Professor Christophe Salomon of the Laboratoire Kastler Brossel, ENS,
France, explained how cooling atoms to close to absolute zero allows
scientists to observe them for relatively long periods of time and so use
them to make high-precision measurements that can then be used to more
precisely define the unit of time itself, the second.
The second is the SI unit of time and has been defined in many different
ways since humans began splitting their days into smaller units. We began by
measuring time according to the position of the sun, essentially the period
of rotation of the earth. However, the earth's rotation is far too variable
for modern science, which requires accuracy and stability of the order of
billionths of a second and smaller to make everything from particle physics
experiments to the latest multimedia games device work on time.
In the 1950s, the atomic clock achieved unprecedented split second accuracy,
but today, physicists are using the successors to this device which are more
accurate by 100 times again. A frequency standard based on microwave
electromagnetic radiation exploits the energy difference when exciting an
atom from its ground to an excited state. This transition occurs in caesium
or rubidium which have been cooled using a laser to within a thousandth of a
thousandth of a degree above absolute zero (0.000001).
Currently, these devices operate at the limit of their capabilities accurate
to less than one second every 50 million years. Theoretically they could be
no more accurate because they operate at the limit of "quantum noise" the
random fluctuations set by the quantum measurement process.
The increasing accuracy with which the atomic transitions from the ground
state to their "excited" state can be measured, mean that ultimately a new
definition of the second will become available. There is no need to redefine
the second now, says Salomon, but once optical clocks surpass the caesium
clocks, which will occur in the future, will the definition of the second be
changed.
Salomon and his colleagues are undertaking tests of fundamental physical
laws using these ultra-stable clocks. By comparing different types of atomic
clock that work on different physical principles they hope to obtain a
clearer picture of the time variation of the fundamental constants of
physics, such as the fine structure constant, ?, discussed by Professor
Barrow (see "Inconstant constants". They also hope to installing an
ultra-stable cold atom clock in space as part of the PHARAO/ACES project
which will launch in 2009. This will also allow the researchers to carry out
much-improved tests of the theory of general relativity. For instance,
experiments to measure the red-shift of distant galaxies and other
astronomical objects, a phenomenon predicted by Einstein's gravitational
theory, will be possible at a much greater accuracy than ever before,
allowing precision to the one part per million level.
Finally, Salomon discussed the prospect of laser-cooled atomic clocks that
would be even more accurate. The newer devices would operate in the optical
domain and have a frequency stability of one in a million, million, million.
Researchers at NPL, NIST, PTB , Paris observatory, Max Planck institute,
Tokyo university, NRC are working on optical clocks. Optical clocks will be
better clocks because they "tick" much faster than microwave clocks, by a
factor 10000. It would thus be easier to detect a small change in the period
of such a clock. Today, optical clocks have an accuracy a factor 3 less than
caesium clocks but soon in the future they will outperform microwave clocks.
Read on...
Combining the constants