Photon scattering errors during stimulated Raman transitions in trapped-ion qubits

We study photon scattering errors in stimulated Raman driven quantum logic gates. For certain parameter regimes, we find that previous, simplified models of the process significantly overestimate the gate error rate due to photon scattering. This overestimate is shown to be due to previous models neglecting the detuning dependence of the scattered photon frequency and Lamb-Dicke parameter, a second scattering process, interference effects on scattering rates to metastable manifolds, and the counter-rotating contribution to the Raman transition rate. The resulting improved model shows that there is no fundamental limit on gate error due to photon scattering for electronic ground state qubits in commonly-used trapped-ion species when the Raman laser beams are red detuned from the main optical transition. Additionally, photon scattering errors are studied for qubits encoded in metastable $D_{5/2}$ manifold, showing that gate errors below $10^{-4}$ are achievable for all commonly-used trapped ions.
Comment: 24 pages, 8 figures, to be submitted to Phys. Rev. A. In this version, we changed the two-qubit gate under consideration. Originally, we considered a gate driven by two perpendicular pairs of Raman beams. In this version, we consider a gate driven by a pair of Raman beams counterpropagating against a third Raman beam