Stop! In the Name of … Physics

Stopping and Freezing a Bullet

Well, not so much a bullet as pretty much any paramagnetic atom (which means most atoms). Reminiscent of Sisyphus cooling only with really big magnetic pulses instead of polarization gradients.

The principle is similar to coilguns being developed by the military to launch projectiles, only “in reverse,” Raizen says. To those atoms with their dipoles aligned opposite the beam’s direction, the pulses are like hills they have to climb, except that each hill disappears before the atoms have a chance to slide “downhill.”

A Look Back at Laser Cooling

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.