Consider a thought experiment where we have a source emitting a single photon, like an atom/molecule going from an excited energy state to its ground state.
We have an infinite number of point detectors forming a continuous sphere around the source.
Is the photon unidirectional and only seen by 1 detector? Is it an omnidirectional wave that is seen by all detectors?
The photon will register is one and only one detector. If you repeat the experiment many times, the photon may not trigger the same detector for every repeat, but every time one and only one detector will trigger.
The exact distribution in space of which detector triggers will depend on the details of the experiment, i.e. the radiation is not uniform in space in the same way the radiation from a classical antenna is not uniform in space. Because the atoms or molecules often have random orientations, many experiments use a particle (which may or may not be a photon) to "open" the detectors and what is measured is the relative orientation of this reference particle and the photon.
Photons are part of the standard model of particle physics. By the way, it should be noted that unlike the other particles in the model, which are each assigned exactly one energy content (in the form of a mass), photons represent a class of particles that have different energy contents (frequency, wavelength).
Photons are produced by the emission of excited states of subatomic particles, but also by the deflection of particles moving through a magnetic field. Photons are indivisible units until they are absorbed by subatomic particles.
Now, it may be advantageous for calculations of nuclear and shell relationships to consider a photon as existing everywhere in space. However, it is not productive to generalize this to the fact that an emitted photon is everywhere in space and only reunites with absorption at one location. It would be better to assign the photon (the photons) a cross-section of action. So that it fits through a slit, or not. With which it interacts with an electron, or not.
Is the photon unidirectional and only seen by 1 detector?
A photon is absorbed by exactly one detector. However, high-energy photons can trigger re-emissions, which in turn can be absorbed by other detectors.
