Bose-Einstein condensate

The following report is from an August 1995 article in the Encarta Yearbook.

Physicists Condense Supercooled Atoms, Forming New State of Matter

A team of Colorado physicists has cooled atoms of gas to a temperature so low that the particles entered a merged state, known as a “Bose-Einstein condensate.” This phenomenon was first predicted about 70 years ago by the theories of German-born American physicist Albert Einstein and Indian physicist Satyendra Nath Bose. The condensed particles are considered a new state of matter, different from the common states of matter—gas, liquid, and solid—and from plasma, a high temperature, ionized form of matter that is found in the sun and other stars.

Physicists have great expectations for the application of this discovery. Because the condensate essentially behaves like one atom even though it is made up of thousands, investigators should be able to measure interactions at the atomic and subatomic level that were previously extremely difficult, if not impossible, to study quantitatively.

The condensate was detected June 5 by a Colorado team led by National Institutes of Standards and Technology physicist Eric Cornell and University of Colorado physicist Carl Wieman. Their discovery was reported in the journal Science on July 14. Cornell and Wieman formed their condensate from rubidium gas.

Several groups of physicists, including the teams in Texas and Colorado and a group at the Massachusetts Institute of Technology, have been working to form pure condensate in recent years. The goal of the investigations has been to create a pure chunk of condensate out of atoms in an inert medium, such as a diffuse, nonreactive gas. The effort began when methods of cooling and trapping became refined enough that it seemed possible to reach the required conditions of temperature and density.

The Colorado team used two techniques: first laser cooling and then evaporative cooling. The laser technique used laser light whose frequency was carefully tuned to interact with the rubidium atoms and gently reduce their speeds. A number of lasers were aimed at the gas to slow the motion of the atoms in different directions.

The Colorado physicists then switched to evaporative cooling. In this method, the gas is “trapped” by a magnetic field that dwindles to zero at its center. Atoms that are moving wander out of the field, while the coldest atoms cluster at the center. Because a few very cold atoms could still escape at the zero field point of the trap, the physicists perfected their system by adding a second slowly circling magnetic field so that the zero point moved, not giving the atoms the chance to escape through it.

Physicists will now begin to explore the properties of the condensate and see what other materials they can use to form it. One unusual characteristic of the condensate is that it is composed of atoms that have lost their individual identities. This is analogous to laser light, which is composed of light particles, or photons, that similarly have become indistinguishable and all behave in exactly the same manner. The laser has found a myriad of uses both in practical applications and in theoretical research, and the Bose-Einstein condensate may turn out to be just as important. Some scientists speculate that if a condensate can be readily produced and sustained, it could be used to miniaturize and speed up computer components to a scale and quickness not possible before.

The prediction that a merged form of matter will emerge at extremely low temperatures is based on a number of aspects of the quantum theory. This theory governs the interaction of particles on a subatomic scale. The basic principle of quantum theory is that particles can only exist in certain discrete energy states.

The exact “quantum state” of a particle takes into consideration such factors as the position of the particle and its “spin,” which can only have certain discrete values. A particle's spin categorizes it as either a boson or a fermion. Those two groups of particles behave according to different sets of statistical rules. Bosons have spins that are a constant number multiplied by an integer (e.g., 0, 1, 2, 3). Fermions have spins that are that same constant multiplied by an odd half-integer (1/2, 3/2, 5/2, etc.). Examples of fermions are the protons and neutrons that make up an atom's nucleus, and electrons.

Composite particles, such as nuclei and atoms, are classified as bosons or fermions based on the sum of the spins of their constituent particles. For instance, an isotope of helium called helium-4 turns out to be a bose particle. Helium-4 is made up of six fermi particles: two electrons orbiting a nucleus made up of two protons and two neutrons. Adding up six odd half-integers will yield a whole integer, making helium-4 a boson. The atoms of rubidium used in the Colorado experiment are bose particles as well. Only bose atoms may form a condensate, but they do so only at a sufficiently low temperature and high density.

At their lab in Colorado, Cornell and Wieman cooled a rubidium gas down to a temperature as close to absolute zero, the temperature at which particles stop moving, as they could get. The slower the particles, the lower their momentum. In essence, the cooling brought the momentum of the gas particles closer and closer to precisely zero, as the temperature decreased to within a few billionths of a degree Kelvin. (Kelvin degrees are on the scale of degrees Celsius, but zero Kelvin is absolute zero, while zero Celsius is the freezing point of water.)

As the temperature, and thus the momentum, of the gas particles dropped to an infinitesimal amount, the possible locations of the atom at any given moment increased proportionally. The goal of the experiment was to keep the gas atoms packed together closely enough that during this process—as their momentum got lower and lower, and their wavelengths got larger and larger—their waves would begin to overlap. This interplay of position and movement in three dimensions with the relative distances between particles is known as the phase-space density and is the key factor in forming a condensate.

In essence, the momentum of the atoms would become so precisely pinpointed (near zero) that their position would become less and less certain and there would be a relatively large amount of space that would define each atom's position. As the atoms slowed to almost a stop, their positions became so fuzzy that each atom came to occupy the same position as every other atom, losing their individual identity. This odd phenomenon is a Bose-Einstein condensate.

As their experimental conditions neared the realm of Bose-Einstein condensation, Cornell and Wieman noticed an abrupt rise in the peak density of their sample, a type of discontinuity that strongly indicates a phase transition. The Colorado physicists estimated that after progressive evaporative cooling of the rubidium, they were left with a nugget of about 2,000 atoms of pure condensate. Cornell and Wieman then released the atoms from the “trap” in which they had been cooling and sent a pulse of laser light at the condensate, basically blowing it apart. They recorded an image of the expanding cloud of atoms. Prior to the light pulse, when the density dropped after the atoms were released, the physicists believed the temperature of the condensate fell to an amazing frigidity of 20 nanoKelvins (20 billionths of one degree above absolute zero).

The image showed a larger, expanding sphere of particles with a smaller, more concentrated elliptical-looking center. Cornell and Wieman observed that when a gas is constrained and then released (in an extreme example, as in a bomb), thermodynamics specifies that it will expand outward equally in all directions regardless of the shape in which it had been contained. This occurs because the particles in that gas, even if the gas was very cold, were moving in all different directions with various energies when the gas was pushed outwards.

This rule of uniform expansion does not hold for a Bose-Einstein condensate. Because the particles were all acting in exactly the same manner at the time of the light pulse, their expansion should give some indication of the shape of the space they had previously inhabited. The uneven, elliptical-looking clump of atoms in the center of the image recorded by Cornell and Wieman thus gave further proof that a condensate had formed.

Bose-Einstein characteristics have been observed in other systems, specifically, in superfluid liquid helium-4 and in superconductors. It is believed that liquid helium-4 at a sufficiently low temperature is composed of two components mixed together, the colder of which is a Bose-Einstein condensate. Liquid helium-4, which at very low temperatures is also a superconductor of heat, behaves in dramatic ways, trickling up the sides of containers and rising in fountains.

Electrical superconductors are also boson-related phenomena. In superconductors, which are also formed by supercooling, electrical resistance disappears. In this case it is the electrons within a substance's atoms, rather than the atoms themselves, that condense. The electrons pair up, together forming a particle of zero spin. These paired electrons merge into an overall substance that flows freely through the superconductor, offering no resistance to electric current. Thus, once initiated, a current can flow indefinitely in a superconductor.