Suppressing and tuning-out Raman transitions in multilevel alkali atoms via multi-path interference

Raman coupling, a two-photon transition between ground states mediated through one or more excited states, is widely used in spectroscopy, laser cooling, and atomic state manipulation. Adiabatic elimination of the upper manifold, which results in an effective two level atom, is commonly used to determine the effective strength of the two photon transition; here we explore the limits of validity for this approach. In many cases, we find that for certain transition pathways, interferences significantly suppress, or even tune out, the transition strengths.

To model this, we use an effective Hamiltonian for multilevel atoms addressed by two beams, which drive electric dipole transitions through multiple allowed excited states. These independent pathways can be of similar magnitude and result in interference effects, both constructive and destructive. Our analysis identifies three possible regimes as a function of detuning, polarizations, and the atomic structure: no destructive interference, destructive interference at specific single-photon detuning, and destructive interference at large detunings. We have identified regimes in which coupling tends to zero in the presence of arbitrary laser powers: a tune-out frequency for Raman transitions. We apply our model to alkali atoms, examining the transition strengths between all possible hyperfine levels for both bosonic and fermionic species. Despite the complicated dependence of transition strengths on various parameters, all coupling schemes belong to one of the three regimes.

We discuss how these effects modify the fidelity in various applications, including an atom-based quantum gate and a quantum memory. Finally, we experimentally measure Raman transition strengths in Rb-87 for different coupling polarizations and geometries, and find good agreement with our model.