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

André Esteves aifesteves at gmail.com
Mon Aug 22 19:04:53 EDT 2016

Some interesting developments in european atomic clocks.



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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