ScienceWise - Sep/Oct 2006

Probing Superatoms

Article Illustration
Researchers Dr Robert Dall (left) with Dr Andrew Truscott with the magnetic gradient coils used to slow atoms en-route to the BEC trap (right).
Article Illustration
The spatial distribution of atoms during the formation of a BEC. As the cloud cools it forms a single quantum superatom (lower image).

We are used to new technology such as lasers appearing on the market quite suddenly and seemingly from nowhere. However, in reality, the science that underpins such devices has often been developed slowly over many decades. Back in the 1960s the first lasers were exotic devices from the realms of theoretical physics that had seemingly little practical application. Today they form the backbone of telecommunications and data storage and have thousands of other uses in devices that would have been once unimaginable.

Another conceptually new technology only just now making its debut is that of the Bose Einstein condensate (BEC). Originally proposed by Satyendra Nath Bose and Albert Einstein way back in the 1920s, it wasn’t until seventy years later that scientists were able to actually create the world’s first Bose Einstein-Condensate in the laboratory.

Essentially the theory is that if you make a cloud of gas atoms cold enough, they will slow down to the point that they all begin to occupy the same quantum state. This means that the atoms become identical in the same way individual photons in a laser beam become identical. Collectively, the atoms in the condensate form an entirely new phase of matter not found naturally anywhere in the universe. Even the coldest depths of space are a billion times too hot for a BEC to exist.

When we try to imagine atoms, most of us recall the billiard ball diagrams of old textbooks, a little red lumpy nucleus with blue electron balls spinning round them. However, this is not a very accurate visual picture of the universe at the quantum level. It would be closer to the truth to imagine the particles as little clouds of fog, which were very hard to pinpoint to any precise location in space. The theoretical physics of BECs is very complex but essentially a BEC forms when many atoms condense into a single bigger quantum fog. They loose their individual identity in the same way raindrops falling into a bucket do. They become a sort of super raindrop, and in the same way, scientists sometimes refer to the BEC as a superatom - an indistinguishable amalgam of many atoms.

These superatoms have many strange properties that promise future technologies beyond anything we could currently imagine. However, in order to unlock their technological potential, scientists need to better understand BECs and especially the process of their formation. Studying the formation process in conventional ground state alkali atom BECs is complicated by the inability to make measurements on individual constituent atoms. Measurements on such systems are limited to averaging over the quantum ensemble.

To get around this, scientists at ANU have recently become one of only four groups in the world to achieve a BEC of metastable helium atoms. This particular atomic configuration of helium has vastly more energy than the ground state and is sufficiently long lived to allow experiments to be performed. The special thing about these metastable excited atoms is that even though they are almost stationary and thus very cold, inside them, stored in the configuration of their metastable state, is a billion billion times as much energy as the same atom would have in its ground state. The practical upshot of this is that when such an atom contacts a metal probe it releases its energy and creates an electron, which can be detected by sensitive electronics. In this way atoms can be detected individually and since atoms in a BEC cloud are all quantum identical, probing one yields a perfect snapshot of the others. The ANU team is hopeful that this newly functioning BEC apparatus will yield vital clues to the mechanism of BEC formation.

The history of physics is full of examples of strange and exotic phenomena that having been developed out of pure curiosity have gone on to spawn unimaginable technological advances. Lasers, X-rays, and transistors all belong to this family and BECs may well be its newest member.

(This research is funded by the ARC Centre of Excellence for Quantum Atom Optics)

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