Entanglement and extreme planar spin squeezing
Giuseppe Vitagliano, Giorgio Colangelo, Ferran Martin Ciurana, Morgan W. Mitchell, R. J. Sewell, Géza Tóth · Physical Review A · 2018
We introduce an entanglement-depth criterion optimized for planar quantum-squeezed (PQS) states. It is connected with the sensitivity of such states for estimating a phase generated by rotations about an axis orthogonal to its polarization. We compare numerically our criterion with the well-known extreme spin-squeezing condition of S\o{}rensen and M\o{}lmer [Phys. Rev. Lett. 86, 4431 (2001)] and show that our condition detects a higher depth of entanglement when both planar spin variances are squeezed below the standard quantum limit. We employ our theory to monitor the entanglement dynamics in a PQS state produced via quantum nondemolition measurements using data from a recent experiment [Phys. Rev. Lett. 118, 233603 (2017)].