Spontaneous Raman scattering out of a metastable atomic qubit

Metastable qubits in atomic systems can enable large-scale quantum computing by simplifying hardware requirements and adding efficient erasure conversion to the preexisting toolbox of high-fidelity laser-based control. For trapped atomic ions, the fundamental error floor of this control is given by spontaneous Raman and Rayleigh scattering from short-lived excited states. We measure spontaneous Raman scattering rates out of a metastable ${D}_{5/2}$ qubit manifold of a single trapped $^{40}\mathrm{Ca}^{+}$ ion illuminated by 976 nm light that is $\ensuremath{-}44$ THz detuned from the dipole-allowed transition to the ${P}_{3/2}$ manifold. This supports the calculation of error rates from both types of scattering during one- and two-qubit gates on this platform, thus demonstrating that infidelities less than ${10}^{\ensuremath{-}4}$ are possible.