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Positronium is a short-lived system in which an electron and its anti-particle are bound together. In 2007, physicists at the University of California, Riverside created molecular positronium, a brand-new substance, in the laboratory. Now they have succeeded in isolating for the first time a sample of spin polarized positronium atoms.

The ultra-high vacuum target chamber used in the experiment. Photo credit: David Cassidy, UC Riverside.

Study results appear this week in the journal Physical Review Letters.

Spin is a fundamental and intrinsic property of an electron, and refers to the electron's angular momentum. Spin polarised atoms are atoms that are all in the same spin state. A collection of spin polarized positronium atoms is needed to make a special form of matter, called the Bose-Einstein condensate (BEC). The BEC, predicted in 1924 and created in 1995, allows scientists to study atoms in a unique manner.

''We achieved our result by increasing the density of the positronium atoms in our lab experiment,'' said David Cassidy, the lead author of the research paper and an assistant researcher working in the laboratory of Allen Mills, a professor of physics. ''At such a high density, positronium atoms get annihilated simply by interacting with each other. But it turns out that not all the positronium atoms get annihilated under these conditions.''

Cassidy explained that positronium atoms come in two types - say, an up type and a down type. The positronium atoms are only annihilated when an up type meets a down type. Two atoms of the same type do not affect each other.