Continuous Real-Time Tracking of a Quantum Phase Below the Standard Quantum Limit
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 Δ<em>ϕ</em><sub>SQL</sub>=1/√<em>N</em> rad.
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Physical Review Letters
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