Gupta–Bleuler formalism  In quantum field theory, the Gupta–Bleuler formalism is a way of quantizing the electromagnetic field. The formulation is due to theoretical physicists Suraj N. Gupta and Konrad Bleuler. Firstly, consider a single photon. A basis of the one-photon vector space (it is explained why it is not a Hilbert space below) is given by the eigenstates formula_1 where formula_2, the 4-momentum is null (formula_3) and the formula_4 component, the energy, is positive and formula_5 is the unit polarization vector and the index formula_6 ranges from 0 to 3. So, formula_2 is uniquely determined by the spatial momentum formula_8. Using the bra–ket notation, this space is equipped with a sesquilinear form defined by  where the formula_10 factor is to implement Lorentz covariance. The metric signature used here is +−−−. However, this sesquilinear form gives positive norms for spatial polarizations but negative norms for time-like polarizations. Negative probabilities are unphysical, not to mention a physical photon only has two transverse polarizations, not four. If one includes gauge covariance, one realizes a photon can have three possible polarizations (two transverse and one longitudinal (i.e. parallel to the 4-momentum)). This is given by the restriction formula_11. However, the longitudinal component is merely an unphysical gauge. While it would be nice to define a stricter restriction than the one given above which only leaves the two transverse components, it is easy to check that this can't be defined in a Lorentz covariant manner because what is transverse in one frame of reference isn't transverse anymore in another. To resolve this difficulty, first look at the subspace with three polarizations. The sesquilinear form restricted to it is merely semidefinite, which is better than indefinite. In addition, the subspace with zero norm turns out to be none other than the gauge degrees of freedom. So, define the physical Hilbert space to be the quotient space of the three polarization subspace by its zero norm subspace. This space has a positive definite form, making it a true Hilbert space. This technique can be similarly extended to the bosonic Fock space of multiparticle photons. Using the standard trick of adjoint creation and annihilation operators, but with this quotient trick, one can formulate free field vector potential operator valued distribution formula_12 satisfying  with the condition  for physical states formula_15 and formula_16 in the Fock space (it is understood that physical states are really equivalence classes of states which differ by a state of zero norm).