler is numerically analysed using the finite time domain difference algorithm. We study the effects of the grating period, grating duty ratio, silica isolation layer thickness, polarization and angle of incident light on the coupling efficiency of the grating. The spatial light propagation electric field images are simulated for resonant and non-resonant wavelengths. The results show that with a grating period of 650 nm, a grating duty cycle of 0.3 and an etching depth of 130 nm, an optimised grating coupling efficiency of ~38% can be obtained using TM(transverse magnetic) polarised light incident along the grating normal angle of 17°, thus effectively coupling spatial light into the lithium niobate subwavelength waveguide film. This is of great reference value for the design and application performance of LiNbO3 micro-nano grating couplers.
中国综合性科技类核心期刊(北大核心)
