Cold atoms and molecules
|
The invention in 1975 of laser cooling using the force of (nearly) resonant laser light spawned a revolution in experimental atomic and quantum
physics. Where conventional spectroscopy was always limited by finite interaction times and thermal velocity distributions, laser cooling allows atomic
samples to be slowed and confined so that their fundamental properties can be examined and measured. Near elimination of the atoms' kinetic energy -
and hence reduction of their ensemble temperature towards absolute zero - allows not only better measurement, however: it also allows the atomic
samples to condense into new phases of matter governed entirely by quantum mechanics, the Heisenburg uncertainty principle and Pauli's exclusion principle.
Bose-Einstein condensation could be observed for the first time, and with exquisite clarity; and Fermi gases followed. The new ultracold
regime has allowed new devices to be devised, using quantum mechanics for quantum-enabled technologies including ultraprecise sensing and an
entirely new kind of information processing. Collisions between ultracold atoms can be controlled and reversible, and their chemical interactions herald
a new regime of superchemistry that is dominated by quantum coherence and controllability. Miniaturization, borrowing fabrication techniques from
the electronics industry, has already allowed the production of prototype atom chips which, it is envisaged, could one day put quantum-based
technologies at the heart of everyday consumer devices. As recognition of these remarkable advances, recent Nobel prizes have been several times awarded
to the cold atom and quantum control pioneers.
|
Nobel prizes:
|
Introduction to laser cooling
The Doppler cooling of atoms and ions, the magneto-optical trap, and sub-Doppler cooling mechanisms all rely upon spontaneous emission to reset
the atomic state after each photon has imparted its impulse, and to carry away entropy as part of the cooling process. The use of spontaneous emission,
however, requires a closed optical cycle – or one whose losses may be easily repumped – and this, together with spectroscopic accessibility and source
species availability, has limited the ultracold regime to alkali metals, the electronically-equivalent alkaline earth ions, and a handful of other
elements blessed by good fortune. The rest of the periodic table, and all molecular species, may be cooled only indirectly (e.g. by sympathetic/buffer
gas cooling), velocity-selected from a beam and decelerated, or – in the case of molecules – formed directly in the ultracold state by association of
already cold atoms. The latter route essentially limits molecules to alkali metal dimers, which are usually formed in a highly excited, loosely-bound
state.
New time-domain techniques
New spatially-dependent techniques
|
