Dr Bill Phillips of the National Institute of Standards and Technology, USA gave a joint talk with Professor Keith Burnett FRS of the University of Oxford and they discussed the links between an intriguing state of matter known as the Bose-Einstein condensate (BEC) and precision experiments.
The BEC was first predicted in 1924 by Albert Einstein. He suggested that a
dense enough gas composed of bosons (atoms that have an even number of
protons and neutrons in their nuclei and so "zero spin"), cooled to a low
enough temperature would undergo a phase change analogous to the change of
phase seen when water vapour condenses on a cold window, for instance. The
phase transition that forms a BEC is the switch from a cold, but energetic,
gas to one in which most of the gas particles drop into the lowest energy
form, or ground state.
Phillips emphasised that cold atoms, produced by laser cooling, are already
a standard tool of precision measurement. For example, laser-cooled atomic
fountain clocks, such as the caesium clocks discussed by Christophe Salomon
(see Timing is everything) now give us the best definition of the second.
However, there might be exceptional circumstances in which still colder
atomic samples are needed. For example, Phillips explained, clocks launched
into space operate in a microgravity environment and so could provide
scientists with longer observation times than are possible on earth but
would require colder atoms. BECs, exist, he explained, for many such
practical purposes, at zero temperature, they are "really, really cold",
and, he added, while cold is good, colder is better. BECs are therefore an
attractive source of such atoms for precision measurements.
Techniques such as "photon recoil measurements to determine h/m (Planck
divided by mass) might also benefit from BEC samples. In addition, added
Burnett, BECs offer scientists a unique opportunity to carry out
measurements that are below the level of random noise in the system,
so-called sub-shot noise. They can do this by exploiting the properties of
"non-classical" or "squeezed" atomic samples, which are prepared by
transferring a BEC into a special system known as an optical lattice
potential, which limits the behaviour of the atoms in the condensate in such
a way that the noise fluctuations are reduced and interactions, or
interference, between the atoms can be measured. Such conditions give much
higher resolution for determination of fundamental physical properties.
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