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Optical Clock Leads to
Drivers caught in slow-moving traf- fic may not realize that they are
experiencing relativistic time dilation.
But thanks to super-accurate optical
clocks, scientists can measure the effects
of relativity on ordinary speeds in the
everyday world—10 m/s, which is 33 kph
or roughly 22 mph.
Four researchers at the National
Institute of Standards and Technology
(NIST; Boulder, Colo., U.S.A.) used
their aluminum-ion-based optical clocks
to measure time dilation to an unprecedented degree (Science 329, 1630). The
team detected gravitational time shifts in
a change of height as small as 33 cm.
The group, led by postdoctoral associate James Chin-wen Chou, conducted its
experiments with two of its newest Al+
optical clocks, which have systematic frequency uncertainties up to 8. 6 3 10–18.
One clock used a magnesium ion as the
signaling “logic” ion—because the Al+
ion couldn’t be cooled directly—while
the other clock used a Be+ ion.
The team set the clocks in separate
laboratories and compared them via a
75-m length of
optical fiber. To
test the effects of
motion on time,
applied a small
electric field to
one of the clocks,
which caused the
Al+ ion to move
away from the null
position. The field
caused the ion to
move—and the clock measured time
more slowly than its stationary twin.
Likewise, the scientists experimented
with time dilation by raising one of the
clocks above the other. At the Earth’s
surface, they expected time to shift by
about a factor of 1. 1 3 10–16 per meter
of change in height.
Chou and colleagues started the
Al-Mg clock 17 cm below the Al-Be
clock and then raised it 33 cm higher.
The raised clock exhibited a fractional
frequency change of about 4. 1 3 10–17.
The ion trap in the NIST aluminum-ion clock. The aluminum ion and part- ner magnesium ion sit in the slit running down the center of the device between the electrodes.
If the optical clocks are improved
so that they can run for long periods
without human monitoring, they could
find applications in geodesy, hydrology
and tests of fundamental physics. The
optical fiber links used in the NIST
experiments provide sufficient accuracy
across lengths of up to about 250 km,
but the researchers say that intercontinental links between clocks may require
free-space optical techniques that are
still being developed.
The Optical Aspects of Graphene Research
Graphene, the one-atom-thick carbon lattice that is the subject
of this year’s Nobel Prize in Physics, has
many potential applications in photonics—from flexible solar cells and thin-film displays to heat dissipation in tiny
Andre Geim and Konstantin Novoselov, both of the University of Manchester
(England), received this year’s physics Nobel for their pioneering work on
graphene, which began only six years
ago. According to Alexander Balandin, an
electrical engineering professor at the University of California at Riverside (U.S.A.)
and an OSA member, the optical aspects
of graphene research are quite intriguing.
Two years ago, Balandin and colleagues were the first to measure the
extremely high thermal conductivity
of single-layer graphene (Nano Lett. 8,
902). Due to the thinness of graphene,
the only way they could do so was to
heat the graphene layer, suspended over
a trench in a substrate, with a laser and
then measure the temperature rise from
the shift in graphene’s Raman spectrum.
Essentially, they converted the optical
instrument—a Raman spectrometer—
into a thermometer.
Because of its high thermal conductivity, graphene could cool down
hot spots in tiny optoelectronic and
photonic devices, Balandin said. Also,
graphene is transparent, absorbing only
a tiny fraction of the light falling on it.
With the proper substrates, it could be
used as a transparent top electrode in
touch-screen displays, flexible displays
and photovoltaic cells.
Balandin and his group are continuing to investigate the thermal properties
of graphene and graphene applications
for thermal management. They are
also working on low-noise graphene
10 | OPN Optics & Photonics News