Practical quantum metrology with large precision gains in the low-photon-number regime

Quantum metrology exploits quantum correlations to make precise measurements with limited particle numbers. By utilizing inter-and intramode correlations in an optical interferometer, we find a state that combines entanglement and squeezing to give a sevenfold enhancement in the quantum Fisher information (QFI)-a metric related to the precision-over the shot-noise limit, for low photon numbers. Motivated by practicality we then look at the squeezed cat state, which has recently been made experimentally, and shows further precision gains over the shot-noise limit and a threefold improvement in the QFI over the optimal Gaussian state. We present a conceptually simple measurement scheme that saturates the QFI, and we demonstrate a robustness to loss for small photon numbers. The squeezed cat state can therefore give a significant precision enhancement in optical quantum metrology in practical and realistic conditions.