Atom’s recoil is sometimes less than photon’s momentum

Kurt Gibble, an associate professor of physics at Pennsylvania State University (University Park, PA), has uncovered a surprising characteristic of atoms absorbing photons in a laser beam; if the beam is focused, an atom near the center of the beam recoils less from absorbing a photon than if the light were an infinite plane wave. This means that the photon actually imparts less momentum (ħkz) to the atom for a wavefront finite in size.

According to Gibble’s calculations, which are based on the Schrödinger equation, the transverse spatial gradient (in the atom cloud) of the dipole energy produces a force that “lenses” the atomic wave function, affecting its phases, but not its average position or width. Thinking of the angular spectrum of the wavefront as a sum of plane waves, Gibble explains that the atom feels the effect of the photons from all of the beams simultaneously. “Surprisingly, it recoils with a speed that is less than it would get from the momentum of any one of the infinitely wide photons,” he says. Gibble’s discovery has implications for the accuracy of atomic clocks, which are based on microwaves. Contact Kurt Gibble at kgibble@phys.psu.edu.