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Portrait of Bose–Einstein condensate

Science · Physics

Bose–Einstein condensate

Predicted 1924–25; first created 1995

Physics — condensed matter and atomic physics

A Bose–Einstein condensate is a state of matter formed when a dilute gas of bosons is cooled to within a whisker of absolute zero and a large fraction of the atoms collapse into the same lowest quantum state, so that the whole cloud behaves as a single quantum object with one shared wavefunction. It was predicted in 1924–25 after Satyendra Nath Bose sent Albert Einstein a paper on the statistics of photons, which Einstein translated, had published and then extended to atoms. Seventy years passed before laser cooling and evaporative cooling made it possible: in June 1995 Eric Cornell and Carl Wieman produced one from rubidium-87 at JILA in Boulder, and Wolfgang Ketterle did the same with sodium at MIT months later. The three shared the 2001 Nobel Prize in Physics. It is often called the fifth state of matter.

How to spot them

  • ▸Sometimes called the fifth state of matter, after solid, liquid, gas and plasma
  • ▸Made at about 170 nanokelvin — colder than anything in nature
  • ▸Thousands of atoms sharing one wavefunction, a single 'superatom'
  • ▸Named jointly for a Dacca physicist whose rejected paper was translated by his famous correspondent
  • ▸Used to slow a light pulse to the speed of a bicycle

Key facts

The fifth state of matter, after solid, liquid, gas and plasma

A cloud of atoms all in the same quantum ground state.

First made at JILA in Boulder on 5 June 1995 from about 2,000 rubidium-87 atoms (1995)

Eric Cornell and Carl Wieman cooled them to roughly 170 nanokelvin.

The medium in which Lene Hau slowed a light pulse to 17 metres per second (1999)

She later stopped light in one altogether and restarted it.

Predicted after a 1924 paper on photon statistics was translated into German by its recipient (1924)

The recipient extended the argument from light quanta to atoms with mass.

The 2001 Nobel Prize in Physics went to Cornell, Wieman and Ketterle for first making it (2001)

Ketterle produced one from sodium at MIT and made the first atom laser.

A state only bosons can form — fermions are forbidden it by the Pauli exclusion principle

Integer-spin particles may pile into one state; half-integer-spin ones may not.

Key ideas

  • As a gas cools, each atom's de Broglie wavelength grows; when the wavelengths become as long as the spacing between atoms, the atoms overlap and lose their separate identities.
  • Bosons positively like to share a state, so once the first few condense the rest follow in an avalanche — the same gregariousness that makes lasers and superfluids work.
  • Evaporative cooling finishes the job: the hottest atoms are let out of the magnetic trap, like blowing on soup, until only the coldest remain.

Quick-fire facts

  • •Named for Satyendra Nath Bose and Albert Einstein; Bose's 1924 paper had been rejected by a journal before Einstein translated it
  • •Paul Dirac named the 'boson' after Bose; Bose himself never won a Nobel Prize
  • •First produced on 5 June 1995 by Eric Cornell and Carl Wieman at JILA, Boulder, using rubidium-87
  • •Wolfgang Ketterle made one from sodium-23 at MIT in September 1995 and produced the first 'atom laser'
  • •The 2001 Nobel Prize in Physics was shared by Cornell, Wieman and Ketterle
  • •Temperatures involved are of order 100 nanokelvin — billionths of a degree above absolute zero
  • •Lene Hau used one to slow light to 17 m/s in 1999 and to bring it to a halt in 2001
  • •The Cold Atom Laboratory on the International Space Station has made condensates in orbit, where free fall lets them last far longer
  • •Superfluid helium-4 is a related but much denser and more strongly interacting case, which is why it took a dilute gas to show the effect cleanly

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