Name: cooling an optically trapped ultracold fermi gas by periodical driving
Uploaded: 2017-03-30T14:00:00
Description: Watch this Scientific Journal Video about Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving at JoVE.com

We thank Ji Liu and Wen Xu for involving in the experimental setup. Le Luo is a member of the Indiana University Center for Spacetime Symmetries (IUCSS). This work was supported by IUPUI and IUCRG.

NOTE: This protocol requires a home-built ultracold atom apparatus including the following equipment: two external cavity diode lasers (ECDL), a locking setup for the ECDL offset frequency locking10, a fiber laser for the ODT, an AOM for laser intensity modulation, an radio frequency (rf) antenna system with a source generator and a power amplifier, an absorption imaging system with a CCD camera, a computer program for timing sequence and data acquisition (DAQ), a computer program for imaging proc...

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We present an experimental protocol for parametric cooling of a noninteracting Fermi gas in a crossed-beam optical trap. The critical steps of this protocol include: First, the optically-trapped Fermi gas needs to be cooled close to the degenerate temperature by lowering the trap depth. Second, a modulation frequency is chosen that is resonant with the anharmonic component of the trapping potential. Third, the intensity of the trapping beam is modulated to cool the atomic cloud and measure the dependence of the cloud ene...

In the past two decades, various cooling techniques have been developed for generating Bose-Einstein condensates (BEC) and degenerate Fermi gases (DFG) from hot atomic vapors1,2,3,4,5. BEC and DFG are novel phases of matter that exist in extremely low temperatures, usually one millionth of a degree above absolute zero temperature, far below those normally found on Earth or in space. To obtain such low temperatures, most cooling methods rely on lowering the trapping potential to evaporatively cool the atoms. However, the lowering scheme also decreases the collision rate of the atoms, which limits the cooling efficiency when the gas reaches the quantum regime6. In this article, we present an &#34;expelling&#34; method to evaporatively cool an ultracold Fermi gas in an ODT without lowering the trap depth. This method is based on our recent study of parametric cooling7, showing several advantages compared to the lowering schemes7,8,9.
The key idea of the parametric scheme is to employ the anharmonicity of the crossed-beam ODT, which makes the hotter atoms near the edge of the trapping potential feel the lower trapping frequencies than the colder atoms in the center. This anharmonicity allows the hotter atoms to be selectively expelled from the trap when modulating the trapping potential at frequencies resonant with the high-energy atoms.
The experimental protocol of parametric cooling requires a pre-cooled noninteracting Fermi gas near the degenerate temperature. To implement this protocol, an acousto-optic modulator (AOM) is used to modulate the intensity of the trapping beams by controlling the modulation frequency, depth and time. To verify the cooling effect, the atomic cloud is probed by absorption imaging of time-of-flight (TOF), where a resonant laser beam illuminates the atomic cloud and the absorption shadow is captured by a charge coupled device (CCD) camera. The cloud properties, such as the atom number, energy, and temperature, are determined by the column density. To characterize the cooling effect, we measure the dependence of the cloud energies on the various modulation times.

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cooling an optically trapped ultracold fermi gas by periodical driving

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.

Using this protocol, we study the dependence of the parametric cooling on the modulation time with the optimized modulation frequency and amplitude, both of which have been determined in our previous publication7. We first prepare a noninteracting Fermi gas of 6Li atoms in the two lowest hyperfine states with a temperature of T/TF &#8776; 1.2. Here, TF = (6N)1/3&#295;&#969;/<em...

Watch this Scientific Journal Video about Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving at JoVE.com

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

We present a parametric driving method to cool an ultracold Fermi gas in a crossed-beam optical dipole trap. This method selectively removes high-energy atoms from the trap by periodically modulating the trap depth with frequencies that are resonant with the anharmonic components of the trapping potential.

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 ...

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