Beyond one-second laser coherence via active optical atomic clock

Chi Shu, Edwin Pedrozo-Peñafiel, Simone Colombo +4 more

Jingbiao Chen

This letter presents the principles and techniques of active optical clock, a special laser combining the laser physics of one-atom laser, bad-cavity gas laser, super-cavity stabilized laser and optical atomic clock. As an example, a compact version of active optical clock based on thermal Strontium atomic beam shows a quantum-limited linewidth of 0.51 Hz, which is insensitive to laser cavity-length noise, and may surpass the recorded narrowest 6.7 Hz of Hg ion optical clock and 27Hz of very recent optical lattice clock. The estimated 0.1Hz one-second instability and 0.27Hz uncertainty are limited only by the relativistic Doppler effect may be improved to 10mHz by using cold atoms.

Sören Dörscher, Ali Al-Masoudi, Marcin Bober +4 more

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.

Vladan Vuletić

F. Riehle

Optical atomic clocks with fractional uncertainties around and below 10−17 [1] and fractional instabilities at one second interrogation time of below 10−15 [2] have outperformed the best microwave clocks like the cesium atomic clock. In optical atomic clocks a highly stable laser is used to interrogate optical transitions in laser-cooled atoms, ions, or molecules. The increase in accuracy and stability of the optical atomic clocks results from the increased line quality factor Q= ν/Δν given by the five orders of magnitude higher frequency of optical transitions as compared to the transition relevant in the microwave domain. To utilize this potential it is furthermore necessary to realize a suitable narrow linewidth of the interrogated line. The linewidth of any optical transition is given by the different broadening mechanisms of the respective quantum transition relevant for particular interrogation process as well as by the coherence properties of the interrogating laser.

Andrew D. Ludlow