[time-nuts] Shrinking Atomic Clocks
Thomas A. Frank
ka2cdk at cox.net
Sun Mar 15 17:46:54 EDT 2009
Time to shrink the atomic clock
14 March 2009 by Anil Ananthaswamy
Magazine issue 2699. Subscribe and get 4 free issues.
ATOMIC clocks, currently the size of fridges, could shrink to the
microscale thanks to a new way of measuring the second. The technique
could also see aluminium displace caesium as the standard of time.
The world's most accurate atomic clocks are at the National Institute
of Standards and Technology (NIST) at Boulder, Colorado. Known as
fountain clocks, they send clouds of caesium atoms through a vacuum
chamber in a magnetic field. Large atoms like caesium and aluminium
have multiple energy levels that are so close together they appear
indistinguishable. The magnetic field separates these levels into two
The chamber is also filled with microwaves, which excite the atoms.
They then emit light as they drop to the lower hyperfine state. The
microwave frequency that maximises this fluorescence is used to
define the length of a second, currently the time it takes for
9,192,631,770 cycles of microwave radiation.
All this takes place in a large vacuum chamber and so fountain clocks
are big devices, about a cubic metre in size. That makes it hard to
keep the magnetic field and the device's temperature uniform over the
whole area, which can lead to errors of measurement.
That's why Andrei Derevianko and Kyle Beloy of the University of
Nevada in Reno and colleagues have come up with the idea of trapping
the atoms in place using lasers. This means their energy states could
be monitored in an area only a few micrometres across, potentially
leading to more accurate measurements. This is difficult to get
right, though, because the lasers distort an atom's energy levels in
a complex way, making it impossible to define a jump that equates to
Derevianko's team overcome this problem by finding a laser frequency
that alters both hyperfine states by exactly the same amount - a
trick that works in aluminium and gallium but not as well in caesium
(www.arxiv.org/abs/0808.2821). "Then, the energy difference between
the levels is the same as if the atoms are in vacuum," says Derevianko.
Using this method, the team has calculated the second to be 1506
million cycles of microwaves for aluminium-27 and 2678 million cycles
Although the atoms can be trapped in an area only a few micrometres
across, the lasers, and cooling and computing equipment will add to
the bulk. Nevertheless, the team say the clocks may be portable and
could be used in space-based experiments that require extremely
accurate timekeeping, such as those for detecting gravitational waves
or for testing Einstein's theories.
Tom Heavner, who works on fountain clocks at NIST, describes the
proposal as forward-thinking and original. "It is a really clever way
to meld together the old-style clocks with new laser technology," he
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