Abstract
The frequency stability achieved by an optical atomic clock ultimately
depends on the coherence of its local oscillator. Even the best ultrastable
lasers only allow interrogation times of a few seconds, at present. Here we
present a universal measurement protocol that overcomes this limitation.
Engineered dynamic decoupling of laser phase noise allows any optical atomic
clock with high signal-to-noise ratio in a single interrogation to reconstruct
the laser's phase well beyond its coherence limit. A compound clock is then
formed in combination with another optical clock of any type, allowing the
latter to achieve significantly higher frequency stability than on its own. We
demonstrate implementation of the protocol in a realistic proof-of-principle
experiment with a phase reconstruction fidelity of 99 %. The protocol enables
minute-long interrogation for the best ultrastable laser systems. Likewise, it
can improve clock performance where less stable local oscillators are used,
such as in transortable systems.
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