Singlet state

In quantum mechanics, a singlet is a quantum state of a system with a spin of 0, such that there is only one allowed value of the spin component, 0.

A pair of spin-1/2 particles can be combined to form one of three states of total spin 1 called the triplet, or a state of spin 0 which is called the singlet. In theoretical physics, a singlet usually refers to a one-dimensional representation (e.g. a particle with vanishing spin). It may also refer to two or more particles prepared in a co-related state, such that the total angular momentum of the state is zero. Singlets and other such representations frequently occur in atomic physics and nuclear physics, where one tries to determine the total spin from a collection of particles.

A single electron has spin 1/2, and upon rotation its state transforms as a doublet, that is, as the fundamental representation of the Lie group SU(2). We can measure the spin of this electron's state by applying an operator to the state, and we will always obtain (or spin 1/2) since the spin-up and spin-down states are both eigenstates of this operator with the same eigenvalue.