Bose-Einstein condensate: Encyclopedia - Bose-Einstein condensate

Bose-Einstein condensate

A Bose-Einstein condensate is a phase of matter formed by bosons cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nK). Under such conditions, a large fraction of the atoms collapse into the lowest quantum state.

Bose-Einstein condensate - Introduction

Bose-Einstein condensates are best known to laymen as extremely low temperature fluids with weird properties, such as spontaneously flowing out of their container. The effect is the consequence of quantum mechanics, which states that systems can only acquire energy in discrete steps. Now, if a system is at such a low temperature that is is in the lowest energy state, it is no longer possible for it to reduce its energy, not even by friction. Therefore, without friction, the fluid will easily defy gravity because of adhesion between the fluid and the container wall, and it will take up the most favorable position, i.e. all around the container.

Bose-Einstein condensate - Theory

The collapse of the atoms into a single quantum state is known as Bose condensation or Bose-Einstein condensation. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein. The result of the efforts of Bose and Einstein is the concept of a Bose gas, governed by the Bose-Einstein statistics, which describes the statistical distribution of identical particles with integer spin, now known as bosons. Bosonic particles, which include the photon as well as atoms such as helium-4, are allowed to share quantum states with each other. Einstein speculated that cooling bosonic atoms to a very low temperature would cause them to fall (or "condense") into the lowest accessible quantum state, resulting in a new form of matter.

This transition occurs below a critical temperature, which for a uniform three-dimensional gas consisting of non-interacting particles with no apparent internal degrees of freedom is given by:

where:

Bose-Einstein condensate - Discovery

In 1938, Pyotr Kapitsa, John Allen and Don Misener discovered that helium-4 became a new kind of fluid, now known as a superfluid, at temperatures below 2.17 kelvins (K) (lambda point). Superfluid helium has many unusual properties, including zero viscosity (the ability to flow without dissipating energy) and the existence of quantized vortices. It was quickly realized that the superfluidity was due to Bose-Einstein condensation of the helium-4 atoms, which are bosons. In fact, many of the properties of superfluid helium also appear in the gaseous Bose-Einstein condensates created by Cornell, Wieman and Ketterle (see below). However, superfluid helium-4 is not commonly referred to as a "Bose-Einstein condensate" because it is a liquid rather than a gas, which means that the interactions between the atoms are relatively strong. The original theory of Bose-Einstein condensation must be heavily modified in order to describe it.

The first "true" Bose-Einstein condensate was created by Cornell, Wieman, and co-workers at JILA on June 5, 1995. They did this by cooling a dilute vapor consisting of approximately 2000 rubidium-87 atoms to below 170 nK using a combination of laser cooling (a technique that won its inventors Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips the 1997 Nobel Prize in Physics) and magnetic evaporative cooling. About four months later, an independent effort led by Wolfgang Ketterle at MIT created a condensate made of sodium-23. Ketterle's condensate had about a hundred times more atoms, allowing him to obtain several important results such as the observation of quantum mechanical interference between two different condensates. Cornell, Wieman and Ketterle won the 2001 Nobel Prize for their achievement.

Bose-Einstein condensate - Unusual characteristics

Further experimentation by the JILA team in 2000 uncovered a hitherto unknown property of Bose-Einstein condensate. Cornell, Wieman, and their coworkers originally used rubidium-87, an isotope whose atoms naturally repel each other making a more stable condensate. The JILA team instrumentation now had better control over the condensate so experimentation was made on naturally attracting atoms of another rubidium isotope, rubidium-85 (having negative atom-atom scattering length). Through a process called Feshbach resonance involving a sweep of the magnetic field causing spin flip collisions, the JILA researchers lowered the characteristic, discrete energies at which the rubidium atoms bond into molecules making their Rb-85 atoms repulsive and creating a stable condensate. The reversible flip from attraction to repulsion stems from quantum interference among condensate atoms which behave as waves.

When the scientists raised the magnetic field strength still further, the condensate suddenly reverted back to attraction, imploded and shrank beyond detection, and then exploded, blowing off about two-thirds of its 10,000 or so atoms. About half of the atoms in the condensate seem to have disappeared from the experiment altogether and are unaccounted for and are not seen either in the cold remnant or the expanding gas cloud. Carl Wieman explained that under current atomic theory this characteristic of Bose-Einstein condensate could not be explained because the energy state of an atom near absolute zero should not be enough to cause an implosion, however, subsequent mean-field theories have been proposed to explain it.

Due to the fact that supernovae explosions are implosions, the explosion of collapsing Bose-Einstein condensate was christened "bosenovas."

Bose-Einstein condensate - Current research

Compared to more commonly-encountered states of matter Bose-Einstein condensates are extremely fragile. The slightest interaction with the outside world can be enough to warm them past the condensation threshold, forming a normal gas and losing their interesting properties. It is likely to be some time before any practical applications are developed.

Nevertheless, they have proved to be useful in exploring a wide range of questions in fundamental physics, and the years since the initial discoveries by the JILA and MIT groups have seen an explosion in experimental and theoretical activity. Examples include experiments that have demonstrated interference between colliding Bose condensates due to wave-particle duality [1], the study of superfluidity and quantized vortices [2], and the slowing of light pulses to very low speeds using electromagnetically induced transparency [3]. Experimentalists have also realized "optical lattices", where the interference pattern from overlapping lasers provides a periodic potential for the condensate. These have been used to explore the transition between a superfluid and a Mott insulator [4], and may be useful in studying Bose-Einstein condensation in less than three dimensions, for example the Tonks-Girardeau gas.

Bose-Einstein condensates composed of a wide range of isotopes have been produced [5].

Related experiments in cooling fermions rather than bosons to extremely low temperatures have created degenerate gases, where the atoms do not congregate in a single state due to the Pauli exclusion principle. To exhibit Bose-Einstein condensate, the fermions must "pair up" to form compound particles (e.g. molecules or Cooper pairs) that are bosons. The first molecular Bose-Einstein condensates were created in November 2003 by the groups of Rudolf Grimm at the University of Innsbruck, Deborah S. Jin at the University of Colorado at Boulder and Wolfgang Ketterle at MIT. Jin quickly went on to create the first fermionic condensate comprised of Cooper pairs [6].

See also

• Bose gas
• Electromagnetically induced transparency
• Fermionic condensate
• Gas in a box
• Slow glass
• Superfluid
• Supersolid
• Super-heavy atom
• Tonks-Girardeau gas

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