If a photon can be described as an excitation in a quantum field, is this the same field for all photons, or does each photon exist in its own field?
I believe it is the same field for all photons in the universe. Similarly there is one field for all up quarks in the universe, another field for all down quarks in the universe and so on for each particle type. In this way the field theory model explains the uncanny consistency of particles; a neutrino made in a supernova millions of miles away is identical (as far as we can tell) to the same species of neutrino created on Earth. This suggests that all neutrinos come from the same 'stuff', and we call this stuff the neutrino field.
I think David Tong mentions as much in this talk:
Probability of photon emission by an atom depends on the occupation number of already existing photons of this sort. The corresponding occupation number is determined with the "boundary conditions". We cannot take the occupation number of all existing photons in the Universe, so time-space "separated" regions have effectively their own photon fields and their own occupation numbers.
On the other hand, when we observe an interference pattern, we don't care of the possible sources. To us it is crucial to have a photon at the same time at the same place whatever is its origin. Superposition happens for photons of a given sort arriving at the same time at the same place. That means the boundary conditions may be "open" for two or more sources.
Let us consider the Hamiltonian of an electromagnetic field in a finite region of space. It is possible to find a suitable unitary transformation to change it in a form same as a Hamiltonian of a independent harmonic oscillators. A photon is a quantum of energy of such an ensemble of independent oscillators. All excitations of this ensemble will be associated with the same field.