Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

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

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 vapors^1,2,3,4,5. 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.....

Protocol

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 processing and data analysis, a pair of electromagnets for the MOT and bias magnetic fields, and an ultrahigh vacuum chamber including a ⁶Li va....

Results

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/T_F ≈ 1.2. Here, T_F = (6N)^1/3ħω/k_B <.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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 1,064 nm CW laser	IPG photonics	YLR-100-1064-LP	Quantity: 1
1,064 nm AOM	IntraAction	ATM-804DA6B	Quantity: 1
1,064 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
1,064 nm plano-concave lens			Quantity: 1 for beam reducer
1,064 nm plano-convex lens			Quantity: 1 for beam reducer
1,064 nm Mirrors			Quantity: depends on apparatus design
1,064 nm Half-wave Plates			Quantity: depends on apparatus design
1,064 nm Lenses			Quantity: depends on apparatus design
1,064 nm Thin Film Polarizer			Quantity: 1
100 W, 1,064 nm Beam Dump			Quantity: 1
100 W, 1,064 nm Power Meter			Quantity: 1
RF Function Generator	Rigol	DG4162	Quantity: 1
RF Power Amplifier	Mini-Circuits	ZHL-100W-GAN+	Quantity: 1