Singlet state

In quantum mechanics, a singlet originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number ${\displaystyle s=0}$, though this meaning has since been generalized to include other situations. The link between the particles may be historical, such as two widely separated particles whose current angular momentum states originated in a single quantum event; or ongoing, such as two particles bound by charge. A set of linked particles that lacks net angular momentum is said to be in a singlet state.

Singlets and the related spin concepts of doublets and triplets occur frequently in atomic physics and nuclear physics, where one often needs to determine the total spin of a collection of particles. Since the only observed fundamental particle with zero spin is the extremely inaccessible Higgs boson, singlets in everyday physics are necessarily composed of sets of particles whose individual spins are non-zero, e.g. 1/2 or 1.

The origin of the term "singlet" is that bound quantum systems with zero net angular momentum emit photons within a single spectral line, as opposed to double lines (doublet state) or triple lines (triplet state). The number of spectral lines ${\displaystyle n}$ in this singlet-style terminology has a simple relationship to the spin quantum number: ${\displaystyle n=2s+1}$, and ${\displaystyle s=(n-1)/2}$.

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