[time-nuts] Coherent optical clock down-conversion for microwave frequencies with 10^−18 instability

[Bill Byrom]

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