Atomic transport dynamics in crossed optical dipole trap

We study the dynamical evolution of cold atoms in crossed optical dipole trap theoretically and experimentally. The atomic transport process is accompanied by two competitive kinds of physical mechanics, atomic loading and atomic loss.The loading process normally is negligible in the evaporative cooling experiment on the ground, while it is significant in preparation of ultra-cold atoms in the space station. Normally, the atomic loading process is much weaker than the atomic loss process, and the atomic number in the central region of the trap decreases monotonically, as reported in previous research. However, when the atomic loading process is comparable to the atomic loss process, the atomic number in the central region of the trap will initially increase to a maximum value and then slowly decrease, and we have observed the phenomenon first. The increase of atomic number in the central region of the trap shows the presence of the loading process, and this will be significant especially under microgravity conditions. We build a theoretical model to analyze the competitive relationship, which coincides with the experimental results well. Furthermore, we have also given the predicted evolutionary behaviors under different conditions. This research provides a solid foundation for further understanding of the atomic transport process in traps. The analysis of loading process is of significant importance for preparation of ultra-cold atoms in a crossed optical dipole trap under microgravity conditions.

Superfluid-Mott-Insulator Transition in an Optical Lattice with Adjustable Ensemble-Averaged Filling Factors

We study the quantum phase transition from a superfluid to a Mott insulator of ultracold atoms in a threedimensional optical lattice with adjustable filling factors.Based on the density-adjustable Bose-Einstein condensate we prepared,the excitation spectrum in the superfluid and the Mott insulator regime is measured with different ensemble-averaged filling factors.We show that for the superfluid phase,the center of the excitation spectrum is positively correlated with the ensemble-averaged filling factor,indicating a higher sound speed of the system.For the Mott insulator phase,the discrete feature of the excitation spectrum becomes less pronounced as the ensemble-averaged filling factor increases,implying that it is harder for the system to enter the Mott insulator regime with higher filling factors.The ability to manipulate the filling factor affords further potential in performing quantum simulation with cold atoms trapped in optical lattices.

Measurement of interacting quantum phases:A band mapping scheme

Band mapping is widely used in various scenarios of cold atom physics to measure the quasi-momentum distribution and band population.However,conventional methods fail in strongly interacting systems.Here we propose and experimentally realize a novel scheme of band mapping that can accurately measure the quasi-momentum of interacting manybody systems.Through an anisotropic control in turning down the threedimensional optical lattice,we can eliminate the effect of interactions on the band mapping process.Then,based on a precise measurement of the quasi-momentum distribution,we introduce the incoherent fraction as a physical quantity that can quantify the degree of incoherence of quantum many-body states.This method enables precise measurement of processes such as the superfluid to Mott insulator phase transition.Additionally,by analyzing the spatial correlation derived from the quasi-momentum of superfluid-Mott insulator phase transitions,we obtain results consistent with the incoherent fraction.Our scheme broadens the scope of band mapping and provides a method for studying quantum many-body problems.

Quantum precision measurement of two-dimensional forces with 10^(-28)-Newton stability

High-precision sensing of vectorial forces has broad impact on both fundamental research and technological applications such as the examination of vacuum fluctuations and the detection of surface roughness of nanostructures.Recent years have witnessed much progress on sensing alternating electromagnetic forces for the rapidly advancing quantum technology-orders of magnitude improvement has been accomplished on the detection sensitivity with atomic sensors,whereas such high-precision measurements for static electromagnetic forces have rarely been demonstrated.Here,based on quantum atomic matter waves confined by a two-dimensional optical lattice,we perform precision measurement of static electromagnetic forces by imaging coherent wave mechanics in the reciprocal space.The lattice confinement causes a decoupling between real-space and reciprocal dynamics,and provides a rigid coordinate frame for calibrating the wavevector accumulation of the matter wave.With that we achieve a stateof-the-art sensitivity of 2.30(8)×10^(-26) N/√Hz.Long-term stabilities on the order of 10^(-28) N are observed in the two spatial components of a force,which allows probing atomic Van der Waals forces at one millimeter distance.As a further illustrative application,we use our atomic sensor to calibrate the control precision of an alternating electromagnetic force applied in the experiment.Future developments of this method hold promise for delivering unprecedented atom-based quantum force sensing technologies.

Probing quantum many-body correlations by universal ramping dynamics

Ramping a physical parameter is one of the most common experimental protocols in studying a quantum system, and ramping dynamics has been widely used in preparing a quantum state and probing physical properties. Here, we present a novel method of probing quantum many-body correlation by ramping dynamics. We ramp a Hamiltonian parameter to the same target value from different initial values and with different velocities, and we show that the first-order correction on the finite ramping velocity is universal and path-independent, revealing a novel quantum many-body correlation function of the equilibrium phases at the target values. We term this method as the non-adiabatic linear response since this is the leading order correction beyond the adiabatic limit. We demonstrate this method experimentally by studying the Bose-Hubbard model with ultracold atoms in three-dimensional optical lattices.Unlike the conventional linear response that reveals whether the quasi-particle dispersion of a quantum phase is gapped or gapless, this probe is more sensitive to whether the quasi-particle lifetime is long enough such that the quantum phase possesses a well-defined quasi-particle description. In the BoseHubbard model, this non-adiabatic linear response is significant in the quantum critical regime where well-defined quasi-particles are absent. And in contrast, this response is vanishingly small in both superfluid and Mott insulators which possess well-defined quasi-particles. Because our proposal uses the most common experimental protocol, we envision that our method can find broad applications in probing various quantum systems.

Field theoretical approach to spin models

We developed a systematic non-perturbative method base on Dyson–Schwinger theory and theΦ-derivable theory for Ising model at broken phase.Based on these methods,we obtain critical temperature and spin spin correlation beyond mean field theory.The spectrum of Green function obtained from our methods become gapless at critical point,so the susceptibility become divergent at Tc.The critical temperature of Ising model obtained from this method is fairly good in comparison with other non-cluster methods.It is straightforward to extend this method to more complicate spin models for example with continue symmetry.

Stabilized DFB laser system with large tuning range

We propose, design, and realize a compact stabilized laser system that can be tuned within 24 GHz automatically. This laser system consists of two distributed feedback （DFB） lasers, one of which is reference and locked to the D2 line of Rb87, the other laser is a slave that is locked to the reference laser via a loop servo. We measured the frequency of the beating signal of the two lasers and generated an error signal, which controlled the frequency of the slave laser to close the loop. We compressed the fluctuation of the beating signal’s frequency to less than 1 MHz. Furthermore, the system can also automatically determine and control whether the slave is red detuned or blue detuned to the reference. The dimensions of our laser system are about 15 cm×20 cm×10 cm. This kind of laser system can be applied in many important applications, such as atomic interferometer and cold atomic clock.