[time-nuts] Optical link connects atomic clocks over 1400 km of fibre

[Dave Brown]

What is the coupling mechanism giving rise to the 50Hz disturbance?
DaveB, NZ


----- Original Message ----- 
From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
To: <time-nuts at febo.com>
Cc: <magnus at rubidium.se>
Sent: Wednesday, August 24, 2016 8:54 AM
Subject: Re: [time-nuts] Optical link connects atomic clocks over 1400 km of 
fibre


> The presentations and posters at 8FSM and EFTF York have been interesting. 
> The PTB link-end is even more stable than the clock, but only in frequency 
> stability.
>
> More links is planned, among those between LNE-SYRTE at Paris Observatory 
> and NPL outside London. Such links aid in the comparison of optical 
> clocks, alongside the PTB portable optical clock, as various realizations 
> of same and different species is realized by various labs. The 
> inter-comparations will be important to narrow down the frequency 
> relationships as well as iron out various systematic shifts of 
> implementations. In the end, this is important as stepping stones towards 
> the redefinition of the SI second in terms of optical clocks.
>
> The active damping being done is quite interesting, but the bandwidth 
> allowed is limited by the length of the span due to the time-delay, so 
> that makes the length of each span limited and inter-related to the 
> bandwidth of compensation.
>
> These links is in principle not very complex, but they are regardless 
> somewhat sensitive. One link experienced excessive 50 Hz disturbance, 
> which they could trace to the fact that for a short distance the fibre was 
> laying alongside the house 400V three-phase feed-cable with quite a bit of 
> current in it.
>
> Fascinating stuff, and that they now can tie together labs for real is a 
> real advancement. Many labs is doing it, and they have different 
> approaches.
>
> Cheers,
> Magnus
>
> On 08/23/2016 01:04 AM, André Esteves wrote:
>> Some interesting developments in european atomic clocks.
>>
>> http://physicsworld.com/cws/article/news/2016/aug/22/optical-link-connects-atomic-clocks-over-1400-nbsp-km-of-fibre
>>
>> http://www.nature.com/articles/ncomms12443
>>
>> The time kept by atomic clocks in France and Germany has been compared
>> for the first time using a new 1400 km optical-fibre link between labs
>> in Paris and Braunschweig. Hailed as the first comparison of its kind
>> made across an international border, the link has already shown that
>> two of the most precise optical atomic clocks in Europe agree to
>> within 5 × 10–17. The link is the first step towards a European
>> network of optical clocks that will provide extremely stable and
>> precise time signals for research in a number of scientific fields
>> including fundamental physics, astrophysics and geosciences.
>>
>> An optical atomic clock works by keeping a laser in resonance with an
>> electronic transition between energy levels in an atom or ion – with
>> the "ticks" of the clock being the frequency of the laser light. As
>> with any clock, it is important to be able to compare the frequencies
>> of two or more instruments to ensure that they are working as
>> expected. Comparisons are also important for basic research,
>> particularly for testing the fundamental physical laws and constants
>> that are involved in the operation of atomic clocks.
>> Both of the clocks are based on the same optical transition in
>> strontium atoms, which are held in optical lattices created by laser
>> light. The clock at the LNE-SYRTE laboratory in Paris operates at an
>> uncertainty of about 4.1 × 10–17 and the clock at the PTB Braunschweig
>> laboratory at 1.8 × 10–17.
>>
>> Gravitational shift
>> If they were side by side, the clocks would tick at exactly the same
>> frequency. However, there is a 25 m difference in the elevation
>> between the two locations, which means that the Earth's gravitational
>> field is not the same for both clocks – causing them to tick at
>> slightly different frequencies. This gravitational redshift was
>> confirmed by the link, which can detect differences in elevation as
>> small as 5 m.
>> The link comprises two commercial-grade optical fibres that run
>> between Paris and Braunschweig. The route is not the shortest distance
>> between the two clocks, but rather takes a significant southward
>> detour via Strasbourg on the French–German border. For every 1020
>> photons that begin the journey, only one would arrive at its
>> destination. This 200 dB attenuation is compensated for by 10 or so
>> special amplifiers along the route. The German portion of the link
>> runs 710 km from Braunschweig to Strasbourg and is dedicated to
>> connecting the clocks. The French portion, however, uses 705 km of an
>> active telecommunications link that also carries Internet traffic. As
>> a result, two different approaches were needed to amplify the clock
>> signals on either side of the border.
>>
>> Second connection
>> The optical clock at PTB Braunschweig is already linked to the Max
>> Planck Institute for Quantum Optics (MPQ) in Garching near Munich.
>> This is done via a 920 km pair of optical fibres, and researchers at
>> the MPQ plan to use the clock signal to make extremely precise
>> spectroscopy measurements. A further expansion of this network would
>> provide researchers in other labs in Europe with access to
>> high-precision clock signals.
>> Applications could include measuring a fundamental physics constant in
>> several different locations – to confirm that the value of the
>> constant is indeed constant. Other possible uses include precision
>> measurements in spectroscopy that look for evidence of physics beyond
>> the Standard Model and making very precise measurements of the shape
>> and density of the Earth.
>> The construction and testing of the link are described in Nature 
>> Communications.
>>
>> About the author
>> Hamish Johnston is editor of physicsworld.com
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