Background-scatter-free measurement protocol for trapped barium ions

The on-off state of a trapped ion qubit is discriminated by measuring the atomic fluorescence rate from the spontaneous decay of a fast dipole transition while the transition is continually driven. Ideally, detection of a single photon at the fluorescence wavelength would suggest that the electron wavefunction has collapsed into the measured state. However, as the driving laser is conventionally at the same wavelength as the ion fluorescence, the background scattered laser light into the photon collection channel is indistinguishable from ion fluorescence, leading to potential false positive results. This probability of yielding false positive results limits state discrimination time, as the photon collection time needs to be long enough to yield a sufficiently high signal-to-noise ratio for high measurement fidelities. In this study, we propose a protocol for trapped barium ions to eliminate the effects of background scattering from the driving laser during state discrimination. We exploit the spontaneous decay branching ratio of the 6P_1/2 level to 6S_1/2 and 5D_3/2 levels of a barium ion to alternate laser driving of the two transitions while collecting the photon of the decay branch in absence of the corresponding laser. We investigate the parameter space for optimal performance and compare the performance of our proposed protocol against the conventional methods.