In our daily experience, most of us deal with three phases of matter: solid, liquid, and gas. A fourth, high-energy phase of matter, plasma, occurs in high energy processes as near as a fire or as far away as the core of a star. For decades, the existence of a fifth, low-energy form of matter, known as Bose-Einstein Condensates (BECs), was only a theoretical possibility. In 2001, the Nobel Prize for Physics went to Eric Cornell, Wolfgang Ketterle, and Carl Wieman, who used lasers, magnets, and evaporative cooling to bring about this fascinating new phase of matter.
BECs have strange properties with many possible applications in future technologies. They can slow light down to the residential speed limit, flow without friction, and demonstrate the weirdest elements of quantum mechanics on a scale anyone can see. They are effectively superatoms, groups of atoms that behave as one.
The theory of BECs was developed by Satyendra Nath Bose and Albert Einstein in the early 1920s. Bose combined his work in thermodynamics and statistical mechanics with the quantum mechanical theories that were being developed, and Einstein carried the work to its natural conclusions and brought it to the public eye. At the time, none of the necessary technology was available to make BECs in the lab: cryonics were extremely limited, and the first laser wasn't even built until 1960. The fine control allowed by modern computers was also a prerequisite. Because of all of these technological hurdles, it wasn't until 1995 that experimenters were able to force rubidium atoms to form this type of condensate.
Phases of Matter
We can distinguish among the phases of matter in several ways. On the most elementary level, solids have both fixed volume and fixed shape; liquids have fixed volume, but not fixed shape; and gases have neither. Solids have stronger intermolecular bond structure than their corresponding liquids, which in turn have stronger intermolecular bond structure than gases. We can also differentiate between phases of matter by considering energy levels. Solids have the lowest energy levels (corresponding with the lowest temperatures), while liquids and gases have increasingly higher levels. At the top end of this scale, we can add plasmas, which are energetic enough to emit all kinds of energy in the form of heat and photons.
Bose-Einstein Condensates represent a fifth phase of matter beyond solids. They are less energetic than solids. We can also think of this as more organized than solids, or as colder -- BECs occur in the fractional micro-Kelvin range, less than millionths of a degree above absolute zero; in contrast, the vacuum of interstellar space averages a positively tropical 3 K. BECs are more ordered than solids in that their restrictions occur not on the molecular level but on the atomic level. Atoms in a solid are locked into roughly the same location in regard to the other atoms in the area. Atoms in a BEC are locked into all of the same attributes as each other; they are literally indistinguishable, in the same location and with the same attributes. When a BEC is visible, each part that one can see is the sum of portions of each atom, all behaving in the same way, rather than being the sum of atoms as in the other phases of matter.
Wavefunctions and Quantum Spin
At the very beginning of the study of quantum mechanics, it was discovered that light could behave either as a wave or as a particle, when before it had only been treated as a wave. This discovery led Pierre de Broglie to theorize that perhaps matter could be treated as a wave, and not just as a particle. This theory was tested and found to be true: matter behaves as both a wave and a particle, depending on how it is observed.
Each atom has a wavefunction that describes its behavior as a wave. This wavefunction can be used to determine the probabilities that the atom will be in a given place or have a certain momentum or other useful properties. Each particle can also be determined to have a spin. While many physics terms mean something other than their everyday usage, "spin" seems to be a behavior that acts just as if the particle is spinning around an axis.
The amount of spin a particle can have depends on the type of particle. Fermions (like electrons) can have spin values that are +/- 1/2, +/- 3/2, +/- 5/2, etc.; bosons (like some isotopes of hydrogen and helium) have spin values that are whole numbers. Fermions obey the Pauli Exclusion Principle, whereas bosons do not. Bosons and fermions can both be composite particles; they don't have to be "indivisible" particles. The same physics will hold for bosons such as photons and K mesons as will hold for hydrogen and helium atoms, as long as the atoms are close to their ground state.
The Pauli Exclusion Principle (which was determined experimentally) states that no two fermion particles can occupy the same state at the same time. They must have some way of being distinguished, whether by location, spin state, or some other property. That means that if one fermion is in a local ground or minimum energy state, the next fermion in the area must be in a higher energy state. For bosons, however, the Pauli Exclusion Principle is irrelevant by definition -- so all of the bosons can be in the same state at the same time. They don't have to be distinguishable from each other. When this happens, a Bose-Einstein Condensate is created.
Creating a Condensate
Because of the specialized conditions under which they can exist, Bose-Einstein Condensates have only been created in laboratories. First, an experimenter takes bosons that have been purified of other elements and puts them in a vacuum. Popular choices for these bosons include specific isotopes of atoms of helium, sodium, rubidium, and hydrogen. Not all isotopes are bosons, and only bosons can form a BEC. The initial method of making a rubidium condensate is the most straightforward, and further methods have been refinements of the same general principles of cooling.
The atoms are first cooled to fractions of a degree Kelvin. They need to be virtually motionless in order to stay in the BEC ground state. Then they are put into a magnetic trap, keeping them in a limited area. The magnetic trap is arranged with eight magnets in what is known as a quadrupole configuration. The magnets we are most familiar with in daily life are dipole magnets: a two-ended field of magnetization with one polarity at one end and the opposite polarity at the other end. A quadrupole configuration looks more like a plus sign, with the opposing points having the same polarity.
When the atoms are in a quadrupole magnetic trap, the way they interact is primarily through their spin; higher order considerations such as magnetostatic interactions are limited by the trap. A laser with a precisely calculated wavelength shines on the atoms, and as the light scatters off the atoms, it takes with it more energy than it brought into the process. The Doppler shift from the higher energy atoms is calculated so that they "see" the laser of the right color, and the atoms that are already lower energy stay unexcited. The energy state of the atoms is, of course, directly related to how quickly they are moving, so the first wavelength used is selected for the fastest atoms present.
The laser's wavelength must be very precisely tuned to the atom. One of the hardest problems physicists face in making BECs is keeping the laser tuned to the right frequency despite outside interference; even a car passing by on the road outside a lab may cause enough vibration to knock the laser out of its desired frequency. To make things worse, as the average speed of the atoms decreases and their energy level goes down, the desired Doppler shift changes, so the laser must be retuned to match the new "high" energy atoms. In order to account for motion from all directions, the lasers shine in on the atoms from opposite points on all three axes. Further, the magnetic trap is combined with an optical trap that pushes atoms back towards the center if they stray too far. This laser set-up is known as "optical molasses."
The atoms are then cooled further through what is known as evaporative cooling. Essentially, evaporative cooling allows the faster, more energetic atoms to escape from the trap, leaving only the slowest, coolest, least energetic atoms behind. Of all the materials used, rubidium was the easiest to make into a BEC because its atoms are the largest -- they achieve low velocities at the highest temperature (energy) because mass relates to energy (hydrogen was the hardest BEC to form, but researchers think it may have superior applications because of its small size). When the atoms get to the point where only ground state atoms are left, they coalesce into a Bose-Einstein Condensate, which behaves like a superatom. The first condensate consisted of 2000 atoms; some condensates have been created that are the size of a dime (several million atoms), but still behave as one giant atom.
Properties and Future Applications
Most research into Bose-Einstein Condensates serves as "basic" research -- that is to say, it is more concerned with knowing more about the world in general than with implementing a specific technology. However, there are several potential uses for BECs. The most promising application is in etching. When BECs are fashioned into a beam, they are like a laser in their coherence. That is to say, both a laser and a BEC beam run "in lock step," guaranteeing that an experimenter can know how a part of the beam will behave at every single location. This property of lasers has been used in the past for etching purposes. A BEC beam would have greater precision and energy than a laser because even at their low kinetic energy state, the massive particles would be more energetic than the massless photons. The major technological concerns with a BEC beam would be getting a clean enough environment for it to function repeatedly and reducing the cost of BEC creation enough to use BECs regularly in beams. However, BEC beams or "atom lasers" could produce precisely trimmed objects down to a very small scale -- possibly a nanotech scale. Their practical limits will be found with experimentation.
In some ways, the atom laser works as the opposite of a laser. A laser can produce more photons from the atoms at hand, but an atom laser can only deal with the number of atoms it starts with. Rather than being knocked into an excited state, as atoms that emit laser photons are, BEC atoms are cooled down to the ground state. Unlike a laser beam, an atom laser beam could not travel far through air and would fall due to gravity. However, these differences can be calculated and accounted for in the future uses of the atom laser.
One of the most commonly known properties of BECs is their superfluidity. That is to say, BECs flow without interior friction. Since they're effectively superatoms, BECs are all moving in the same way at the same time when they flow, and don't have energy losses due to friction. Even the best lubricants currently available have some frictional losses as their molecules interact with each other, but BECs, while terribly expensive, would pose no such problem.
One of the problems physicists run into when teaching quantum mechanics is that the principles are just counter-intuitive. They're hard to visualize. But videos of BEC blobs several millimeters across show wave-particle duality at a level we can comprehend easily. We can watch something that acts like an atom, at a size we could hold in our hands. MIT researchers have produced visible interference fringe patterns from sodium BECs, demonstrating quantum mechanics effects on the macroscale. That alone is worth notice.
Perhaps most interestingly, BECs have been used to slow the speed of light to a crawl -- from 186,282 miles per second (3x108 m/s) in a vacuum to 38 miles per hour (17 m/s) in a sodium BEC. No other substance so far has been able to slow the speed of light within orders of magnitude of that speed. Although so far this discovery has not been applied to any technological problems, researchers at Harvard suggest that it might make possible revolutions in communications, including possibly a single-photon switch.
The Bose-Einstein Condensate is to matter as the laser is to light -- the analogy is precisely that simple. It took twenty years from the invention of the laser until its technological applications began to take off. At first, lasers were considered too difficult to make to ever find use in everyday applications; now, they're everywhere. The characteristics of BECs, specifically their response to sound and other disturbances, are still under investigation, but they hold the promise of many curious developments to come.
Marissa K. Lingen is a freelance writer living in Hayward, CA. Her background is in physics, but she's currently also interested in Finland, early (pre-transistor) computing, and moose.
Links
The BEC Homepage.
An article on atom lasers.