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

[Bill Byrom]

Thanks, Bruce! That's a copy of that same Science article. I guess that NIST got permission to post it on their website, since they were the sponsor of the study. 
--
Bill N5BB


On Thu, Jun 4, 2020, at 6:32 PM, Bruce Griffiths wrote:
> https://tf.nist.gov/general/pdf/3093.pdf
> is likely more accessible than the sciencemag link
> 
> Bruce
> > On 05 June 2020 at 11:15 Bill Byrom <time at radio.sent.com> wrote:
> > 
> > 
> > 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
> > 
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