A quantum mechanical 'tune up'
for better measurement
exploiting the weird quantum behavior of atoms, physicists
at the Commerce Department's National Institute of Standards
and Technology (NIST) have demonstrated a new technique
that someday could be used to save weeks of measurements
needed to operate ultraprecise atomic clocks. The technique
also could be used to improve the precision of other
measurement processes such as spectroscopy.
The technique, described in today's issue of Science,
effectively turns atoms into better frequency sensors.
Eventually, the technique could help scientists measure
the ticks of an atomic clock faster and more accurately.
Just as a grandfather clock uses the regular swings
of a pendulum to count off each second of time, an atomic
clock produces billions of ticks per second by detecting
the regular oscillations of atoms. The trick to producing
extremely accurate atomic clocks is to measure this
frequency very precisely for a specific atom.
In the latest experiment, the scientists used very brief
pulses of ultraviolet light in a NIST-developed technique
to put three beryllium ions (charged atoms) into a special
quantum state called entanglement. In simple terms,
entanglement involves correlating the fates of two or
more atoms such that their behavior--in concert--is
very different from the independent actions of unentangled
atoms. One effect is that, once a measurement is made
on one atom, it becomes possible to predict the result
of a measurement on another. When applied to atoms in
an atomic clock, the effect is that n entangled atoms
will tick n times faster than the unentangled atoms.
Currently, scientists at NIST and other laboratories
make many thousands of measurements of the ticks of
unentangled atoms and average these results to get highly
accurate atomic clocks (currently keeping time to better
than one second in 40 million years).
If entangled atoms could be used in a clock, the same
or better results could be achieved with far fewer separate
measurements. The current experiment demonstrates this
new approach to precision measurement with three ions;
however, the researchers are looking forward to entangling
even more ions to take greater advantage of the technique.
"Even if we could implement this new technique
with only 10 ions, in the clock business that's really
important because the clocks must be averaged for weeks
and even months," says NIST physicist Dave Wineland,
leader of the research group. "The time needed
to do that would be reduced by a factor of 10."
In the experiment reported in Science, scientists entangled
the ions with two laser beams, using a technique originally
developed for quantum computing applications. The ions
are hit with another series of laser pulses and their
fluorescence (emitted light, which represents the ions'
quantum state) is measured for a specific period of
time. The duration of the steps, number of ions, and
other experimental conditions are controlled carefully
to ensure all the ions are in the same state when they
are measured, so that either all or none fluoresce,
which simplifies the readout.
The research was supported in part by the Advanced Research
and Development Activity and the National Security Agency.
As a non-regulatory agency of the U.S. Department of
Commerce's Technology Administration, NIST develops
and promotes measurement, standards and technology to
enhance productivity, facilitate trade and improve the
quality of life.
Institute of Standards and Technology (NIST)