where σ = (σ 1, σ 2, σ 3) = (σ x, σ y, σ z) is a vector of the Pauli matrices, E is the energy operator, p = (p 1, p 2, p 3) = (p x, p y, p z) is the 3-momentum operator, I 2 denotes the 2 × 2 identity matrix, the zeros (in the second line) are actually 2 × 2 blocks of zero matrices.