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Inquantum mechanics, adoublet is a composite quantum state of a system with an effectivespin of 1/2, such that there are two allowed values of the spin component, −1/2 and +1/2. Quantum systems with two possible states are sometimes calledtwo-level systems. Essentially all occurrences of doublets in nature arise fromrotational symmetry; spin 1/2 is associated with thefundamental representation of theLie groupSU(2).
The term "doublet" dates back to the early 19th century, when it was observed that certainspectral lines of an ionized, excited gas would split into two under the influence of a strongmagnetic field, in an effect known as theanomalous Zeeman effect. Such spectral lines were observed not only in the laboratory, but also in astronomicalspectroscopy observations, allowing astronomers to deduce the existence of, and measure the strength of magnetic fields around the Sun, stars and galaxies. Conversely, it was the observation of doublets in spectroscopy that allowed physicists to deduce that theelectron had a spin, and that furthermore, the magnitude of the spin had to be 1/2. See the history section of the article onSpin (physics) for greater detail.
Doublets continue to play an important role in physics. For example, thehealthcare technology ofmagnetic resonance imaging is based onnuclear magnetic resonance. In this technology, a spectroscopic doublet occurs in a spin-1/2atomic nucleus, whose doublet splitting is in the radio-frequency range. By applying both a magnetic field and carefully tuning a radio-frequency transmitter, the nuclear spins will flip and re-emit radiation, in an effect known as theRabi cycle. The strength and frequency of the emitted radio waves allows the concentration of such nuclei to be measured. Another potential application is the use of doublets as the emitting layer in light emitting diodes (LEDs). These materials have the advantage of having 100% theoretical quantum efficiency based on spin statistics whereas singlet systems and triplet systems have significantly lower efficiencies or rely on noble metals such as Pt and Ir to emit light.[1]