Optimal Dose-Limited Phase Estimation without Entanglement

Phase estimation is one of the most important facets of quantum metrology, with applications in sensing, microscopy, and quantum computation. When estimating a phase shift in a lossy medium, there is an upper bound on the attainable information per particle sent through the phase shift. Previously, only entanglement-enhanced measurements have been shown to saturate this bound. We introduce a measurement scheme which can saturate the bound without relying on entanglement.