APS/Erich J. Mueller

Figure 1: Scheme for generating spin-orbit coupling in a neutral, ultracold atomic gas. Two counterpropagating laser beams couple two spin states by a resonant stimulated two-photon Raman transition: an atom in a spin-up ($↑$) state is excited to a virtual level by absorbing a photon from the left beam, then flips to the spin-down ($↓$) state by emitting another photon into the right beam. The lasers are detuned by a frequency $δ$ from an excited multiplet. This stimulated Raman process results in a momentum kick to the atom, leading to single-particle eigenstates where spin and momentum are entangled.