Everything about Bose Einstein Condensate totally explained
A
Bose–Einstein condensate (BEC) is a
state of matter of
bosons confined in an external
potential and cooled to
temperatures very near to
absolute zero (or ). Under such
supercooled conditions, a large fraction of the atoms collapse into the lowest
quantum state of the external potential, at which point quantum effects become apparent on a macroscopic scale.
This state of matter was first predicted by
Satyendra Nath Bose in 1925. Bose submitted a paper to the
Zeitschrift für Physik but was turned down by the peer review. Bose then took his work to
Einstein who recognized its merit and had it published under the names Bose and Einstein, hence the hyphen.
Seventy years later, the first gaseous condensate was produced by
Eric Cornell and
Carl Wieman in 1995 at the
University of Colorado at Boulder NIST-
JILA lab, using a gas of
rubidium atoms cooled to 170
nanokelvin (nK) (or ).
Eric Cornell,
Carl Wieman and
Wolfgang Ketterle at
MIT were awarded the 2001
Nobel Prize in Physics in Stockholm, Sweden.
Introduction
"Condensates" are extremely low-temperature fluids which contain properties and exhibit behaviors that are currently not completely understood, such as spontaneously flowing out of their containers. The effect is the consequence of quantum mechanics, which states that systems can only acquire energy in discrete steps. If a system is at such a low temperature that it's in the lowest energy state, it's no longer possible for it to reduce its energy, not even by
friction. Without friction, the fluid will easily overcome
gravity because of
adhesion between the fluid and the container wall, and it'll take up the most favorable position, all around the container.
Bose-Einstein condensation is an exotic quantum phenomenon that was observed in dilute atomic gases for the first time in 1995, and is now the subject of intense theoretical and experimental study.
Theory
The slowing of atoms by use of cooling apparatuses produces a singular quantum state known as a
Bose condensate or
Bose–Einstein condensate. This phenomenon was predicted in 1925 by generalizing Satyendra Nath Bose's work on the
statistical mechanics of (massless)
photons to (massive) atoms. (The Einstein manuscript, believed to be lost, was found in a library at
Leiden University in 2005.) 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
demonstrated 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:
»
which is greater than that of
singly-charged
vortices, indicating that these multiply-charged vortices are
unstable to decay. Research has, however, indicated they are
metastable states, so may have relatively long lifetimes.
Unusual characteristics
Further experimentation by the
JILA team in 2000 uncovered a hitherto unknown property of Bose–Einstein condensates. 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 seemed to have disappeared from the experiment altogether, not being 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 couldn't be explained because the energy state of an atom near absolute zero shouldn't be enough to cause an implosion; however, subsequent mean-field theories have been proposed to explain it.
Because
supernova explosions are implosions, the explosion of a collapsing Bose–Einstein condensate was named "
bosenova", a pun on the musical style
bossa nova.
The atoms that seem to have disappeared almost certainly still exist in some form, just not in a form that could be detected in that experiment. Two likely possibilities are that they formed molecules consisting of two bonded rubidium atoms, or that they somehow received enough energy to fly away fast enough that they left the observation region before they could be observed.
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've 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 condensates due to
wave-particle duality, the study of
superfluidity and quantized
vortices, and the
slowing of light pulses to very low speeds using
electromagnetically induced transparency.
Vortices in Bose-Einstein condensates are also currently the subject of analogue-gravity
research, studying the possibility of modeling black holes and their
related phenomena in such environments in the lab.
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, and may be useful in studying Bose–Einstein condensation in fewer than three dimensions, for example the
Tonks-Girardeau gas.
Bose–Einstein condensates composed of a wide range of
isotopes have been produced.
Related experiments in cooling
fermions rather than
bosons to extremely low temperatures have created
degenerate gases, where the atoms don't congregate in a single state due to the
Pauli exclusion principle. To exhibit Bose–Einstein condensation, the fermions must "pair up" to form compound particles (for example
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 composed of
Cooper pairs.
In 1999, Danish physicist
Lene Vestergaard Hau led a team from
Harvard University who succeeded in slowing a beam of light to about 17 metres per second and, in 2001, was able to momentarily stop a beam. She was able to achieve this by using a superfluid.
Hau and her associates at Harvard University have since successfully transformed light into matter and back into light using Bose-Einstein condensates. Details of the experiment are discussed in an article in the journal
Nature, 8 February 2007.
Use in popular science
A prominent example of the use of Bose-Einstein condensation in
popular science is at the
Physics 2000 web site
developed at the
University of Colorado at Boulder. In the context of popularizations, atomic BEC is sometimes called a
Super Atom.
In popular culture
The game
Mass Effect which is developed by BioWare has a weapon upgrade called Cryo Rounds. The description states that "Cooling lasers collapse ammunition into small Bose-Einstein condensate - a mass of super-cooled subatomic particles - capable of snap-freezing impacted objects."
Further Information
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