Physicists demonstrate transfer of ultraprecise time signals over a wireless optical channel

May 1, 2013
NIST researchers transferred ultra-precise time signals over the air between a laboratory on NIST's campus in Boulder, Colo., and nearby Kohler Mesa. Signals were sent in both directions, reflected off a mirror on the mesa, and returned to the lab, a total distance of approximately 2 km. The two-way technique overcomes timing distortions on the signals from turbulence in the atmosphere, and shows how next-generation atomic clocks at different locations could be linked wirelessly to improve distribution of time and frequency information and other applications. Credit: Kelly Talbott, NIST

By bouncing eye-safe laser pulses off a mirror on a hillside, researchers at the National Institute of Standards and Technology (NIST) have transferred ultraprecise time signals through open air with unprecedented precision equivalent to the "ticking" of the world's best next-generation atomic clocks.

Described in Nature Photonics, the demonstration shows how next-generation atomic clocks at different locations could be linked wirelessly to improve geodesy (altitude mapping), distribution of time and frequency information, satellite navigation, radar arrays and other applications. Clock signals of this type have previously been transferred by fiber-optic cable, but a wireless channel offers greater flexibility and the eventual possibility of transfer to and from satellites.

The stability of the transferred infrared signal matched that of NIST's best experimental atomic clock, which operates at . Infrared light is very close to the frequencies used by these clocks, and both are much higher than the in conventional atomic clocks currently used as national time standards. Operating frequency is one of the most important factors in the precision of optical , which have the potential to provide a 100-fold improvement in the accuracy of future time standards. But the signals need to be distributed with minimal loss of precision and accuracy.

The signal transfer demonstration was performed outdoors over a two-way wireless link using two combs. A frequency comb generates a steady stream of ultrashort with a spacing that can be synchronized perfectly with the "ticks" of an optical atomic clock. In the experiment, the two combs were synchronized to the same stable , which serves as a stand-in for an optical atomic clock. Each comb pulse was sent from one of two locations on NIST's campus in Boulder, Colo., reflected off a mirror on a mesa behind the campus, and returned to the other site, traveling a total distance of two kilometers.

Researchers measured travel times for pulses traveling in opposite directions between the two sites. The cumulative timing differences and frequency instabilities were infinitesimal, just one million-billionths of a second per hour, a performance level sufficient for transferring optical clock signals.

The transfer technique overcomes typical wireless signal problems such as turbulence in the atmosphere—the phenomenon that makes images shimmer when it's very hot outside. Because turbulence affects both directions equally, it can be cancelled out. The transfer technique can also withstand signal losses due to temporary obstruction of the light path. The method should be able to operate at much longer distances, possibly even over future ground-to-satellite optical communication links as an added timing channel, researchers say.

The combs potentially could be made portable, and the low-power infrared light is safe for eyes. The research is funded in part by the Defense Advanced Research Projects Agency.

Explore further: Atomic clock signals may be best shared by fiber-optics

More information: F.R. Giorgetta, W.C. Swann, L.C. Sinclair, E. Baumann, I. Coddington, N.R. Newbury. Optical two-way time and frequency transfer over free-space. Nature Photonics. Published online April 28.

Related Stories

Atomic clock signals may be best shared by fiber-optics

March 2, 2007

Time and frequency information can be transferred between laboratories or to other users in several ways, often using the Global Positioning System (GPS). But today's best atomic clocks are so accurate—neither gaining nor ...

Optical Atomic Clock: A long look at the captured atoms

February 5, 2008

Optical clocks might become the atomic clocks of the future. Their "pendulum", i.e. the regular oscillation process which each clock needs, is an oscillation in the range of the visible light. As its frequency is higher than ...

Atomic clock comparison via data highways

April 27, 2012

( -- Optical atomic clocks measure time with unprecedented accuracy. However, it is the ability to compare clocks with one another that makes them applicable for high-precision tests in fundamental theory, from cosmology ...

Redefining time

April 30, 2012

Atomic clocks based on the oscillations of a cesium atom keep amazingly steady time and also define the precise length of a second. But cesium clocks are no longer the most accurate. That title has been transferred to an ...

Recommended for you

Team extends the lifetime of atoms using a mirror

October 13, 2015

Researchers at Chalmers University of Technology have succeeded in an experiment where they get an artificial atom to survive ten times longer than normal by positioning the atom in front of a mirror. The findings were recently ...

A particle purely made of nuclear force

October 13, 2015

Scientists at TU Wien (Vienna) have calculated that the meson f0(1710) could be a very special particle – the long-sought-after glueball, a particle composed of pure force.


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet May 02, 2013
Great...until it rains
not rated yet May 02, 2013
Great...until it rains

Yeah, no different than the FLIR systems we use now, just more accurate. It's funded by DARPA, so they probably are looking for ways to improve their existing IR systems. I know they've been working on improved all-weather systems as well, but those are typically low resolution, so they can't see things like power cables. Not exactly good for flying a helicopter around an urban area at night. High resolution IR is really what you want for that. This isn't anything totally new; more like an improvement on what we already do on helicopters, tanks, etc.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.