Single-atom photon recoil momentum

Single-atom photon recoil momentum in a dispersive medium has been measured. Photons have no mass, but they do carry momentum, h/λ, where h is Planck’s constant and λ is the wavelength of the light in vacuum. In a dispersive medium, light’s momentum separates into electromagnetic momentum and mechanical momentum of the medium. Therefore, there has been some confusion concerning the medium’s recoil when a photon is absorbed. A group at MIT has now verified that the momentum transferred to the absorbing atom is nh/λ, with n the index of refraction. The physicists used two identical laser beams sent into a dilute Bose–Einstein condensate of rubidium atoms. The first beam placed a small fraction of the atoms into a particular momentum state within the BEC. After a delay, the second beam created more identically moving atoms that interfered with the initial batch. The resulting beat note provided the momentum recoil measurement. That the recoil momentum is actually proportional to the index of refraction provides an important correction for high-precision measurements using cold atoms. (G. K. Campbell et al., Phys. Rev. Lett. 94, 170403, 2005 http://dx.doi.org/10.1103/PhysRevLett.94.170403 )