Bose-Einstein Condensate: What It Is and How It Is Made

In a Bose-Einstein condensate, or BEC, a gas of atoms sits only a few hundred billionths of a degree over absolute zero. There the atoms stop acting as individuals. They fall into the same quantum state and behave, on paper, like a single atom. That is why people sometimes call it a superatom. It also goes by the name “fifth state of matter”, which is a common label.

Our Recap on states of matter covers how matter moves between solid, liquid and gas. Here the change is of another kind: it concerns the quantum state the atoms are in.

Bosons and fermions

Particles with whole-number spin are called bosons, and those with half-integer spin are fermions. At low temperatures bosons tend to pile into the lowest-energy quantum state. Fermions keep away from each other. They cannot occupy the same state, so some of them have to take higher-energy ones.

Electrons in the shells of atoms are fermions, which accounts for the order of the periodic table.

Bose and Einstein, 1924

In 1924 the Indian physicist S. N. Bose did a statistical calculation on the kind of particles that would later be named after him, bosons, and more specifically on photons. He sent the work to Einstein, who saw its value, translated it into German and got it published.

Einstein went further. He extended the theory to particles with mass and, in two papers close together, predicted that if enough particles get close and slow down enough, they drop together into the lowest-energy state. That collective state is the condensate. NIST dates the prediction to 1924.

Why extreme cold is needed

The Royal Swedish Academy of Sciences explains it through the wavelength associated with atoms, the same idea as the matter waves of de Broglie. If a gas of cold atoms is dense enough, that wavelength becomes comparable to the distance between the atoms. The matter waves start to feel each other and coordinate their state. This is condensation.

At realistic densities the atoms have to move at only a few millimetres per second. That corresponds to temperatures around 100 nanokelvin, one tenth of a millionth of a kelvin, measured from absolute zero, which is -273.15 °C. The starting material is a dilute gas of alkali atoms.

Similar behaviour had already been seen in more complicated systems. Superconductors carry pairs of electrons and have no electrical resistance, and superfluids have no internal friction.

How a condensate is made

Two cooling stages run one after the other. The first is laser cooling, proposed in 1975 by Hänsch and Schawlow for neutral atoms: photons are absorbed only when they head toward the atom, and that slows it down. The second is evaporative cooling, which works in a purely magnetic trap. Wieman pointed to this route around 1990, with lasers in a magneto-optical trap first and then a magnetic trap for the evaporation.

Practical example: think of a cup of hot coffee cooling down. The fastest molecules leave the liquid, and the average energy of those that stay goes down. In the trap the same thing happens to atoms: the fastest ones are allowed to escape, and the gas left behind is colder.

5 June 1995

The JILA group, a joint institute of NIST and the University of Colorado Boulder, led by Cornell and Wieman, reached the condensation threshold on 5 June 1995 at 10:54. They began with rubidium gas at room temperature, slowed and caught in a trap of laser light. Laser cooling got the atoms down to roughly 10 millionths of a degree, still too warm for condensation, with some 10 million atoms in the trap. In the magnetic trap the atoms cooled further as the hottest ones were removed. One last improvement was needed: Cornell’s upgraded magnetic trap, with a rotating field, kept the coldest atoms from slipping out of the centre. The team started at about 170 nK and, by making the evaporation more efficient, ended with a pure condensate at 20 nK.

The result was a condensate of about 2,000 rubidium-87 atoms that lasted 15 to 20 seconds. On a video camera it showed up as a small sphere of rubidium about 20 microns across.

NIST notes that the search had lasted 71 years and that many physicists compared it to the quest for the Holy Grail.

What a condensate shows

Ketterle’s group was independent of the Colorado one and used sodium. It published its condensate four months after JILA’s. His condensates held hundreds of times more atoms, which made it possible to measure their properties.

One of these is interference. Two separate condensates expanding into each other form sharp fringes, the sign of coherent matter waves.

The 2001 Nobel Prize in Physics went to the realisation of Bose-Einstein condensation in dilute alkali gases and to early studies of how condensates behave. According to the Swedish Academy, the extreme conditions of a BEC in dilute gases are found nowhere else in the universe.