Philosophical Society of Washington

Minutes of the 2131st Meeting

Speaker: Paul D. Lett, Atomic Physics Division, National Institutes of Science and Technology
Topic: “Atoms and Light: From Laser Cooled Atoms to Atom Lasers”

President McDiarmid called the 2131st meeting to order at 8:15 p.m. on April 27, 2001. The Recording Secretary read the minutes of the 2130th meeting and they were approved.

The speaker for the 2131st meeting was Paul D. Lett. The title of his presentation was “Atoms and Light: From Laser Cooled Atoms to Atom Lasers”.

The speaker began by noting that the Philosophical Society of Washington was founded in 1871, and incorporated by an act of Congress in 1901. The National Institutes of Science and Technology is also celebrating a centennial this year. Precision measurements in the tradition of Joseph Henry have always been part of the mission of NIST. LASERs in particular are generally known for their power and precision, being used for such things as welding and eye surgery. They can also be used for cooling. The field of laser cooling and trapping of neutral atoms has developed into an exciting branch of atomic physics, and was recognized by the award of the 1997 Nobel Prize in physics to Steve Chu at Stanford, Claude Tannoudji at Paris, and Bill Phillips at NIST.

In quantum mechanical systems the lowest energy level does not have zero energy. Light absorbed by a quantum mechanical system causes a change in energy proportional to the frequency of the light. Light can be said to exert a force in two ways, by radiation pressure or scattering force like reflection, and by gradient or induced dipole force like refraction. By carefully tuning a LASER to the absorption frequency of suspended atoms doppler-shifted by their motion toward the LASER, they can be brought to a state of essentially zero relative motion. Atoms not moving or moving away from a LASER will not absorb the light because they cannot absorb light of the doppler-shifted frequency. The LASER light in resonance with the doppler-shifted atoms allows them to abosrb against the light pressure and brings them to a halt. Six LASER beams mounted in three mutually perpendicular axes and tuned to this differential doppler-shfted absorption frequency can be used to create a magneto-optical trap, MOT or "optical molasses", for atoms bringing their relative motion and thus their temperature to essentially zero.

There are several applications for LASER cooled atoms. One application especially sgnificant for NIST is the measurement of time. The 1986 standard of time is based on the 9 192 631 770 Hz absorption of cesium 133 atoms. Normal atomic motion of about 3 meters/sec at the measurment temperature had previously limited the precision of this measurement. Another application is that we now have the abilty to produce a Bose-Einstein Condensate, or BEC. A BEC is a coherent cloud of atoms with a spread of momentum much less than the recoil of one photon. Such states could not be produced artificially until we had this means of producing zero atomic motion. Quantum particles obey either of two statistics depending on their intrinsic quantum state referred to as spin. Quantum particles with half-integral spins, that is spins of 1/2, 3/2, etc., obey Fermi-Dirac statistics and are called fermions. The electron, proton and neutron are fermions. Quantum particles with integral spins, that is spins of 0, 1, 2, etc., obey Bose-Einstein statistics and are called bosons. Photons are bosons. Quantum state coupled fermion particles such as atoms can behave as either fermions or bosons depending on the sum of their aggregate spin and their energy state. A BEC is a cloud of identical bosons all with a very small difference in momentum. The size of the cloud is equal to Planck's constant divided by the difference in momentum. The atoms within a BEC become indistinguishable by quantum mechanics. Using a combination of evaporative, magnetic and LASER cooling it had been possible to create a Bose-Einstein condensate containing 5x106 atoms in a volume less than (50 microns)3; that is a density of 1014 atoms per cc.

These experiments lead to the question of what a temperature of absolute zero really means. Does it mean the state of an aggregate mass with no kinetic energy spread, or the state from which no further energy could be removed? Experiments are underway which would use the atoms built up in a BEC to produce beams of coherent atoms. Such beams might be used as atomic fountain clocks or to construct quantum computers.

Mr. Lett kindly answered questions from the floor. President McDiarmid thanked Mr. Lett for the society, and welcomed him to its membership. Mr. Garavelli spoke briefly on the benefits of PSW membership. The President made the announcements about the next meeting, parking, and refreshments, and adjourned the 2131st meeting to the social hour at 9:32 p.m.

Attendance: 35
Temperature: 22.1°C
Weather: clear

Respectfully submitted,
John S. Garavelli
Recording Secretary

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