Ultracold helium gas tests the boundaries of physics
In a small cloud of helium atoms, physicists from FOM and the LaserLaB of VU University Amsterdam have tested the theory of quantum electrodynamics to an accuracy of 12 significant figures. Quantum electrodynamics describes the structure of atoms and is one of the most fundamental theories of physics. For this experiment, the researchers cooled helium gas to one millionth of a degree above absolute zero. They then captured it in the focal point of a laser beam and subsequently irradiated it for a long period of time with a second laser beam of a very precisely determined colour. The measurement they made is 1000 times more accurate than the theory can currently predict. With the result, the researchers could even determine the size of a helium atom nucleus. The researcher's findings were published on 8 July in the renowned journal Science.
Theory of the helium atom
The basic laws of physics are often tested in large particle accelerators, such as that of CERN in Geneva, using extremely high levels of energy. Recent developments in the area of ultrastable lasers and accurate atomic clocks have made it possible to perform such tests at a small scale due to the extreme precision with which observations in such a set-up can be made. Quantum electrodynamics predicts the exact colours of light that can be absorbed by a helium atom with unprecedented accuracy.
Unique laser equipment
In the infrared part of the helium spectrum, the theory predicts an extremely weak spectral line. This line is one-hundred thousand billion times weaker than normal lines in helium and had therefore never been observed. Such an observation requires a large quantity of laser light with a very precisely set colour that must be shone on the atoms for many seconds. This is only possible if the helium atoms are brought to a standstill in an experimental set-up that uses specialised lasers linked to an atomic clock.
Ultracold
The predicted line cannot be observed in a gas at room temperature because then the atoms move quickly in all directions. Lasers were therefore used to cool the helium atoms to one millionth of a degree above absolute zero. They were then captured in the intersection of two highly focused laser beams. Researchers then shone a lot of light from the ultrastable laser onto the atoms and very accurately measured the absorption. They did that for both isotopes of helium, helium-3 and helium-4, which at these temperatures did not behave as distinct particles but as waves. The measurements were made to an accuracy of 12 significant figures. This is 1000 times more accurate than the theory can currently calculate but is, nevertheless, in line with it. Theoretical physicists have therefore got some work to do. From the difference in the measurements between the two isotopes, the size of the nucleus of the helium-3 atom, which has one less neutron than the helium–4 atom (the alpha-particle), could be determined to an accuracy of 4 attometres (1 attometre is a billionth of a billionth of a metre). This is more accurate than measurements that could be achieved with particle accelerators.
Reference
The article Frequency metrology in quantum degenerate helium: direct measurement of the 2 3S1 → 2 1S0 transition was published on 8 July 2011 in Science. The authors of the article are: Rob van Rooij (FOM), Joe Borbely (FOM), Juliette Simonet (ENS Paris), Maarten Hoogerland (Auckland, New Zealand), Kjeld Eikema (VU), Roel Rozendaal (FOM) and Wim Vassen (VU).
Photo material
You can download photo material from:
http://www.nat.vu.nl/~wim/Cold_Atoms/metrologie2011.html and http://www.nat.vu.nl/~wim/Cold_Atoms/cold2.html
Further information
For further information please contact:
Dr Wim Vassen, VU University Amsterdam, +31 20 598 79 49;
Press Information Department VU, +31 20 598 56 66;
Communication Department FOM, +31 30 600 12 18.