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Atoms That Behave Like Light
October 1, 2010   
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A group of Polish scientists are helping develop a new field of research in physics called quantum-atom optics. The aim is to explore unusual physical phenomena during which atoms start behaving like light.

The Polish physicists are part of an international research group at the Institute of Optics in Paris that is conducting research on collisions occurring in matter cooled to exotic states called Bose-Einstein condensates. The research has shown that, under experimental conditions, atoms of helium reveal their quantum nature and start displaying features typical of light.

The Polish scientists taking part in the experiment are Prof. Marek Trippenbach, and Jan Chwedeńczuk, Ph.D., from the University of Warsaw’s Institute of Theoretical Physics; Piotr Deuar, Ph.D., from the Polish Academy of Sciences’ Institute of Physics; and Paweł Ziń, Ph.D., from the Andrzej Sołtan Institute for Nuclear Studies in Âwierk near Warsaw.

Altered states

“Working in this research group, we are trying to develop a theoretical model to account for phenomena observed by our French colleagues and involving atoms colliding in Bose-Einstein condensates,” says Ziń.

Bose-Einstein condensates are sometimes described as a new state of matter, in addition to plasma and the three states of matter which occur on Earth: solid, liquid and gas. Bose-Einstein condensates were predicted as a theory by physicists Satyendra Nath Bose and Albert Einstein in 1924, but it was only in 1995 that the first condensate was produced in a laboratory by a group of scientists from the University of Colorado at Boulder. The scientists used a laser to cool a gas of rubidium atoms to 170 nanokelvin (a nanokelvin is one-billionth of a Kelvin). A similar condensate was produced in Poland in 2007 at the National Laboratory of Atomic, Molecular and Optical Physics in the north-central city of Toruń.

It is easier to understand the physical nature of Bose-Einstein condensates if they are compared to light, Ziń says. The first physical description of light was provided by Maxwell’s equations, which treated light as an electromagnetic wave. It was only later discovered that light could be also regarded as a packet of particles called photons.
Roughly the same goes for the Bose-Einstein condensate, Ziń says, but in this case the discoveries were made in the reverse order. It turns out that, at ultra-low temperatures, atoms of gas spontaneously assume a state where they can be collectively described in terms of the bosonic field.

The term “condensate” brings to mind something thick, but in reality Bose-Einstein condensates are clouds of gas millions of times thinner than air. “The term does not refer to the condensation of matter, but the fact that all atoms ‘condense’ into the same quantum state, which means that each atom behaves identically,” says Ziń.

Colliding condensates

The international team of researchers at the Institute of Optics in Paris, headed by French physicist Alain Aspect, is studying collisions in Bose-Einstein condensates. The condensates in Paris are produced from metastable atomic helium cooled to temperatures of millionth of a Kelvin. The metastability of a helium atom means an excited state with certain stability. The energy of the metastable excited state is higher than the lowest energy of the ground state permitted by the laws of physics. A condensate cloud several micrometers in size is produced in a vacuum chamber and is subsequently released from a magnetic trap. Then, laser impulses push two opposite sections of the cloud toward each other causing pairs of atoms to collide. The atoms are knocked out of the cloud and several hundred milliseconds later they fall onto an array of micro-channel plate (MCP) detectors. The detectors record single atoms with a probability of 10 percent. The detector array takes a “photograph” of the distribution of the atoms at a given moment. With a series of such “photographs,” physicists can recreate the spatial structure of colliding clouds and see where the atoms traveled.

“We are now studying the correlations between colliding atoms,” says Ziń. “The course of the collisions seems to comply with the laws of classical physics, but the detectors show that the colliding atoms hit such spots on the array where their presence can only be accounted for with quantum phenomena.”

Quantum reality

The world of quantum objects is formed by bosons and fermions, which are two classes described with different laws of statistics. Fermions, which include electrons, seek to retain their individuality whenever they are in the same quantum state. Bosons, which include photons and helium atoms studied in the Paris experiment, manifest a tendency to form groups sharing an identical quantum state.

Taking physicists by surprise, astronomers Robert Hanbury Brown and Richard Q. Twiss found in 1956 that photons which were chaotically emitted by thermal sources such as sodium vapor lamps or stars reached detectors not as homogenous beams, but in groups. The phenomenon, called the HBT effect (after Hanbury Brown and Twiss), has been found not only in photons, but also other bosons and since metastable helium atoms too are bosons, the HBT effect is observed during collisions in Bose-Einstein condensates.

The long-term goal of the Paris experiments is to build pairs of single atoms occupying a precisely defined quantum state. Sources of this kind would become atomic counterparts of sources of single photons. Over the past several years, such sources have revolutionized quantum optics, laying the groundwork for research on new physical phenomena and finding application in commercially available quantum cryptography, which is a data transmission method in which the confidentiality of messages is 100 percent guaranteed by laws of physics.

In the future, researchers will be able to use quantum-atom optics to test the most fundamental quantum features of reality, such as locality of interaction and the influence of observation on observed objects, according to Prof. Grzegorz Wrochna, director of the Andrzej Sołtan Institute for Nuclear Studies.

“We do not expect that the knowledge we are acquiring today will be used in practice anytime soon,” says Wrochna. “But we are confident that this knowledge will be used by our children and grandchildren and benefit them enormously, just like we use the results of our predecessors’ work in thousands of devices, frequently in ways they could not even imagine all those decades ago.”
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