Bose-Einstein Condensate: The fifth state of matter

By Jesse Emspak Contributions from Tia Ghose, March 28, 2924. Livescience.com

A Bose-Einstein condensate is a strange form of matter in which extremely cold atoms demonstrate collective behavior and act like a single "super atom."

In the 1920s, Satyendra Nath Bose and Albert Einstein first conceived of a strange form of matter in which individual atoms clump together and behave like a single super atom. But scientists Eric A. Cornell and Carl E. Wieman only demonstrated it seven decades later, in ultracold rubidium atoms. Here, a series of images show, from left to right, increasing density as those rubidium atoms begin to form a BEC. (Image credit: NIST/JILA/CU-Boulder - NIST Image)

The Bose-Einstein condensate (BEC) is one of the five primary states of matter. In it, atoms reach such low energies that the rules of quantum mechanics dictate that they stop acting as individual atoms and behave like a single "super atom."

A Bose-Einstein condensate forms only when materials are cooled to within a hair of absolute zero. At that temperature the atoms are hardly moving relative to each other; they have almost no free energy to do so. The atoms then begin to clump together, and enter the same energy states. They become identical, from a physical point of view, and the whole group starts behaving as though it were a single atom.  

Gases, liquids, solids and plasmas have been studied for decades, if not centuries, but Bose-Einstein condensates weren't created in the laboratory until the 1990s. To make a Bose-Einstein condensate, you start with a cloud of diffuse gas. Many experiments start with atoms of rubidium. Then you cool it with lasers, using the beams to take energy away from the atoms. After that, to cool them further, scientists use evaporative cooling. 

"With a [Bose-Einstein condensate], you start from a disordered state, where kinetic energy is greater than potential energy," Xuedong Hu, a professor of physics at the University at Buffalo, told Live Science. "You cool it down, but it doesn't form a lattice like a solid." 

Instead, the atoms fall into the same quantum states, and can't be distinguished from one another. At that point the atoms start obeying what are called Bose-Einstein statistics, which are usually applied to particles you can't tell apart, such as photons, or light packets. 

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