Bose–Einstein Condensates

A Fifth State of Matter

Solid, liquid, gas, and plasma are familiar. A Bose–Einstein condensate is something else. When certain atoms are cooled to within a whisker of absolute zero, they stop behaving as separate particles. Their quantum waves overlap and merge until the entire cloud acts as a single object, a giant matter wave large enough to see with a camera.

Einstein predicted this in 1924 and 1925. See Bose–Einstein Statistics.

Seventy Years of Waiting

The prediction could not be tested with the technology of his century's first half. The atoms have to be made colder than anything in nature, colder than the depths of intergalactic space, while remaining a gas.

The method, when it came, used two stages. Lasers slow the atoms, and then the most energetic ones are allowed to escape from a magnetic trap, like steam carrying heat away from a cup of coffee. In June 1995, at a laboratory in Boulder, Colorado, Eric Cornell and Carl Wieman cooled a couple of thousand rubidium atoms to well below a millionth of a degree and saw them collapse into the single lowest quantum state. Wolfgang Ketterle at MIT did the same with sodium a few months later, with far more atoms. The three shared the Nobel Prize in 2001.

What It Is For

A condensate is to atoms what a laser is to light: a source in which every particle is in step. That makes it exquisitely sensitive to anything that disturbs it.

• Atom interferometers split a matter wave and recombine it, and can measure gravity, rotation, and acceleration with extreme precision. They are being developed for navigation without satellites and for surveying what lies underground.
• Quantum simulators arrange cold atoms in patterns of laser light to imitate the behavior of electrons in materials too complex to calculate.
• Fundamental tests use condensates to check whether different kinds of atoms fall at the same rate, the principle at the root of general relativity.

In Orbit

On Earth a condensate falls out of its trap in a fraction of a second. In free fall it can be studied for far longer. NASA's Cold Atom Laboratory, launched to the International Space Station in 2018, produced the first Bose–Einstein condensate in orbit that summer, and has since operated an atom interferometer in space.

How Direct Is the Link?

Direct. Einstein predicted the phenomenon from the mathematics, and it was found exactly as described. Reaching it took seven decades and several Nobel Prizes' worth of other people's ingenuity.