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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Aug 23 16:54:41 EDT 2016

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.


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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