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

[Bruce Griffiths]

https://tf.nist.gov/general/pdf/2995.pdf
may be also of some interest.
Its about optimising the linearity of high speed photodiodes.
These are used (amongst other applications )as mixers for converting optical combs to microwave signals.

Bruce 
> On 05 June 2020 at 11:35 Bill Byrom <time at radio.sent.com> wrote:
> 
> 
> 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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