Overview of neutral atomic and molecular cooling and trapping shows an energy
scale spanning 12 orders of magnitude in kinetic energy E of
atomic motion, expressed in temperature units
T = E/kB (kB is the
Boltzmann constant). The length scale shows the corresponding de Broglie
wavelength  = h/p
(h is the Planck constant), where momentum
p = (2mE)1/2. The line to guide the eye
is calculated using the mass m of the Na atom. Typical atomic
dimensions are on order of 1 a0 = 0.0529 nm,
the Bohr radius of the H atom, whereas Bose-Einstein condensates can have
dimensions on the order of 100 µm. The figure also indicates typical
orders of magnitude for familiar energy scales associated with atomic fine
structure, hyperfine structure, natural radiative broadening, and optical or
magnetic shifts. Evaporative cooling of trapped atoms can dramatically lower
the temperature and reach the limit of quantum degeneracy, where the phase
space density  , defined
as the number of particles per cubic thermal de Broglie wavelength, is of
order unity. If the atoms are bosons, then Bose-Einstein condensation occurs
when reaches the
critical value of 2.6. |
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