TY - JOUR AU - Athreya Shankar AU - Graham Greve AU - Baochen Wu AU - James Thompson AU - Murray Holland AB - collective spin composed of N pseudospins. Quantum nondemolition measurements of a lossy cavity mode interacting with an atomic ensemble are used to directly probe the phase of the collective atomic spin without converting it into a population difference. Unlike traditional Ramsey measurement sequences, our scheme allows for real-time tracking of time-varying signals. As a bonus, spin-squeezed states develop naturally, providing real-time phase estimation significantly more precise than the standard quantum limit of ΔϕSQL=1/√N rad. BT - Physical Review Letters DA - 2019-06 DO - 10.1103/PhysRevLett.122.233602 N2 - collective spin composed of N pseudospins. Quantum nondemolition measurements of a lossy cavity mode interacting with an atomic ensemble are used to directly probe the phase of the collective atomic spin without converting it into a population difference. Unlike traditional Ramsey measurement sequences, our scheme allows for real-time tracking of time-varying signals. As a bonus, spin-squeezed states develop naturally, providing real-time phase estimation significantly more precise than the standard quantum limit of ΔϕSQL=1/√N rad. PY - 2019 EP - 233602 T2 - Physical Review Letters TI - Continuous Real-Time Tracking of a Quantum Phase Below the Standard Quantum Limit UR - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.233602 VL - 122 SN - 0031-9007 ER -