A Portable Magneto-Optical Trap

In the fall of 1996 Thad Walker and Ian Nelson gave a physics colloquium at the University of Wisconsin-Madison during which they demonstrated laser cooling and trapping of atoms in front of a live audience.  The demonstration was very enthusiastically received (including a lengthy parody in that year's Christmas colloquium).  It was clear that live trapping and cooling of atoms would be a real hit with audiences.  But there was a major limitation to the apparatus .  The setup required a roughly 4 ft X 6 ft optical table, with dozens of mirrors, lenses, and waveplates to be aligned to produce the six trapping laser beams required for the demonstration.  This meant the apparatus had to be preassembled on a movable table and then wheeled into the lecture hall ~30 minutes before the lecture.  Obviously, the mass and complexity were still too great for the apparatus to be portable.

Three key technological developments were needed to make a portable trap a reality.  The first, inexpensive and lightweight microwave modulation of diode lasers, had been developed by Paul Feng and was already in routine use in our research laboratory.

Subsequently, building on some ideas of Won Ho-Je in Korea, Rob Williamson and Paul Voytas developed a pyramidal trap that only requires one laser beam, with a resulting drastic simplification in the number and complexity of the optical elements.

Finally, Carl Wieman's group invented the DAVLL (Dichroic-Atomic-Vapor Laser Lock) method for simplified locking of a laser to an atomic resonance line (Applied Optics.vol.37, no.15; 20 May 1998; p.3295-8).

Exploiting these new technologies, in the fall of 1998 Ray Newell, a Physics graduate student, designed and built a portable magneto-optical trap.  It is modular so that it can be broken down to fit inside a portable equipment case.  It has been demonstrated to work, but its portability and reliability have yet to be tested.  The total cost was about $14K, and the weight is less than 50 pounds.

The system can be broken down into three main components:

An optical rail holding the laser, beam shaping optics and the spectroscopy system



A vacuum chamber containing a pyramidal mirror assembly, a Rubidium source and a small ion pump



A circuit box incorporating all the necessary driving and control electronics.






For more details, download Ray Newell's Master's Thesis.