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 |k,εμ〉 where k, the 4-momentum is null (k2=0) and the k0 component, the energy, is positive and εμ is the unit polarization vector and the index μ ranges from 0 to 3. So, k is uniquely determined by the spatial momentum
where the
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
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
with the condition
for physical states |χ〉 and |ψ〉 in the Fock space (it is understood that physical states are really equivalence classes of states which differ by a state of zero norm).
It should be emphasised that this is not the same thing as
Note that if O is any gauge invariant operator,
does not depend upon the choice of the representatives of the equivalence classes, and so, this quantity is well-defined.
This is not true for non-gauge-invariant operators in general because the Lorenz gauge still leaves residual gauge degrees of freedom.
In an interacting theory of quantum electrodynamics, the Lorenz gauge condition still applies, but