Measuring quantum nature of atoms

 
How do you measure a stellar diameter or the diameter of a cloud of ultracold atoms at a temperature of one millionth of a degree above absolute zero? These questions do not seem to have anything in common. However, both may be answered by measuring the light or particles intensity with a sensitive detector.

To determine a stellar diameter
Stars appear to us as points of light (radio stars), but if they are large enough and not too distant, they show a seeming diameter. This seeming diameter is the small angle between the rays of light that are coming from the opposite stellar rims. If you know the distance to the star, the angle provides you immediately with the stellar diameters.
From the first half of the twentieth century onwards, astronomers were able to measure the angle by using interference, the well-known effect of rays of light that are able to reinforce but also to extinguish each other. In interference experiments the electricity fields of light are added up. These fields may have a positive sign, but also a negative one.

In the middle of the twentieth century researchers ran into the perturbing effects caused by turbulence in the earth’s atmosphere. Consequently, reliable interference measurements are difficult.

Exactly fifty years ago the astronomers Robert Hanbury Brown and Richard Twiss applied their knowledge in the field of radar that they acquired in World War II, to perceptible radiation and they discovered a new method to measure the seeming stellar diameters. They showed that it is also possible to observe the intensity of light (a standard to the number of light particles that arrives per unit of time) on two detectors from the very beginning, instead of measuring an interference pattern.
If you compare the intensities measured on two detectors that are sufficiently close together, as being a function of the time difference between two measurements, then it appears that more light particles per unit of time is being measured if the time difference between the two measurements follows closely. This was called the Hanbury Brown and Twiss (HBT) effect. It is also possible to calculate the seeming stellar diameters from the measurement of the HBT effect. This solved the problem of the turbulences in the earth’s atmosphere and made it possible to give a detailed definition of stellar diameters, for example, of the well-known and close by stars Sirius and Wega.

Cold atoms, therefore HBT effect
The discussion on the interpretation of the measurements by Hanbury Brown and Twiss was a firm one in the fifties and sixties of the twentieth century. The commotion had to do with the interpretation of the HBT effect on the image of either wave or particle. Quantum physics tells us that light as well as a wave looks like a bunch of light particles (photons). Scientists could easily understand the HBT effect described as ‘wave’. However, if light would be considered to be a bunch of photons, then it would all have been quite complicated.
The HBT effect as an image of photons being particles implies that the chance of measuring two photons shortly after each other, will increase if the two detectors are being put together closely enough.

In the case of one single detector that is small enough, you may notice that photons from ordinary thermal sources (such as a star), will arrive shortly after each other. How do photons know about each other when to hit a detector? After all, they interact not with each other but only with itself, as the famous physicist Paul Dirac had already observed before!. The discussion led to the development of quantum optics, the description of optical phenomena through quantum theory. Roy Glauber was awarded the Nobel Prize in Physics 2005 for this.

What has all this to do with ultracold atoms? Well, atoms may be described as either particles or as a wave, the famous dualism of wave and particles. This is hardly noticeable in everyday practice. The wavelength that is part of a particle with mass m and a velocity v, is inversely proportional to mv, of which the proportional factor is the Planck constant. This Planck constant is so small that the wave nature of atoms is noticeable only at extremely low velocities. However, low velocities correspond to low temperatures and so, ultracold atoms should also show the HBT effect. In 2005 French scientists demonstrated convincingly the HBT effect for waves of matter ( mass particles) by ultracold helium-4 atoms.

Helium-4 behaves like light
The nucleus of helium-4 consists of two protons and two neutrons. This means that helium-4, together with the two electron revolving around them, contains an even number of elementary particles; this is called a boson in jargon. As photons are also bosons, it was expected that helium-4 should behave like light. This actually appeared to be the case.
However, helium has a second isotope, helium-3, and only one neutron in the nucleus. In jargon this is called a fermion (an odd number of elementary particles). Although both helium isotopes differ only in one small detail (a neutron), every manual mentions that helium-3 atoms should behave differently. Practically, this means that helium-4 atoms are inclined to search for each other and that helium-3 atoms want to avoid each other.

Helium-3 and helium-4 behave differently
Tom Jeltes and John McNamara, researchers at the FOM-foundation, and Wim Hogervorst and Wim Vassen, researchers of the Laser Centre at the Vrije Universiteit (LCVU) in Amsterdam have been able to demonstrate this phenomenon for the first time, together with French fellow-researchers.

The research has been executed in Amsterdam; the Frenchmen brought their extremely sensitive detector. With the aid of the laser cooling method the researchers cooled atoms of helium-4 or helium-3 gas to a temperature of one millionth degrees above absolute zero. Then, the gas has been trapped in a geometry of magnetic fields that take care of the gas remaining in its place; it will form a cloud that is smaller than 0,1 millimetre in diameter, of which the small diameter will finally determines the success of the measurement. When the magnetic field is switched off, under the influence of gravity the cloud (containing about 100,000 atoms) just falls on the detector that is 63 centimetres below. The hour of arrival of the single atoms is measured at about 10,000 points to this detector. Helium-4 shows a result similar to that of light measured by Hanbury Brown and Twiss: the atoms will arrive shortly after each other at an increased probability. This had already been demonstrated by the French team one year earlier. Something special was the fact that an opposite behaviour was observed for helium-3. It was found that the atoms avoid each other. This behaviour of either bunching or antibunching is strongly dependent on the temperature and diameter of the cloud in the magnetic trap. This has been convincingly and quantitavely demonstrated for the first time. But is this not a strange thing? There is not an interaction between Helium-3 atoms and yet, they avoid each other. This is absolutely a quantum effect and measured on distances of one millimetre, which is macroscopically large for a quantum phenomenon!

New experiments are now possible
This experiment demonstrates quantum effects at a macroscopic scale and makes it possible to test theories from manuals among utter conditions without approximation. Cold atoms have shown their potentiality before, witness the observation by Bose-Einstein condensation (bosonic, awarded the Nobel Prize in Physics 2001) and measuring the frequencies at an extremely sensitive level (Nobel Prize in Physics 2005. The HBT effect (bosonic as well as fermionic) can now be used further in more exotic systems, such as on atoms that are trapped in so-called optical grids. The atoms have been evenly ordered in distances of microns (one thousandth of a millimetre); in this way artificial grids arise at a distance that is a 10,000 times larger than in solid matter. Physicists try to gain a better understanding of superfluidity and superconductivity (flow of fluids without friction or electrons without electrical resistance, respectively). Both phenomena are notoriously hard to understand because of the powerful interaction in solid matter.

For more information, please contact Dr. Wim Vassen, phone: +31 (0)20 598 79 49.

More information:
http://www.nat.vu.nl/~wim/Cold_Atoms/HBT.html   and http://www.nat.vu.nl/~wim/Cold_Atoms/HBT_NL.html