Here σ σ = ( σ σ x , σ σ y , σ σ z ) are the Pauli operators collected into a vector for convenience, and p ^ ^ = − − i ℏ ℏ ∇ ∇ is the momentum operator in position representation. The state of the system, | ψ ψ ⟩ ⟩ (written in Dirac notation), can be considered as a two-component spinor wavefunction, or a column vector (after choice of basis):