Research

Ultracold atoms at Willamette University:

   We are interested in studying the properties of atoms that have been slowed to a crawl by using a clever combination of laser light and magnetic fields, the magneto-optical trap. Instead of moving with several hundred or thousand meters per second, which is the typical speed of an atom or molecule at room temperature, these ultracold atoms move with only a few centimeters per second. This allows us to look at them more closely and study their properties.

   We are working with rubidium, an alkali-metal atom, and calcium, an alkaline-earth atom. Both species have been cooled and trapped individually, but the combination of both atomic species has not yet been studied. We are particularly interested int he study of collisions between these two different atomic species at ultracold temperatures with the ultimate goal of creating ultracold molecules consisting of these two atoms. 

Student Involvement:

   Students of all levels, from Freshmen to Seniors, get the opportunity to be involved in every aspect of this state-of-the-art research project. They get exposed to lasers, atom-light interaction, ultra-high vacuum systems, electronics and computer control of a complex experiment. Individual projects vary depending on student interest and background. If you are interested in joining our research group please send an email to Prof. Kleinert.

The workhorse: The Magneto-Optical Trap (MOT):

   The invention of the magneto-optical trap in 1987 [1], for which the Physics Noble Prize was awarded 10 years later [2], has been one of the major achievements in the field of atomic physics as it reduces the temperature of gaseous atoms from several hundred Kelvin to just a few 10-100 micro-Kelvin. This has lead to an explosion of high-precision spectroscopic data, the formation of Bose-Einstein Condensates and Fermi-degenerate gases, the creation of the atom laser and the formation of ultracold homo- and heteronuclear molecules, to name just a few. While initially only alkali-metal atoms were trapped due to their relatively simple level structure - they only contain a single electron in the valence shell -, by now several alkaline-earth, rare-earth metals and metastable gases have also been trapped. An up to date list of groups working with ultracold atoms and molecules can be found here.

   So how does this trap work? For now let us assume a two-level atom, an atom that has only a ground state (this is the state in which the electron spends most of its time) and one excited state. If a photon of just the right amount of energy hits the atom it can be absorbed as shown below:

  Note that while the absorption will always happen from the same direction (from the laser beam), the emission can happen in any direction (spontaneous emission). Thus, each time the atom absorbs a photon it feels a "kick" opposing the direction of propagation of the laser beam, and each time it emits a photon it feels a kick in a random direction. Over many absorption-emission cycles, the atom will gain a net momentum opposing the direction of propagation of the laser beam.
   The MOT uses a combination of 3 pairs of counter-propagating laser beams, all slightly red-detuned from the atomic transition as shown below.

If an atom is moving toward or away from a laser beam, the frequency that it "sees" is up- or down-shifted, respectively, due to the Doppler effect. Thus, red-detuned laser beams will be shifted into resonance for atoms moving toward them. Those atoms will thus preferentially absorb photons from the laser beam opposing its direction of motion and will get net momentum kicks that slow it down and push it back toward the center. This can effectively cool atoms down to several hundred micro-Kelvin and confines them to a volume that is given by the overlap volume of the laser beams.

   To confine atoms even more and increase the density of the ultracold ensemble, a magnetic field is added. If the magnetic field increases linearly with increasing distance from the center and vanishes at the center (something that can be created with an Anti-Helmholtz pair of coils), then the atomic energy levels mimic this changing magnetic field (Zeeman effect) as shown in the figure below:

[1] E. L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Pritchard, Trapping of neutral sodium atoms with radiation pressure, Phys. Rev. Lett. 59, 2631 (1987)

[2] S. Chu, The manipulation of neutral particles, Rev. Mod. Phys. 70, 685 (1997); C. Cohen-Tannoudji, Manipulating atoms with photons, Rev. Mod. Phys. 70, 707 (1997); W. D. Phillips, Laser cooling and trapping of neutral atoms, Rev. Mod. Phys. 70, 721 (1997)