Swans on Tea

Physical Review Letters is celebrating its Z=79 anniversary, and highlighting important letters. This week is:

Letters from the Past — A PRL Retrospective: This week’s Milestone Letter was originally published in 1970

Acceleration and Trapping of Particles by Radiation Pressure
A. Ashkin Phys. Rev. Lett. 24, 156 (1970)

This was a description of radiation pressure on transparent latex spheres, which felt a force when placed in laser light that had a gradient — the refraction gives rise to a force, or pressure, because the power is asymmetric across the sphere — the light changes direction, so the sphere must recoil, and the amount of recoil doesn’t balance. This is a precursor to a dipole force trap, which traps atoms at a field maximum (or minimum) of a light field, e.g. from focused laser. It also lays out radiation pressure by near-resonant scattering

The absorption and isotropic reradiation by spontaneous emission of resonance radiation striking an atom results in an average driving force or pressure in the direction of the incident light

which was the idea that led to laser cooling and optical molasses.

There is also a brief summary of the laser cooling history there this month, Landmarks: Laser Cooling of Atoms that takes you through the milestones up until the Nobel prizes for laser cooling and Bose-Einstein condensates.