Baby BEC

Overview

Our goal is to develop a portable BEC atom-chip vacuum cell system for the practical application of ultra-cold atom physics. These BEC systems have to be miniaturized such that they are easy to use by engineers and non-experts. The miniaturization is accomplished via a simple, compact vacuum system, a batch fabricated atom-chip, and optical fiber inputs for the cooling and imaging laser beams.

The whole vacuum system compact (30cm x 30cm x 15cm), and the ultra-high vacuum (UHV) in the cell chamber is matained by a small, 8 L/s, ion-pump and non-evaporable getter. A lithographically patterned copper on an aluminum-nitride chip is used to seal the vacuum system, provide the electrical feedthroughs, and create the magnetic trap potentials. These components allow for a scalable, interchangeable system that can be adapted to a variety of applications and is capable of futher miniaturization as smaller components are developed.

In the future, such a system will be the basis for devices such as an atomic quantum information processing unit, integrated atom interferometers for inertial navigation and other sensing applications.

The picture at right (BEC Time-of-Flight) is the first BEC we created in December 2003.

This past month, an article appeared in the online edition of EETimes magazine. Read it here: "Hot research harnesses cold atoms".

Technical Synopsis

The following summarizes the steps to achieve BEC:

1. Ultraviolet Light Induced Atomic Desorption (UV LIAD). As we know, a traditional BEC system takes double chamber design, where a MOT is loaded from a high atomic vapor pressure chamber and then it is transferred to a UHV vauum chamber which is required by RF evaporation. In our single chamber system, we are using UV LIAD technology to modify cell pressure temporarily instead of spatially. By sending an UV light pulse, the atoms chemically adsorbed on the cell inner wall are kicked away by LIAD effect which increases the pressure typically a factor of 100. After a mirror MOT is loaded, the UV is switched off and the pressure is recovered back very quickly (about 5 seconds) to its original UHV level for keeping atoms long enough in a Z wire chip trap to achieve RF forced evaporation.
2. Mirror MOT. As shown in the Mirror MOT configuration, only 4 laser beams are required to create a surface MOT due to the reflections from the mirror on the chip. The Mirror MOT is loaded during 3 second UV pulse. After UV is switched off, the Mirror MOT is hold for 5 seconds by keeping lasers and magnetic fields on to let the UHV vacuum recovered. In this way, we typically load 6 to 10 million atoms with a temperature 200 to 300 micro K.
3. Chip U MOT. After atoms are loaded into the above Mirror MOT, the quadruple magnetic field generated from the external coils as shown in part 2 is replaced by the magnetic field created by the current Iu through the U wire and a y-directional bias field. By adjusting the bias field, cooling laser frequency and repumping power, the chip U MOT is moved closer to the surface and experience a Compression. At the end of this CMOT stage, atoms are brought at about 500 um below the surface with a temperature of 100 uK. The magnetic trap has a lifetime about 4 to 5 seconds.
4. PGC (Polarization Gradient Cooling) and OP (Optical Pumping). After 1.7 ms PGC, atoms are fatherly cooled down below 30 micro K, then atoms are pumped into a single spin state |F=2, M=2> by 100 micro s optical pumping.
5. Z wire (and U wire) Chip micro trap. After optical pumping, About 2-3 million atoms are loaded into the Z-trap by switching Iz = 4 A and By=14 Gauss. The loading efficiency is about 30 to 40%. Exactly, there are 2 type of wire trap: U wire and Z wire. As discussed in part 3, U-trap is a quadruple trap which has a zero magnetic field point which causes spin flip loss. Z-trap has only a non zero minimum so that it is suitable for RF evaporation. Both type of traps are shown in the bellow, but only Z-trap is used to achieve BEC on chip.
6. Compression. After Z-trap initial loading, Iz quickly reduces from 4 A to 2.75 A, and y bias field increases from 14 Gauss to 60 Gauss in a very short amount of time (200 ms). As shown in the left figure, the red curvature is the initial trap potential, and the black one is the final trap. The compression not only increase collision rate dramatically but also reduce current though Z wire to protect the chip. After compression, atoms have a temperature about 300 micro K, but a higher collision rate >100Hz which make RF run-away forced evaporation possible within 4 to 5 second magnetic trap lifetime.
7. RF Forced evaporation and BEC! BEC is achieved after only 4 second RF evaporation. Atom number of a pure BEC is about 2,000, and the transition temperature is about 200 nK.

MEMS MOT

Students:

• Brian McCarthy

Solder self-assembly can be used to expand the versatility of a commercial foundry, like MEMSCAP's PolyMUMPs process. These foundries are attractive for prototyping MEMS as they can offer consistent, low cost fabrication runs by sticking to a single process and integrating multiple customers on each wafer. However, this standardization limits the utility of the process for a given application.

Solder self-assembly, gives back some of this versatility and expands the envelope of surface micromachining capability in the form a simple post-process step. Here it is used to create novel micromirrors and micromirror arrays as well as to delve into the field of ultracold atom optics where the utility of MEMS as an enabling technology for atom control is explored. Two types of torsional, electrostatic micromirrors are demonstrated, both of which can achieve ±10° of rotation.

The first is a novel out-of-plane micromirror that can be rotated to a desired angle from the substrate. This integrated, on-chip assembly allows much simple packaging technology to be used for devices that require a laser beam to be steered off-chip. Planar micromirror arrays that use solder self-assembly to tailor the electrode gap height are also demonstrated. With these design, no special fabrication techniques are required to achieve large gap heights and micromirrors with a variety of gap heights can even be fabricated on the same chip. Finally, solder self-assembly is used to explore how complex microscale structures can be used to control ultracold atoms. For this study, a MEMS version of a magneto-optical trap, the basis for most ultracold atomic systems, is used to control Rb atoms. In doing so, it provides a path for the successful integration of a number of MEMS devices in these types of systems.