Browsing by Author "de Melo, Leonardo"

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    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 Science
    Open 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.
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    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 Science
    We 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.