Physicists in the US have created a highly-stable optical clock
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Wed, 06 Dec 2006 22:08:05 GMT
Optical clocks strike again
http://physicsweb.org/articles/news/10/12/4/1
6 December 2006
Physicists in the US have created a highly-stable optical clock that
could push the precision of these new timekeepers beyond that of
conventional atomic clocks. Optical clocks keep time by using visible
light to probe the unwavering energy levels within atoms. While they
have the potential to be much more precise than microwave-based atomic
clocks, they have suffered from stability problems due to the motion of
the atoms. This new optical clock gets around this problem by trapping
strontium atoms in an optical lattice thereby restricting their motion
(Science 314 1430).
Optical clocks are based on a specific transition between atomic energy
levels that involves the absorption of laser light at a very precise
frequency. A laser is used to stimulate the transition and, once
absorption begins, a feedback mechanism stabilizes the laser light at
the precise absorption frequency. A device called a "femtosecond
comb" is then used to measure the frequency, which is the ticking
of the clock.
Unfortunately this process can be easily disturbed by the motion of
atoms, which is a key challenge facing designers of optical clocks. Jun
Ye and colleagues at JILA at the University of Colorado have now
managed to reduce these motion-related effects by trapping strontium
atoms in a one-dimensional optical lattice -- a periodic structure of
atoms that are held in place by interfering laser beams.
According to Ye, the optical lattice allows the probing laser light to
interact coherently with the atoms for a longer period of time. "We are
the first group to demonstrate that coherent interactions can last for
nearly one second", he says.
The clock operates at 430 THz and up to 4.3 × 10^14 cycles can be
counted during one measurement, which boosts the precision of the
measurement. Ye's clock has a precision of 2 Hz in 430 THz, or
about five parts in 10^15. This makes it less precise than a mercury ion
atomic clock created by NIST in the US and state-of-the-art atomic
clocks -- both of which can achieve one part in 10^15 precision.
However, an important feature of this new clock is that it can deliver
a strong and stable signal. This could open the door to measurements
longer than one second, which could ultimately push the precision to
one part in 10^17. Atomic clocks currently measure over about one day
and have reached their practical limit at about one part in 10^15
precision.
.
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