Introduction: Laser Cooling and Trapping

In 1873, James Clerk Maxwell postulated a force on a solid body due to the absorption or reflection of an electromagnetic wave. Einstein continued this idea in 1917 when he theorized that an electromagnetic radiation field influenced the motion of molecules.¹ In 1933, Frisch provided the first experimental evidence of a momentum transfer between light and atoms.² The use of this momentum transfer to accelerate or decelerate particles was first proposed by Ashkin in 1970.³ Yet, it was not until tunable lasers were developed, that Wineland et al.⁴ and Neuhauser et al.⁵ demonstrated laser cooling of trapped ions. Efforts to extend this work to neutral atoms were hampered, though, by Ashkin's proof that a pure scattering force could not be used to trap atoms (Optical Earnshaw Theorem).⁶

In 1985 several groups succeeded in slowing beams of neutral atoms^7,8 and in 1986 Chu et al. created an optical molasses to cool atoms in three dimensions.⁹ When Pritchard et al.¹⁰ postulated a method for circumventing the OET the experimental realization soon followed. In 1987, Raab et al. succeeded in trapping neutral atoms using radiation pressure.¹¹ The field of laser cooling and trapping has produced many dramatic results over the past ten years reaching an apex with the creation of a Bose-Einstein condensate in 1995 by a group led by Carl Wieman and Eric Cornell.¹²

Cold atoms produced using laser cooling and trapping techniques have many applications for basic science and industry: high-resolution spectroscopy, studies of collisional interactions between atoms,¹³ precision measurements of atomic electric dipole moments and atomic parity violation,¹⁴ more precise atomic clocks,¹⁵ and new wavelength standards for optical communications.¹⁶

The field of laser cooling and trapping, however, has not been limited to graduate research institutions. In the undergraduate laboratory, we have used Doppler cooling and a magneto-optic trap (MOT) to create a cold, dense cloud of cesium atoms. Filling, density, and temperature of a MOT have been determined. In conjunction with this experiment, Windows-based software has been developed to simulate cooling and trapping. The program graphically depicts the position and velocity of a variable number of atoms being cooled and trapped. The algorithm accounts for absorption, spontaneous and stimulated emission, Doppler and Zeeman shifts, and radiation trapping. The user has the ability to adjust time step, atom velocity, laser intensity and detuning, and magnetic field gradient. The program can demonstrate optical molasses, a MOT, and a dark MOT. Simulations can be recorded and played back in the same graphical environment, along with movies from the actual experiment. The simulation is a useful tool for explaining and investigating Doppler cooling of atoms and the magneto-optic trap. Such a program will allow undergraduates without access to a cooling and trapping apparatus to obtain computational results and compare them with published results.

This text provides a brief overview of the theoretical, experimental, and computational aspects of laser cooling and trapping. The work and results reported in this text represent a two semester study of the field and is intended to be a learning tool for undergraduate physics majors interested in atomic physics. Special thanks goes to Dr. Robert A. Cline who served as the undergraduate thesis advisor throughout the project. Further thanks goes to undergraduates Andrew J. Borleske, Clay Lenhart, and Mike Lee for their previous work with the apparatus. Finally, special thanks goes to my father for Beta testing Cool, and to my mother who had to listen to most of our conversations.