Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

The overall goal of this procedure is to cool an optically trapped ultracold Fermi gas to the microKelvin regime by periodically modulating the intensity of the trapping laser beam. This cooling method can address the key questions in ultracold atom research such as generating lower temperature quantum gas for simulating nova phenomena in nature. The main advantage of a parametric cooling is that it can cool ultracold Fermi optically, some six atoms with very high cooling efficiency.

This complicated procedure will be implemented by our post-doc Jiaming Li.The apparatus for this experiment is already in place. There are several key components starting with the oven. The oven will output lithium-6 at the required flux.

After leaving the oven, the lithium travels through a Zeeman slower and crossover coils. From there, the atoms reach the experiment chamber. The chamber is between coils that have either a Helmholtz or anti-Helmholtz magnetic field.

A photo diode monitors the fluorescent signal through the chamber window. There is also a CCD camera to image the atom clouds through an infrared cutoff filter during the cooling and pumping phase. Surrounding the chamber are lasers and optical elements for magneto-optical and optical dipole traps to cool the atoms.

Much of the experiment is computer controlled by a timing file entered into the control software. The atoms will first be cooled and trapped with a Magneto-optical Trap or MOT. The trap laser is a 671 nanometer single-frequency external cavity diode laser with 500 milliwatts of output power.

An overview of is associated optical elements is provided in this schematic. The slowing beam is in purple. The beams for the Magneto-optical Trap are labeled.

A second 671 nanometer external cavity diode laser at 35 milliwatts is used for absorption imaging. The MOT laser frequency is stabilized with a digital laser current modulation method and the imaging laser is frequency locked to the MOT laser with an offset locking method. Next, prepare the laser locking acousto-optic modulator in the atomic vapor cell.

Then turn on the frequency lock controller and open its software. Turn on the laser grading scanning. To start the external cavity diode laser modulation, click the on button under modulation.

Now move to the laser to turn on the external cavity laser emission. Make slight adjustments of the current to the laser to tune the frequency. The goal is to observe the lock-in error signal of the lithium-6 D2 line.

Once the signal is seen, use the software to set the lock point at the desired transition. Lock the laser frequency to this transition. Then adjust the lock point to the center of the transition.

Before proceeding, prepare the imaging laser according to the offset lock method. For absorption imaging, modify the output of the imaging laser by sending its beam through elements arranged according to this schematic. Note the acousto-optic modulator connected to an arbitrary pulse generator.

At the computer set the imaging pulse duration and the imaging time separation. When everything is ready for trapping, turn on the oven heaters and wait for the oven temperature to enter the operational region. On entering the operational region, turn on the cooling fans for the Zeeman slower.

Then slowly increase the current of the slower to 9.2 amps. Next, turn on the current of the two crossover coils to seven amps and one amp. Continue by manually opening the shutter to unblock the Zeeman slower laser beam.

Then make sure the magnetic field coils are being cooled before turning on their power supply. At the computer, set the currents to produce a field gradient of about 22 gauss per centimeter. The next cooling steps take place in the dynamic MOT followed by an optical dipole trap as depicted in this schematic.

It is this trap that is responsible for generating ultracold Fermi gases. For the trap, an arbitrary function generator drives a Fiber Laser. The beam travels through a driven acousto-optic modulator.

It then travels through a polarizer and crosses the vacuum chamber twice. After the static MOT is established, the control program steps the experiment through several phases over time. First, the dynamic MOT phase begins to cool the atoms.

As the MOT loading proceeds, the atoms in the experiment chamber fluoresce and the fluorescence is visible at the chamber window and monitored by the photo diode. The image is from the end of a test of the dynamic MOT phase when the temperature is about 300 microKelvin. Immediately after the MOT phase, have the timing control software change the magnets to a Helmholtz field configuration and increase the bias field from zero to 330 gauss.

Then use the software to prepare a 50 to 50 mixture of the two lowest lithium-6 hyperfine states and tune the locked laser to resonance with the atoms at 527.3 gauss. During setup for experimental control, program the fiber laser function generator to create a laser pulse that the timing software will start 14 milliseconds before the end of the MOT phase. The pulse increases the trap depth to its maximum value denoted by U not, then back to one-tenth maximum.

The laser cooling stage takes 0.5 seconds. This absorption image of the trap is from a test of the system through the end of the laser cooling phase. In the timing program, wait for 30 milliseconds before starting the second stage of evaporative cooling.

For the second stage, start with acousto-optic modulator at 80%of the saturated Rf power and program it to lower the trap depth from one-tenth maximum to one-hundredth of its maximum. In this experiment, the curve followed by the acousto-optic modulator voltage is an exponential and the trap depth reaches its lowest value after 1.5 seconds. During this phase, the magnetic field remains at 330 gauss.

This absorption image of the cold atoms, is from just after the completion of evaporative cooling. And the start of the parametric phase, have the timing program increase the magnetic field to 527.3 gauss. After a pause of 100 milliseconds, use the acousto-optic modulator to modulate the trap depth sinusoidally.

Complete the program by having the arbitrary pulse generator abruptly turn off the trapping beams. Allow the particles to ballistically expand for 300 microseconds before performing absorption imaging. These are time of flight absorption images of lithium atoms prepared in the two lowest hyperfine states at different modulation times.

In all images, the modulation frequency and amplitude are the same optimized values found in previous studies. The atomic clouds show a significant decrease in the axial cloud size as the modulation time increases. This indicates the absolute temperature is continually reduced by parametric cooling.

In this plot, the blue data circles are for the atomic cloud energy along the axial direction in units of the Fermi energy. The red squares are for the atomic cloud energy along the radial direction, also in units of the Fermi energy. Note the scales are different.

The difference in energy between the two energies initially is due to the fast-trap lowering for evaporative cooling. After parametric cooling, the axial energy is reduced significantly with time, eventually going below the Fermi energy. In contrast, the radial energy increases slightly.

The anisotropy is due to the dominant anharmonicity of the crossed-beam optical dipole trap being along the axial direction. Here is how the number of atoms in the cloud evolves as a function of time. The plot demonstrates that atoms are expelled from the trap during parametric cooling.

The moderation frequency and the amplitude are pretty important rule in this parametric cooling. The optimal values of them are determined before this experiment. The cooling method mainly depend on the harmonicity of the trap potential so we can engineer it by designs of optical dipole trap.

The method can provide insight into dissipation processes in ultracold atomic gases and be applied to generate an isotropic cold atom samples for studying thermalization processes in a many-body quantum system. Why implement this procedure? It is a key to choose the right modulation frequency according to anharmonicity of our optical trap.

We expect that it is possible to expand the current method to other species of ultracold atoms, such as potassium-40. After this video, you should have a good understand of how to use this parametric cooling to cool down an ultracold atomics. Working with lasers can be extremely hazardous and precautions such as wearing laser safety goggles should always be taken while performing this procedure.