JumpStart - Physical Science

Laser Cooling and Atomic Physics*

Atomic physics is the study of the structure of isolated atoms and their interactions with external stimuli, which include other atoms, surfaces, electromagnetic fields, temperature, pressure, and light. Measurements of these atoms can be remarkably precise if they are well-isolated from external environmental influences, such as collisions with atmospheric gases or with the walls of a container. A common method of isolation is to release the atoms into a high-vacuum chamber. However, the isolation achieved in this method is not perfect. Atoms at room temperature move with a great velocity, which causes them to collide with the chamber walls in a rather short time. This strongly limits the precision of any measurements that can be obtained.

At the atomic level, phenomena in the physical world usually happen at very short length scales and on very fast time scales, making these phenomena difficult to observe. A major difficulty has been overcoming the high-speed movement exhibited by an atom at room temperature. Scientists have searched for methods to slow and trap atoms to make them observable over longer periods.

Laser cooling technology has provided new methods for extending observation times for experiments in atomic physics. Laser cooling uses lasers to slow individual atoms by bombarding them with light of a certain frequency that will exchange momentum with the atoms. This slowing of clouds of atoms allows scientists more time to observe the atoms' behavior. However, even laser cooling has its limits; when very slow atoms are released into the chamber, they quickly accelerate due to the influence of gravity.

Another strategy for studying atoms involves taking measurements of very cold atoms in an atom trap, which makes use of magnetic or optical forces to loosely contain the atoms and prevent them from falling. When this technique is used in future microgravity experiments, the observation time will be further increased because the cold, slow atoms will not fall out of the range of the observer's view as quickly as they do under the influence of Earth's gravity. The forces used to manipulate the atoms and maintain the trap, however, can interfere with some types of atomic measurements. In microgravity, the forces necessary to manipulate the atoms can be weaker or can be eliminated, and this should lead to even more precise experiments.

The trapping of atoms and laser cooling are also central to improved atomic clocks. When a single laser-cooled atom is trapped, it can be released into the clock mechanism, where it is then stimulated by a second laser to make a transition between two of its internal states. This change of states is usually a change in the motions of electrons within the atom. The electrons shift back and forth, resembling the motion of a pendulum, with a predictable frequency that becomes the time standard for the clock. Atomic clocks will work better in microgravity because the laser-cooled atoms can be manipulated and observed for a significantly longer period of time.

Laser cooling techniques have also been used to cause a cloud of atoms to condense into the Bose-Einstein state, a new state of matter similar to superfluid helium. The Bose-Einstein condensate occurs when atoms at a particular temperature and pressure, on the removal of some energy, fall into lock-step with one another.

The three images above, produced at millisecond intervals, show a cloud of
sodium atoms cooling into the Bose-Einstein condensate, a state in which a
majority of the atoms in a substance suddenly rearrange themselves into a
compact formation. The atoms can be maintained in the condensate formation
for longer observation periods in microgravity than in Earth's gravity, creating
better opportunities for scientists to learn more about the properties of the
condensate.

The properties of the Bose-Einstein condensate are still being examined. Studying this phenomenon in microgravity means that larger condensates can be supported in an experiment apparatus for longer periods of time, which in turn should lead to increased understanding of this unique state of matter.

Another use for laser cooling techniques has been found in aiding scientists in the quest to determine an intrinsic electric dipole moment for an atom or elementary particle. The intrinsic electric dipole moment is defined as the difference in the position of the center of the positive charge and the center of the negative charge in a particle or atom. To date, no dipole moments have been observed for the fundamental particles in experiments that have used conventional thermal atomic methods for analysis. According to the most widely accepted model for explaining physics at small length scales, an intrinsic atomic electric dipole moment would only be detectable with techniques that produce results ten orders of magnitude more accurate than those currently used. Competing models that predict larger values for an electric dipole moment could be ruled out with less precise measurements. Scientists expect that laser cooling will provide an order of magnitude improvement in ground-based measurements of this parameter and expect to obtain another order of magnitude improvement when such experiments are taken to microgravity.

The existence of an electric dipole moment for one of the fundamental particles would constitute a violation of time-reversal symmetry, one of the basic principles of modern physics. According to this principle, any observable event happening as time moves forward (our normal sense of time sequence) would be exactly reversed as time moved backward. Some experiments have provided hints that time-reversal is not a good symmetry because of known instances that some argue are evidence of time-reversal violation, but no universally accepted evidence for such a violation has yet been obtained. The detection of an electric dipole moment at levels greater than those predicted by the standard model of physics would provide unambiguous evidence that the current explanation of phenomena related to the fundamental forces of electromagnetic interaction and weak and strong nuclear interaction is not completely accurate and requires modification.