Time over time timekeeping’s accuracy improved dramatically during the last millennium.
a chinese water clock (oldest printed illustration shown at left) could lose around 10 minutes a
day. within five years, scientists hope to have an atomic clock that loses less than one second
over the universe’s lifetime. soUrces: s.a. diddams E T AL/SCIENCE, c. oates
Accuracy of clocks through history
Cesium fountain clocks
Optical lattice clock
Early cesium atomic clocks
Chinese water clock
Huygens pendulum
Harrison chronometer
Shortt clock
Quartz crystal
1018
1015
1012
109
106
103
While devices like the NIST-F1 use
atomic signals of microwave frequency
with billions of cycles per second,
newer clocks, including Oates’, rely
on light waves beating a million times
faster. The new approach “is like having
a ruler with more divisions,” says Tom
O’Brian, NIST’s chief of the divisions of
Time and Frequency and of Quantum
Physics. “The pace of improvement is
skyrocketing.”
A further development, the lattice
clock, has been imagined only in the
last decade, with rapid progress in the
last five years. For now, related devices
called single ion optical clocks, which
key in on solitary electrically charged
atoms such as aluminum, are the most
accurate. However, lattice clocks’ use
of many atoms simultaneously, with a
strong combined signal, appears to give
these clocks the ultimate edge.
Katori says his team in Tokyo hopes to
have the first clock with one part in 1018
accuracy working within five years. A look
at how the record-setting mercury clock
would work reveals the basics of all contemporary neutral-atom lattice clocks.
At a glance, the proposed clock is a
bewildering laser beam jubilee —but
there is underlying order.
The action starts with a system of
cooling lasers that bathe a thin vapor of
mercury atoms in what is called “optical
molasses” to slow their motion. Temperatures hit a few millionths of a degree
above absolute zero, a coolness at which
each atom drifts roughly at the walking
speed of ants. But even at that slow speed,
the motion causes a slight blur in the
atoms’ collective optical signals.
The cooling lasers propel the chilled
atoms gently into a zone where another
laser system’s beams cross one another.
The interacting light waves, sometimes
doubled up by mirror systems, form a
tiny three-dimensional array of shimmering energy fields.
This is the lattice. Its standing waves
rise and fall but do not propagate. When
the fields’ energies are diagrammed,
they take on a pattern that looks a bit
like the hollows in egg cartons. These
nodes trap and hold the atoms — ideally
one atom per energy well — in perfectly
aligned ranks. The entire array of atoms
is levitated in a tiny near-vacuum about
100 micrometers across, roughly the
thickness of a page in a glossy magazine
like this one. Most important, the trapping lasers whose beams produce the lattice will be set to a “magic frequency” — a
recent breakthrough in the field — to grip
the atoms in place while not distorting
the shapes of their electron clouds.
All that is preamble to the key step. A
clock laser will, a bit faster than once per
second, illuminate the atoms, adjusting
itself as needed to match the frequency
at which they most easily absorb and
emit light. Lasers may be popularly con-
sidered the essence of precision optics
and purity of color. But at the esoteric
edge of the timekeeper’s craft, they are
too wobbly to keep time by themselves.
Thus the clock laser’s orderly light
waves are paced by the atomic metro-
nome — just as a drill sergeant keeps
troops in precise cadence.
