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

[Magnus Danielson]

I think they speculated in the Kerr effect, so that the voltage swings 
would modulate the fiber. I don't consider it verified, so I do not take 
cross-examination on it. I can ask thought.

Cheers,
Magnus

On 08/23/2016 11:31 PM, Dave Brown wrote:
> 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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