Using a cloud of helium atoms, an Auckland scientist, part of an international team, has tested the
validity of Quantum Electrodynamics, one of the most fundamental physics theories that describes
the structure of atoms, to twelve-digit accuracy. Working in a laboratory at the Free University in
Amsterdam (the Netherlands), the team cooled helium gas to a temperature just a millionth of a
degree above absolute zero, captured it in the focus of a laser beam, and illuminated it with a second
laser beam with an extremely well-defined colour. Their measurements are a factor of a thousand
more accurate than theory can predict. Even the size of the nucleus of the helium atom could be
determined. The team has published the results in Friday's edition of the keynote journal Science
Magazine.
A schematic representation of the experimental setup: The ultra-cold atoms are initially captured
in a magnetic field (green coils)and then in the intersection of two laser beams (red). The
absorption is measured with a part of the laser light that has a slightly different colour (AOM). A
small part of the laser light is transmitted to a different laboratory by an optical fibre for an exact
determination of its colour.
Theory of the helium atom
Most tests of the fundamental physical laws take place in large particle accelerators, such as CERN
in Geneva, at very high energies. Recent developments in the field of ultra-stable lasers and
accurate atom clocks enable these kind of tests in small-scale experiments, as extreme accuracy can
be achieved by these technologies. Quantum Electrodynamics predicts the exact colours of light that
are absorbed by the helium atom with great accuracy.
Unique laser equipment
In the infrared part of the helium spectrum the theory predicts an extremely weak spectral line. This
line is a hundred thousand billion times weaker than “normal” lines in helium, and has hence never
been observed. It requires a large amount of laser light, that can illuminate the atoms at a welldefined colour for many seconds. This is only possible with specialised lasers, which are referenced
to an atomic clock. To achieve the long interaction times between atoms and laser light, the atoms
have to be brought to a standstill.
Ultracold
The predicted line cannot be observed in a room temperature gas, as atoms move rapidly in every
direction. Helium atoms were therefore cooled by laser cooling to a millionth of a degree above
absolute zero, and subsequently captured in the intersection of two strongly focused laser beams.
The team then illuminated the atoms with high intensity light from an ultra-stable laser, and
precisely measured the absorption. They did this for both isotopes of helium, helium-3 and helium4, which at these temperatures behave more like waves than like particles. The measurements were
carried out with a twelve-digit accuracy, a factor of a thousand more accurate than theory can
predict at this time, but in agreement with it. This means there is work to be done for the theoretical
physicists. From the difference in the measurements for the two isotopes, the size of the helium-3
nucleus, which has one fewer neutrons than helium-4 (the α-particle), could be determined with an
accuracy of 4 attometres (one attometre is one billionth of one billionth of a metre). This is more
accurate than can be achieved with particle accelerators.
Note:
The report “Frequency Metrology in Quantum Degenerate Helium: Direct Measurement of the 2 3S1
→2 1S1 Transition” appeared in the July 8th edition of Science Magazine. Authors of the paper are
Rob van Rooij (Netherlands Foundation for Fundamental Research (FOM)), Joe Borbely (FOM),
Juliette Simonet (Ecole Normale Superieure, Paris), Maarten Hoogerland (University of Auckland),
Kjeld Eikema (Free University (VU) Amsterdam), Roel Rozendaal (FOM) and Wim Vassen (VU).
Images can be downloaded from:
http://www.nat.vu.nl/~wim/Cold_Atoms/metrologie2011.html
http://www.nat.vu.nl/~wim/Cold_Atoms/cold2.html