通过定期行驶冷却光学被困超冷费米气体
Summary
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.
Abstract
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.
Introduction
在过去的二十年中，各种冷却技术已经被开发用于产生玻色-爱因斯坦凝聚（BEC）和从热原子蒸气1，2，3，4，5简并费米气体（DFG）。 BEC和DFG是存在于非常低的温度下物质的新颖阶段，通常是比绝对零度的温度的百万分之一，远远低于那些通常在地球上或在空间中。为了获得这样低的温度下，最冷却方法依赖于降低捕获电位蒸发冷却的原子。然而，该方案降低也降低了原子的碰撞率，当气体到达量子政权6这限制了冷却效率。在本文中，我们提出了一种“驱逐”方法来蒸发冷却超冷费米气体中的ODT而不降低陷阱深度。此方法是基于我们最近参数冷却7，示出相比于降低方案7，8，9几个优点的研究。
参数方案的关键思想是采用的交叉光束ODT，这使得邻近所述俘获电位的边缘的较热原子感觉下俘获的频率比在中心较冷原子的非谐。此非谐性允许调节在频率谐振与高能量原子俘获电位时被选择性地从阱排出的热原子。
参数冷却的实验方案需要接近退化温度预冷却的互不影响的费米气体。为了实现该协议，声光调制器（AOM）来调制由controllin捕获光束的强度克的调制频率，深度和时间。要验证的冷却效果，原子云是由时间 – 飞行时间（TOF）的吸收成像，其中谐振激光束照射原子云和吸收阴影由电荷耦合器件（CCD）照相机捕获探测。云性质，如原子数，能量和温度，由列密度确定。为了表征的冷却效果，我们测量云能量上的各种调制倍的依赖性。
Protocol
Representative Results
Discussion
我们提出的用于交叉光束光学阱的非相互作用费米气体的冷却参数的实验协议。此协议的关键步骤包括：首先，将光学捕获费米气体需要通过降低陷阱深度接近冷却至退化温度。第二，调制频率选择即谐振与俘获电位的非调谐组件。第三，俘获光束的强度被调制，以冷却原子云并测量云能量对调制时间的依赖性。
与收集降低方案相比，所述参数冷却方案提供了一种选择性的?…
Disclosures
The authors have nothing to disclose.
Acknowledgements
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.
Materials
| 500 mW 671 nm ECDL | Toptica | TA Pro | Quantity:1 |
| 35 mW 671 nm ECDL | Toptica | DL-100 | Quantity:1 |
| 671 nm AOM | Isomet | 1206C | Quantity:3 |
| 671 nm AOM Driver | Isomet | 630C-110 | Quantity:3 |
| 100 W 1064 nm CW laser | IPG photonics | YLR-100-1064-LP | Quantity:1 |
| 1064 nm AOM | IntraAction | ATM-804DA6B | Quantity:1 |
| 1064 nm AOM Driver | IntraAction | ME-805EH | Quantity:1 |
| Arbitrary Function Generator | Agilent | 33120A | Quantity:3 |
| Digital I/O Board | United Electronic Industries | PD2-DIO-128 | Quantity:1 |
| System Design Platform | National Instruments | LabVIEW | Quantity:1 |
| Analog Voltage Output Device | Measurement Computing | USB-3104 | Quantity:1 |
| CCD Camera | Hamamatsu | Orca R2 | Quantity:1 |
| Arbitrary Pulse Generator | Quantum Composer | 9618+ | Quantity:1 |
| Analog Voltage Output Device | Measurement Computing | USB-3104 | Quantity:1 |
| 20 A power supply | Quantity:1 | ||
| 10 A power supply | Quantity:1 | ||
| 120 A power supply | Quantity:2 | ||
| Cooling Fans | Quantity: depends on apparatus design | ||
| 671 nm Mirrors | Quantity: depends on apparatus design | ||
| 671 nm Half-wave Plate | Quantity: depends on apparatus design | ||
| 671 nm Quarter-wave Plate | Quantity: depends on apparatus design | ||
| 500 mW Beam Shutter | Quantity: depends on apparatus design | ||
| 671 nm Lenses | Quantity: depends on apparatus design | ||
| Faraday Isolator | Quantity: 2, one for each ECDL | ||
| 671 nm Polarizing Beam Splitter | Quantity: depends on apparatus design | ||
| Photodetector | Thorlabs | SM05PD1A | Quantity:1 |
| Multiplexer | Analog Devices | ADG409 | Quantity: 1 |
| Multiplexer | Analog Devices | ADG408 | Quantity: 2 |
| 1064 nm plano-concave lens | Quantity:1 for beam reducer | ||
| 1064 nm plano-convex lens | Quantity:1 for beam reducer | ||
| 1064 nm Mirrors | Quantity: depends on apparatus design | ||
| 1064 nm Half-wave Plates | Quantity: depends on apparatus design | ||
| 1064 nm Lenses | Quantity: depends on apparatus design | ||
| 1064 nm Thin Film Polarizer | Quantity:1 | ||
| 100 W, 1064 nm Beam Dump | Quantity:1 | ||
| 100 W, 1064 nm Power Meter | Quantity:1 | ||
| RF Function Generator | Rigol | DG4162 | Quantity:1 |
| RF Power Amplifier | Mini-Circuits | ZHL-100W-GAN+ | Quantity:1 |
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