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arXiv:1501.03709 · Physical Review X

Phase Locking a Clock Oscillator to a Coherent Atomic Ensemble

Ralf Kohlhaas, Andréa D Bertoldi, Etienne Cantin, Alain Aspect, Arnaud Landragin, Philippe Bouyer

Published Apr 27, 2015• 57 citations
arXivHTMLPDF

Abstract

Time measurements and navigation rely on atomic interferometers and atomic clocks. A new technique can improve the stability of such clocks by coupling a classical oscillator to an ensemble of atoms in a superposition state using repeated, coherence-preserving measurements.
DOI
10.1103/physrevx.5.021011

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We experimentally demonstrated that the stability of an atomic clock improves at its fastest rate τ −1 (where τ is the averaging time) when the phase of a local oscillator is genuinely compared to the continuous phase of many atoms in a single trap (an atomic phase lock). For this demonstration, we developed a simple method that repeatedly monitors the atomic phase while retaining its coherence by observing only a portion of the whole ion cloud. Using this new method, we measured the continuous phase over three measurement cycles, and thereby improved the stability scaling from to τ −1 during the three measurement cycles. This simple method provides a path by which atomic clocks can approach a quantum projection noise limit, even when the measurement noise is dominated by the technical noise.
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Atom interferometry based sensors provide today the most precise measurements of time, inertial forces and magnetic fields. In the common approach, a superposition state is interrogated for a given time interval and finally destructively measured. Conversely, we can repeatedly probe non-destructively the same quantum system and demonstrate efficient measurement schemes using feedback. First, we protect a spin polarized atomic ensemble from the decoherence induced by a synthetic noise. After the noise action, the state of the atomic system is read out with negligible projection using a non-destructive probe, and later corrected with a coherent manipulation to restore the initial state. The efficiency of the feedback scheme is studied versus the strength of the measurement and a maximum is found from the trade-off between information gain and probe destructivity. Our feedback controller is then applied to stabilize a classical local oscillator on a collective quantum state, and this is used in an atomic clock configuration to demonstrate experimentally that in some contexts a hybrid phase and frequency lock can surpass conventional atomic clocks which rely only on a frequency lock.