Abstract

We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey–Bordé interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01P13 intercombination line in neutral Ca40 is used as a frequency discriminator, with an extended cavity diode laser at 423nm probing the ground state population after a Ramsey–Bordé sequence of 657nm light–field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×1014 at 1s and 2.0×1014 at 64s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×1014τ, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 1014 range frequency instability.

© 2010 Optical Society of America

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