matter in the form of a gas of atoms, molecules, or elementary particles that have been so chilled that their motion is virtually halted and as a consequence they lose their separate identities and merge into a single entity. A Bose-Einstein condensate, the fifth state of matter, is formed at low temperatures when a significant number of the elementary particles classified as bosons (see Bose-Einstein statistics) collapse into the same quantum state. A similar condensate that consists of fermions (see Fermi-Dirac statistics) instead of bosons is known as a fermionic condensate, the sixth state of matter.
Such condensates were predicted by Albert Einstein in 1924 based on the system of quantum statistics formulated by the Indian mathematician Satyendra Nath Bose. Quantum theory asserts that atoms and other elementary particles can be thought of as waves. Einstein proposed that as atoms approach absolute zero (−273.15 degrees Celsius), the waves expand in inverse proportion to their momentum until they fall into the same quantum state and finally overlap, essentially behaving like a single atom. The phenomenon could not be observed, however, until techniques were developed to reduce temperatures to within 20 billionths of a degree above absolute zero. In 1995, Eric A. Cornell and Carl E. Wieman isolated a rubidium Bose-Einstein condensate under laboratory conditions; they shared the 2001 Nobel Prize in physics with Wolfgang Ketterle for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates.
A fermionic condensate is far more difficult to achieve because the Pauli exclusion principle prohibits two or more fermions from occupying the same quantum state. In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer suggested that electrons, which are fermions, could form what are now known as Cooper pairs, which act like bosons; such pairings might make a fermionic condensate possible. Murray Holland much later suggested that fermions could pair up at higher temperatures by subjecting them to a magnetic field. In 2003, Deborah Jin and Rudolf Grimm were able to get fermionic atoms to bond together to form molecular bosons and thus form a Bose-Einstein condensate, but not a fermionic condensate. Later that year, applying a time-varying magnetic field to potassium atoms, Jin achieved Cooper pairings and the subsequent formation of a fermionic condensate.
It is believed that these state of matter have never existed naturally anywhere in the universe, since the low temperatures required for their existence cannot be found, even in outer space. Condensates may be useful in the study of superconductivity and superfluidity and in refining measurements of time and distance.