[time-nuts] Nature: Hyper-precise atomic clocks face off to redefine time

[Anders Wallin]

Here are some details on the gravity measurements:
http://projects.npl.co.uk/itoc/project-structure/reg/gravity-observations/

AFAIK this campaign is done with GPS-PPP and TWSTFT for frequency
comparison. The troposphere makes it hard to reach 1e-17 level for the
satellite links - even with a week of averaging time.
The next step is coherent frequency transfer over optical fibers, where
they claim down to maybe 1e-18 or 1e-19 ADEVs for the link itself for a day
of averaging.

Anders


On Sat, Jun 6, 2015 at 3:19 AM, Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

> Can someone explain to me how this is going to work in
> light of the fact that each clock is in a different
> gravitational field?  Or is accuracy not the measurement,
> but rather stability?  No, that can't be because any
> lab that wants to measure stability merely needs to build
> two or three copies of their favorite clock and insure
> against synchronization.  They in principle shouldn't
> need to compare against a dissimilar type of clock.
> Therefore, we are back to the gravity issue.
>
> When we worked on the 5071A, we barely had enough sensitivity
> to notice a few parts in 10^13 between Santa Clara and
> Boulder (~5000 feet).
>
> Rick Karlquist N6RK
>
> On 6/3/2015 12:18 AM, Hal Murray wrote:
>
>> Nice picture: A strontium-ion optical clock housed at the National
>> Physical
>> Laboratory in Teddington, UK.
>>
>> Over the past decade, various laboratories have created prototype optical
>> atomic clocks, which use different elements such as strontium and
>> ytterbium
>> that emit and absorb higher-frequency photons in the visible spectrum.
>> This
>> finer slicing of time should, in principle, make them more accurate: it is
>> claimed that the best of these clocks gain or lose no more than one second
>> every 15 billion years (1E18 seconds) -- longer than the current age of
>> the
>> Universe -- making them 100 times more precise than their caesium
>> counterparts. Optical clocks are claimed to be the best timekeepers in
>> existence, but the only way to verify this in practice is to compare
>> different models against each other and see whether they agree.
>>
>> Starting on 4 June, four European laboratories will kick off this testing
>> process -- the National Physical Laboratory (NPL) in Teddington, UK; the
>> department of Time-Space Reference Systems at the Paris Observatory; the
>> German National Metrology Institute (PTB) in Braunschweig, Germany; and
>> Italy's National Institute of Metrology Research in Turin. Between them,
>> the
>> labs host a variety of optical clocks that harness different elements in
>> different experimental set-ups.
>>
>>
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