Browsing by Author "Luo, Le"
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Item Beam pointing stabilization of an acousto-optic modulator with thermal control(OSA, 2019-04) Zhang, Xiao; Chen, Yang; Fang, Jianxiong; Wang, Tishuo; Li, Jiaming; Luo, Le; Physics, School of ScienceDiffraction beams generated by an acousto-optic modulator (AOM) are widely used in various optical experiments, some of which require high angular stability with the temporal modulation of optical power. Usually, it is difficult to realize both angular stability and high-power modulation in a passive setup without a servo system of radio-frequency compensation. Here, we present a method to suppress the angular drift and pointing noise only with the thermal management of the AOM crystal. We analyze the dependence of the angular drift on the refractive index variation and find that the angular drift is very sensitive to the temperature gradient, which could induce the refractive index gradient inside the AOM crystal. It reminds us that such angular drift could be significantly suppressed by carefully overlapping the zero temperature gradient area with the position of the acousto-optic interaction zone. We implement a water-cooling setup and find that the angular drift of an AOM is reduced over 100 times during the thermal transient and the angular noise is also suppressed to one-third of the non-cooled case. It should be emphasized that this thermal control method generally used to suppress the beam drift in both the diffraction and the perpendicular-to-diffraction directions. The refractive index thermal coefficient of tellurium dioxide crystal at 1064 nm determined by this angular drift-temperature model is 16×10 −6 K −1, consistent with previous studies. This thermal control technique provides potential applications for optical trapping and remote sensoring that demand for intensity ramps.Item Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving(Journal of Visualized Experiments, 2017-03-30) Li, Jiaming; de Melo, Leonardo F.; Luo, Le; Department of Physics, School of ScienceWe present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs the anharmonicity of the ODT, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, we selectively excite the hotter atoms out of the trap while keeping the colder atoms in the trap, generating parametric cooling. This experimental protocol starts with a magneto-optical trap (MOT) that is loaded by a Zeeman slower. The precooled atoms in the MOT are then transferred to an ODT, and a bias magnetic field is applied to create an interacting Fermi gas. We then lower the trapping potential to prepare a cold Fermi gas near the degenerate temperature. After that, we sweep the magnetic field to the noninteracting regime of the Fermi gas, in which the parametric cooling can be manifested by modulating the intensity of the optical trapping beams. We find that the parametric cooling effect strongly depends on the modulation frequencies and amplitudes. With the optimized frequency and amplitude, we measure the dependence of the cloud energy on the modulation time. We observe that the cloud energy is changed in an anisotropic way, where the energy of the axial direction is significantly reduced by parametric driving. The cooling effect is limited to the axial direction because the dominant anharmonicity of the crossed-beam ODT is along the axial direction. Finally, we propose to extend this protocol for the trapping potentials of large anharmonicity in all directions, which provides a promising scheme for cooling quantum gases using external driving.Item Experimental Trapped-ion Quantum Simulation of the Kibble-Zurek dynamics in momentum space(SpringerNature, 2016-09-16) Cui, Jin-Ming; Huang, Yun-Feng; Wang, Zhao; Cao, Dong-Yang; Wang, Jian; Lv, Wei-Min; Luo, Le; del Campo, Adolfo; Han, Yong-Jian; Li, Chuan-Feng; Guo, Guang-Can; Department of Physics, School of ScienceThe Kibble-Zurek mechanism is the paradigm to account for the nonadiabatic dynamics of a system across a continuous phase transition. Its study in the quantum regime is hindered by the requisite of ground state cooling. We report the experimental quantum simulation of critical dynamics in the transverse-field Ising model by a set of Landau-Zener crossings in pseudo-momentum space, that can be probed with high accuracy using a single trapped ion. We test the Kibble-Zurek mechanism in the quantum regime in the momentum space and find the measured scaling of excitations is in accordance with the theoretical prediction.Item Observation of parity-time symmetry breaking transitions in a dissipative Floquet system of ultracold atoms(Springer Nature, 2019-02-20) Li, Jiaming; Harter, Andrew K.; Liu, Ji; de Melo, Leonardo; Joglekar, Yogesh N.; Luo, Le; Physics, School of ScienceOpen physical systems with balanced loss and gain, described by non-Hermitian parity-time [Formula: see text] reflection symmetric Hamiltonians, exhibit a transition which could engender modes that exponentially decay or grow with time, and thus spontaneously breaks the [Formula: see text]-symmetry. Such [Formula: see text]-symmetry-breaking transitions have attracted many interests because of their extraordinary behaviors and functionalities absent in closed systems. Here we report on the observation of [Formula: see text]-symmetry-breaking transitions by engineering time-periodic dissipation and coupling, which are realized through state-dependent atom loss in an optical dipole trap of ultracold 6Li atoms. Comparing with a single transition appearing for static dissipation, the time-periodic counterpart undergoes [Formula: see text]-symmetry breaking and restoring transitions at vanishingly small dissipation strength in both single and multiphoton transition domains, revealing rich phase structures associated to a Floquet open system. The results enable ultracold atoms to be a versatile tool for studying [Formula: see text]-symmetric quantum systems.Item Observation of two PT transitions in an electric circuit with balanced gain and loss(Springer, 2020-08) Wang, Tishuo; Fang, Jianxiong; Xie, Zhongyi; Dong, Nenghao; Joglekar, Yogesh N.; Wang, Zixin; Li, Jiaming; Luo, Le; Physics, School of ScienceWe investigate 𝓟𝓣-symmetry breaking transitions in a dimer comprising two LC oscillators, one with loss and the second with gain. The electric energy of this four-mode model oscillates between the two LC circuits, and between capacitive and inductive energy within each LC circuit. Its dynamics are described by a non-Hermitian, 𝓟𝓣-symmetric Hamiltonian with three different phases separated by two exceptional points. We systematically measure the eigenfrequencies of energy dynamics across the three regions as a function of gain-loss strength. In addition to observe the well-studied 𝓟𝓣 transition for oscillations across the two LC circuits, at higher gain-loss strength, transition within each LC circuit is also observed. With their extraordinary tuning ability, 𝓟𝓣-symmetric electronics are ideally suited for classical simulations of non-Hermitian systems.Item Parametric Cooling and Itinerant Ferromagnetism in a Degenerate Fermi Gas(2018-12) de Melo, Leonardo F.; Cheng, Ruihua; Luo, Le; Greene, Chris; Joglekar, Yogesh; Petrache, HoriaPresented in this thesis is the construction of an apparatus to produce optically trapped lithium-6 atoms in the two lowest hyperfine states, the observation of cooling the trapped atoms by parametric excitation, and a study on the searching for itinerant ferromagnetism in a two-dimensional Fermi gas. In the parametric cooling experiment, a technique is developed to cool a cold atomic Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap. This method employs the anharmonicity of the optical dipole trap, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, hotter atoms are selectively excited out of the trap while keeping the colder atoms in the trap, generating a cooling effect. An analytical study of itinerant ferromagnetism in a two-dimensional atomic Fermi gas is presented, based on the past experiments done with three-dimensional Fermi gases. Here, the formation of repulsive polarons in a strongly-interacting Fermi gas is used as an initial condition. Then the observation of itinerant ferromagnetism is realized by detection of ferromagnetic domains in the two-dimensional gas. Additionally, an experiment and simulation is performed on the effect of velocity-changing collisions on the absolute absorption of lithium-6 vapor in an argon buffer gas. The dependence of probe beam absorption is observed by variation of beam intensity and spatial evolution. The simulation of an effective three-level energy model with velocity-changing collisions determines a collision rate that agrees with transmission data collected.Item Parametric cooling of a degenerate Fermi gas in an optical trap(APS, 2016-04) Li, Jiaming; Liu, Ji; Xu, Wen; de Melo, Leonardo; Luo, Le; Department of Physics, School of ScienceWe demonstrate a technique for cooling a degenerate Fermi gas in a crossed-beam optical dipole trap, where high-energy atoms can be selectively removed from the trap by modulating the stiffness of the trapping potential with anharmonic trapping frequencies. We measure the dependence of the cooling effect on the frequency and amplitude of the parametric modulations. It is found that the large anharmonicity along the axial trapping potential allows one to generate a degenerate Fermi gas with anisotropic energy distribution, in which the cloud energy in the axial direction can be reduced to the ground state value.Item Sub-megahertz frequency stabilization of a diode 2 laser by digital laser current modulation(2015-05) Li, Jiaming; Liu, Ji; DeMelo, Leonardo; Luo, Le; Lai, Tianshu; Wang, Zixin; Department of Physics, School of ScienceDigital laser current modulation (DLCM) is a convenient laser stabilization scheme whose major advantages are simplicity and inexpensiveness of implementation. However, there is a tradeoff between the SNR of the error signal and the laser linewidth due to the direct laser frequency modulation. In this paper, we demonstrated that DLCM can reduce the FWHM linewidth of a tunable diode laser down to 500 kHz using the modulation transfer spectrum of 𝐷2 line of a Li6 atomic vapor. For this purpose, a theoretical model is provided to analyze the DLCM-based modulation transfer spectrum. From the analysis, we experimentally explored the modulation effect on the DLCM spectrum to minimize the laser linewidth. Our result shows the optimized DLCM can stabilize a diode laser into the sub-megahertz regime without requiring acousto-optic and electro-optic modulators.Item Three-Body Recombination near a Narrow Feshbach Resonance in 6 Li(APS, 2018) Li, Jiaming; Liu, Ji; Luo, Le; Gao, Bo; Physics, School of ScienceWe experimentally measure and theoretically analyze the three-atom recombination rate, L3, around a narrow s-wave magnetic Feshbach resonance of 6Li−6Li at 543.3 G. By examining both the magnetic field dependence and, especially, the temperature dependence of L3 over a wide range of temperatures from a few μK to above 200 μK, we show that three-atom recombination through a narrow resonance follows a universal behavior determined by the long-range van der Waals potential and can be described by a set of rate equations in which three-body recombination proceeds via successive pairwise interactions. We expect the underlying physical picture to be applicable not only to narrow s wave resonances, but also to resonances in nonzero partial waves, and not only at ultracold temperatures, but also at much higher temperatures.