A singlet state refers to a quantum state of a system, particularly in the context of quantum mechanics and quantum information theory. In a singlet state, two particles, such as electrons, are entangled in such a way that their total spin is zero. This means that if one particle has a spin of +1/2, the other must have a spin of -1/2, and vice versa.
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A singlet state is the one-dimensional rotation representation; this construction supplies the angular-momentum-zero case for two equal spins.