We report the observation of synthesized spin-orbit coupling (SOC) for ultracold spin-1 87Rb atoms. Different from earlier experiments where a one dimensional (1D) atomic SOC of pseudo-spin-1/2 is synthesized with Raman laser fields, the scheme we demonstrate employs a gradient magnetic field (GMF) and ground-state atoms, thus is immune to atomic spontaneous emission. The strength of SOC we realize can be tuned by changing the modulation amplitude of the GMF, and the effect of the SOC is confirmed through the studies of: 1) the collective dipole oscillation of an atomic condensate in a harmonic trap after the synthesized SOC is abruptly turned on; and 2) the minimum energy state at a finite adiabatically adjusted momentum when SOC strength is slowly ramped up. The condensate coherence is found to remain very good after driven by modulating GMFs. Our scheme presents an alternative means for studying interacting many-body systems with synthesized SOC.
We report the first experimental observations of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the pump-laser detuning, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned light, the familiar distinctive multi-order, matter-wave scattering pattern is clearly visible, whereas with bluedetuned light superradiance is strongly suppressed. In the limit of a dilute atomic gas, however, symmetry is restored. [7][8][9][10][11][12][13][14][15][16] have studied this light/matter-wave interaction process that is of significant importance to the fields of cold atomic physics, cold molecular physics, nonlinear optics and quantum information science.The widely-accepted theory [1] of matter-wave superradiance is based on spontaneous Rayleigh scattering and the buildup of a matter-wave grating enhanced by subsequent stimulated Rayleigh scattering. This intuitive picture, which correctly models late-stage superradiant growth when red-detuned light is used, captures many important aspects of this intriguing matter-light interaction process. However, the simple grating viewpoint and most rate-equation-based theories neglect propagation dynamics of the internally-generated optical field. In fact, the initial study [1] explicitly assumed that the optical fields traveled at the speed of light in vacuum and therefore did not affect scattering at later times. To date, no report in the literature has contradicted that statement [17]. However, we have recently shown theoretically [18,19] that the internally-generated field propagates ultra slowly and plays an important role in the genesis of superradiance with BECs. We also note that most previous theories effectively treated the BEC as a thermal gas by neglecting the extra factor of the mean-field potential seen by the scattered atoms due to the exchange term in the Hamiltonian. As we will show, this unique property of BECs profoundly impacts superradiant scattering and leads to the pump detuning asymmetry reported here.In this Letter we present the first experimental ob- servation of a red-blue detuning asymmetry in matterwave superradiance. We demonstrate astonishingly efficient suppression of superradiance when the pump laser is blue detuned that cannot be explained by current theoretical frameworks. However, using our new theoretical framework [18] we propose a possible explanation for the detuning asymmetry based on an induced optical-dipole potential that results from the ultra-slow propagation velocity and gain characteristics of the generated field. The experimental data reported here were obtained using two 87 Rb BECs created with very different experi-
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