A 920 km optical fiber link for frequency metrology at the 19th decimal place

With residual uncertainties at the 10^-18 level, modern atomic
frequency standards constitute extremely precise measurement
devices. Besides frequency and time metrology, they provide
valuable tools to investigate the validity of Einstein's theory of
general relativity, to test a possible time variation of the
fundamental constants, and to verify predictions of quantum
electrodynamics. Furthermore, applications as diverse as geodesy,
satellite navigation, and very long base-line interferometry may
benefit from steadily improving precision of both microwave and
optical atomic clocks. Clocks ticking at optical frequencies slice
time into much finer intervals than microwave clocks and thus
provide increased stability. It is expected that this will result
in a redefinition of the second in the International System of
Units (SI). However, any frequency measurement is based on a
comparison to a second, ideally more precise frequency. A single
clock, as highly developed as it may be, is useless if it is not
accessible for applications. Unfortunately, the most precise
optical clocks or frequency standards can not be readily
transported. Hence, in order to link the increasing number of
world-wide precision laboratories engaged in state-of-the-art
optical frequency standards, a suitable infrastructure is of
crucial importance. Today, the stabilities of current satellite
based dissemination techniques using global satellite navigation
systems (such as GPS, GLONASS) or two way satellite time and
frequency transfer reach an uncertainty level of 10^-15 after one
day of comparison . While this is sufficient for the comparison of
most microwave clock systems, the exploitation of the full
potential of optical clocks requires more advanced techniques. This
work demonstrates that the transmission of an optical carrier phase
via telecommunication fiber links can provide a highly accurate
means for clock comparisons reaching continental scales: Two 920 km
long fibers are used to connect MPQ (Max-Planck- Institut für
Quantenoptik, Garching, Germany) and PTB (Physikalisch-Technische
Bundesanstalt, Braunschweig, Germany) separated by a geographical
distance of 600 km. The fibers run in a cable duct next to a gas
pipeline and are actively compensated for fluctuations of their
optical path length that lead to frequency offsets via the Doppler
effect. Together with specially designed and remotely controllable
in-line amplication this enables the transfer of an ultra-stable
optical signal across a large part of Germany with a stability of 5
x 10^-15 after one second, reaching 10^-18 after less than 1000
seconds of integration time. Any frequency deviation induced by the
transmission can be constrained to be smaller than 4 x 10^-19. As a
first application, the fiber link was used to measure the 1S-2S two
photon transition frequency in atomic hydrogen at MPQ referenced to
PTB's primary Cs-fountain clock (CSF1). Hydrogen allows for precise
theoretical analysis and the named transition possesses a narrow
natural line width of 1.3 Hz. Hence, this experiment constitutes a
very accurate test bed for quantum electrodynamics and has been
performed at MPQ with ever increasing accuracy. The latest
measurement has reached a level of precision at which
satellite-based referencing to a remote primary clock is limiting
the experiment. Using the fiber link, a frequency measurement can
be carried out directly since the transmission via the optical
carrier phase provides orders of magnitude better stability than
state-of-the-art microwave clocks. The achieved results demonstrate
that high-precision optical frequency dissemination via optical
fibers can be employed in real world applications. Embedded in an
existing telecommunication network and passing several urban
agglomerations the fiber link now permanently connects MPQ and PTB
and is operated routinely. It represents far more than a
proof-of-principle experiment conducted under optimized laboratory
conditions. Rather it constitutes a solution for the topical issue
of remote optical clock comparison. This opens a variety of
applications in fundamental physics such as tests of general and
special relativity as well as quantum electrodynamics. Beyond that,
such a link will enable clock-based, relativistic geodesy at the
sub-decimeter level. Further applications in navigation, geology,
dynamic ocean topography and seismology are currently being
discussed. In the future, this link will serve as a backbone of a
Europe-wide optical frequency dissemination network.