Li Jiaming, de Melo Leonardo F, Luo Le
Department of Physics, Indiana University-Purdue University Indianapolis (IUPUI).
Department of Physics, Indiana University-Purdue University Indianapolis (IUPUI);
J Vis Exp. 2017 Mar 30(121):55409. doi: 10.3791/55409.
We 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.
我们提出了一种通过在交叉光束光学偶极阱（ODT）中参数驱动原子运动来冷却冷费米气体的方法。我们的方法利用了ODT的非谐性，其中阱边缘较热的原子感受到捕获势的非谐分量，而阱中心较冷的原子感受到谐性分量。通过用与非谐分量共振的频率调制阱深度，我们选择性地将较热的原子激发到阱外，同时将较冷的原子保留在阱内，从而产生参数冷却。该实验方案始于由塞曼减速器加载的磁光阱（MOT）。然后将MOT中预冷的原子转移到ODT中，并施加偏置磁场以创建相互作用的费米气体。接着我们降低捕获势，以在简并温度附近制备冷费米气体。之后，我们将磁场扫描到费米气体的非相互作用区域，在该区域中可以通过调制光学捕获光束的强度来体现参数冷却。我们发现参数冷却效应强烈依赖于调制频率和幅度。通过优化频率和幅度，我们测量了云能量对调制时间的依赖性。我们观察到云能量以各向异性的方式变化，其中轴向能量通过参数驱动显著降低。冷却效应仅限于轴向，因为交叉光束ODT的主要非谐性沿轴向。最后，我们建议将该方案扩展到所有方向具有大非谐性的捕获势，这为使用外部驱动冷却量子气体提供了一个有前景的方案。
