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Title:  Bose–Einstein condensation: Where many become one and so there is plenty of room at the bottom 
Authors:  Kumar, N. 
Keywords:  Bose–Einstein condensation bosonic stimulation, quantum gas statistics. 
Issue Date:  25Dec2005 
Publisher:  Indian Academy of Sciences, Bangalore, India 
Citation:  Current Science, 2005, Vol.89, p20932100 
Abstract:  Classically identical particles become quantum mechanically
indistinguishable. Satyendra Nath Bose taught
us, in 1924, how to correctly count the distinct microstates
for the indistinguishables, and for a gas of light
quanta (later photons), whose number is not conserved,
e.g., can vary with temperature, he gave a proper
derivation of Planck’s law of black body radiation.
Einstein, in 1925, generalized the Bose statistics to a
quantum gas of material particles whose number is
now fixed, or conserved, e.g., 4He, and thus opened a
new direction in condensed matter physics: He showed
that for low enough temperatures (~1 Kelvin and below),
a macroscopic number of the particles must accumulate
in the lowest oneparticle state. This degenerate gas
with an extensively occupied single oneparticle state
is the Bose–Einstein condensate, now called BEC.
(Fragmented BEC involving a multiplicity of internal
states of nonscalar Bose atoms is, however, also realizable
now). Initially thought to be a pathology of an
ideal noninteracting Bose system, the BEC turned out
to be robust against interactions. Thus, the Bose–Einstein
condensation is a quantum phase transition, but
one with a difference – it is a purely quantum statistical effect, and requires no interparticle interaction for its
occurrence. Indeed, it happens in spite of it. The condensate
fraction, however, diminishes with increasing
interaction strength – to less than ten per cent for 4He.
The BEC turned out to underlie superfluidity, namely
that the superfluid may flow through finest atomic capillaries
without any viscosity. Interaction, however, seems
essential to superfluidity. But, the precise connection between
BEC and the superfluidity remains elusive. Thus,
for example, we may have superfluidity in twodimensions
where there is no condensate! Seventy years later now,
the BEC has come alive with the breakthrough in 1995
when nearideal BEC was created in dilute alkali gases
of 87Rb and 23Na atoms cooled in the gaseous state down
to nanokelvins and localized in a trap. There are reasons
why we ought to be mindful of the BEC – if only
because here even the interaction between the particles
is tunable at will – the sign as well as the strength of it.
BEC has now become an ideal laboratory for basic and
condensed matter experiments, and for high resolution
applications. Properly viewed, it is indeed a new
state of matter. This article is about the saga of BEC
that really began with Einstein in 1925. 
Description:  Open Access. 
URI:  http://hdl.handle.net/2289/2239 
ISSN:  00113891 
Copyright:  2005 Indian Academy of Sciences, Bangalore, India. 
Appears in Collections:  Research Papers (TP)

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