Atom Optics, Coherence and Ultra Cold Atoms

Laser Cooling and Trapping of Neutral Atoms

The control of atomic motion by laser light is a field which has expanded very rapidly over the last few years. The possibility of reaching extremely low atomic kinetic-energy temperature, in the microKelvin range, is the most spectacular achievement.

We have been working on cooling and trapping experiments of metastable neon atoms. We have succeeded in observing a 2 dimensional pattern of double slit interference with ultracold neon atoms. The matter wave interferometer has extremely high sensitivity compared to conventional light interferometers, and is one of the promising applications of cold atoms. This is a joint research project with Prof. Fujio Shimizu at the University of Tokyo.

The Principles of Laser Cooling

An atom with mass m, velocity v and resonant frequency n0 is illuminated by a counter-propagaining laser beam with frequency nL. When the laser frequency satisfies the condition n0=nL(1+ v/c), the atom absorbs a photon. Its momentum changes the atomic velocity. After many cycles of absorption and emission, the velocity of the atom becomes nearly zero. The direction of the spontaneous emission is symmetrically distributed, so that there is no net contribution from spontaneous emission. In each cycle the atom loses energy hnL( v/c ) which corresponds to the Doppler shift. The kinetic energy of the atom can be lower than 1 mK.

Magneto-Optical Trap

The pair of coils of opposing current generates a spherical quadrupole field. Laser beams counter-propagating along x,y, and z axis cross each other at the center of the quadrupole field. The laser light is circularly polarized. If the frequency is tuned below resonant frequency, atoms feel a net time-averaged force towards the center.

Atom Interferometer

An atom has wave properties as well as particle properties. The wavelength of an atomic wave is given by l=h/mv (m: mass of atom, v: velocity, h: Planck's constant), and for laser cooled atoms the wavelength can be as long as several tens of nanometer. The interference pattern can be observed with a slit of 1 to 2 um width.



Atom Optics


Very slow atoms can be deflected by a very weak force. The second order Stark effect due to the applied electric field is sufficient to change the trajectory of atoms. Atoms were focused one dimensionally by a static electric field and two dimensionally by an alternating electric field. A cylindrical microwave cavity was used as a 2-D lens for ultracold atoms.

The electric field of TM01 mode of cylindrical cavity is axially symmetric and the intensity is maximum at the center. The force towards the center acts on atoms passing through the cavity. This work is supported by Grant-in-Aid for Developmental Scientific Research on atom manipulation from The Ministry of Education, Science, Sports and Culture of Japan.

Lithium Trap

One of the goals of laser cooling research is the observation of a quantum effect which is expected to occur when the de-Broglie wavelength is nearly the same as the average distance between the trapped atoms. Lithium is a candidate to observe this effect.

Our group has succeeded in trapping 6Li and 7Li, and measured quadratic collisional loss rate of 7Li in the magneto-optical trap. The maximum density we could obtain was 1011 1/cm3 with a temperature of about 200 ľK. We are trying to get higher density.