where c is the speed of light in free space and π π is the conjugate canonical momentum of the vector potential A . The general expression for the orbital angular momentum of light is L = 1 c ∫ ∫ d 3 x π π μ μ x × × ∇ ∇ A μ μ , where μ μ = { 0 , 1 , 2 , 3 } denotes four indices of the spacetime and Einstein's summation convention has been applied. To quantize light, the basic equal-time commutation relations have to be postulated, [ A μ μ ( x , t ) , π π ν ν ( x ′ , t ) ] = i ℏ ℏ c g μ μ ν ν δ δ 3 ( x − − x ′ ) , [ A μ μ ( x , t ) , A ν ν ( x ′ , t ) ] = [ π π μ μ ( x , t ) , π π ν ν ( x ′ , t ) ] = 0 , where ℏ ℏ is the reduced Planck constant and g μ μ ν ν = d i a g { 1 , − − 1 , − − 1 , − − 1 } is the metric tensor of the Minkowski space.