Is a neutron deflected sideways by a laser beam?

Question

Is a freely moving neutron deflected sideways when a laser beam is directed at it from the side? It would be great if the question could be considered from the two points of view that the laser beam is EM radiation and that the beam consists of a stream of individual photons.

Answer 1

To first approximation, no. In QFT, the photon is a particle which mediates interactions between charged particles; since the neutron is uncharged, it doesn't interact with photons.

The neutron does have a nonzero magnetic moment, and can in principle have an interaction with the magnetic field of a laser. But that's tiny. The neutron's magnetic moment is about 50 nano-eV per tesla. If you are imagining a beam of thermal neutrons with milli-eV energies, and you have a laser whose energy density corresponds to tesla-scale magnetic fields, then the "transverse kinetic energy" imparted to a neutron which does interact with the laser would be a million times smaller than the thermal-scale kinetic energy. A minuscule deflection, for an unreasonably strong laser field.

In my PhD experiment, we had neutrons which were polarized perpendicular to the beam direction by a weak $(10^{-3}\rm\,T)$ magnetic field. We reversed the neutron spins by passing them through an oscillating field parallel to their motion. The resonant frequency was about 30 kHz, which is somewhat slower than an optical laser. The change in the neutrons' kinetic energies due to this interaction was negligible in our part-per-billion precision measurement.

So to a second approximation, also no.