Entanglement-free, Heisenberg-limited Phase Measurement
Brendon L. Higgins, Howard M. Wiseman, Geoff J. Pryde, Dominic W. Berry, Stephen D. Bartlett · 2007
We demonstrate the first experimental Heisenberg-limited scaling of the phase variance in optical phase measurements. Our algorithm replaces complicated entangled states – widely thought to be essential for Heisenberg scaling – with single photons, multiple passes through a phase shift, and adaptive measurement. We show that although a single-photon version of Kitaev’s phase estimation algorithm has standard-quantum-limited variance, our generalized technique is within a small constant factor of the ultimate quantum limit.