By the postulates of quantum mechanics, an experiment designed to measure the electron spin on the x, y, or z axis can only yield an eigenvalue of the corresponding spin operator (S x, S y or S z) on that axis, i.e. ⁠ ħ / 2 ⁠ or – ⁠ ħ / 2 ⁠. The quantum state of a particle (with respect to spin), can be represented by a two-component spinor : ψ ψ = ( a + b i c + d i ) .