Mobile atomic clock used as an altimeter

<p>Optical atomic clocks are complex
devices and, until recently, were found
only in the laboratories of some large research
institutes. Greater accuracy can be
achieved by relying on an optical transition
that can be excited using visible light
in the atom rather than on a transition
triggered by microwaves – as is the case
in a cesium clock. Optical clocks have
brought us one step closer to detecting
differences in height as small as one centimeter.</p>

<p>For the first measurement
campaign,
PTB’s mobile strontium
optical clock
was placed in its car
trailer and driven
to the Modane Underground
Laboratory
(France), which
is located halfway
through the Fréjus
tunnel on the border
between France and
Italy. There, a team
from PTB and from
NPL, the national metrology institute of Great Britain,
operated the clock and transmitted its
frequency via a 150 km fiber-glass link
to INRIM, the national metrology institute
of Italy, in Turin, where a second
atomic clock was used to measure the
frequency of the strontium clock. A second
(subsequent) comparison of the two
clocks at INRIM allowed the change in
frequency of the strontium clock to be
determined via the height difference between
LSM and INRIM, which amounts
to about 1000 meters. A relative change
in frequency of approx. 1&nbsp;&middot;&nbsp;10^–13 was then
observed. By multiplying this change in
frequency by the squared speed of light,
one obtains the underlying change in potential.
The exact difference in the gravitational
potential had previously been determined
by the University of Hannover
with conventional geodesic measurement
methods. The results of the two measurements
were consistent.</p>

<p>For the new method to become competitive
compared to established measurement
methods, the portable clocks
will have to be improved further. However,
it is expected that the new method will be able to cover long distances with high
spatial resolution and without loss of accuracy.
Measurements of the gravitational
potential improved in this way may
help detect effects such as the displacement
of ice sheets and general changes
in mass (e.g. of ocean water) more accurately.
Such data are crucial for models,
since they can contribute to a better understanding
of global climate change and
to predicting changes.</p>