This was published in the 22 May 2020 issue of Science (AAAS journal). For AAAS members, the direct link is: https://science.sciencemag.org/content/368/6493/889
They make use of a fiber-based OFC (optical frequency comb) and state-of-the-art photodetectors to transfer optical clock stability to a 10 GHz microwave signal. This downconversion from optical to microwave was done with an error of no more than 10-19 (1 x 10 ^-19). The best available optical clock stability is around 10-18 (1 x 10^-18) at a couple of hundred seconds averaging time. This specific experiment compared two independent Yb (Ytterbium) optical lattice clocks running at about 259 THz. One Yb clock drove a 208 MHz comb generator, while the other Yb clock drove a 156 MHz comb generator. Then: 208 MHz x 48th harmonic = 9.984 GHz 156 MHz x 64th harmonic = 9.984 GHz The phase between these 9.984 GHz signals was compared in a mixer phase detector. The fractional frequency instability observed was 10-16 (1 x 10^-16) over a 1 second interval. The frequencies I listed above are approximate -- they actually measured a 1.5 MHz beat note between the ~10 GHz signals. This allowed them to achieve a relative timing error of 900 attoseconds (rms). The optical phase measurements between the two Yb clocks at 259 THz indicated a frequency offset (Yb1 - Yb2) of 0.0000064 Hz, and the microwave ~10 GHz comparison was consistent with that offset (2.5 +/- 0.6) x 10-20 (10^-20). The abstract is: > Optical atomic clocks are poised to redefine the Système International (SI) > second, thanks to stability > and accuracy more than 100 times better than the current microwave atomic > clock standard. However, > the best optical clocks have not seen their performance transferred to the > electronic domain, where > radar, navigation, communications, and fundamental research rely on less > stable microwave sources. > By comparing two independent optical-to-electronic signal generators, we > demonstrate a 10-gigahertz > microwave signal with phase that exactly tracks that of the optical clock > phase from which it is derived, > yielding an absolute fractional frequency instability of 1 × 10−18 in the > electronic domain. Such faithful > reproduction of the optical clock phase expands the opportunities for optical > clocks both technologically > and scientifically for time dissemination, navigation, and long-baseline > interferometric imaging. I have a Science subscription and can read this paper, but I can't distribute it here. You can also see discussion of this achievement by NIST (with assistance by the University of Virginia) at Physics World: https://physicsworld.com/a/microwave-timing-signals-get-hundredfold-boost-in-stability/ You may need to request a free account at Physics World to read this article. -- Bill Byrom N5BB _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
