Optical pumping

Optical pumping uses resonant light to redistribute populations among atomic or molecular energy levels, often creating a polarized ensemble or a nonequilibrium state.

Optical selection rules: Rules derived from symmetries and angular momentum that determine which transitions light can drive. They decide which energy-level transitions the pumping light can induce.
Atomic energy levels: The discrete energies available to an atom’s electrons and nucleus. Pumping acts by moving population among these discrete states.
Optically pumped magnetometer: A magnetic-field sensor that detects changes in optically prepared atomic polarization. It turns pump-induced spin polarization into a measurement of magnetic fields.
Alfred Kastler: French physicist who developed optical pumping and received the 1966 Nobel Prize in Physics. His work established the method and its use in studying atomic resonance.
Electrical pumping: The excitation of a medium by electrical current or discharge to produce excited-state populations. Unlike optical pumping, it uses charged-particle collisions rather than resonant light for excitation.
Zeeman effect: The splitting of atomic or molecular energy levels in a magnetic field. Magnetic fields separate sublevels that circularly polarized light can populate selectively.
Hyperfine structure: Small energy splittings caused by interactions between electronic and nuclear angular momenta. Hyperfine levels provide the distinct states addressed in many pumping schemes.
Atomic clock: A clock that measures time using the stable frequency of an atomic transition. Optical pumping prepares selected atomic states before clock transitions are interrogated.
Jean Brossel: French physicist whose experiments with Alfred Kastler advanced optical pumping and atomic spectroscopy. His collaboration helped turn the proposed method into a practical experimental technique.
Thermal equilibrium: A state in which populations of energy levels follow the distribution set by temperature. Optical pumping drives populations away from this equilibrium distribution.

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