Photons

• The propagation of light (e.g. in free space or in a waveguide) is that of a wave field. The quantum-mechanical field amplitude arising at some point in space and time is the superposition of contributions which correspond to different possible paths for light. These contributions can constructively or destructively interfere with each other, and this is the basis of the well-known optical interference effects. A pure particle picture is difficult to reconcile with the observations, because e.g. in the classical double-slit experiment an ordinary particle would have to go through one of the two slits, and the other slit would be irrelevant; it could not be explained why particles can reach certain locations behind the double slit only when one of the slits is blocked, but not when both are open (destructive interference).
• When light interacts with atoms or other particles, only amounts of energy which are integer multiples of the photon energy h ν can be transferred to or from the light field. This can be easily interpreted as absorption or emission of some number of photons. Such processes are possible only if the particles involved (e.g. atoms) are able to accept such amounts of energy, i.e., only if they have quantum-mechanical energy levels with an energy difference corresponding to the photon energy, or in some cases some integer multiple of it (→ two-photon absorption). A pure wave picture could explain these energy constraints as resonance effects, but can not explain the quantization of exchanged energy.
• The energy quantization is also apparent in the interaction with sensitive photodetectors, which allow for photon counting, i.e. to register single photon absorption events. This finds applications in various areas of science and technology.
• Photons have zero rest mass, and can therefore not be slowed or brought to rest. There are phenomena of “slow light”, but such phenomena occur only in media, where the electromagnetic field strongly interacts with matter. There is then more than only an electromagnetic excitation, and a pure photon picture does not properly describe reality.
• Photons can carry angular momentum of two different forms: resulting from their spin and also as an orbital angular momentum related to the corresponding electric field profile.
• Due to their boson nature, multiple photons (unlike electrons) “like” to populate the same mode of the radiation field. This can be seen e.g. in the process of stimulated emission (and is thus also very important for lasers), but also in the energy spectrum of thermally excited radiation (black body radiation).
• Photons can occur in entangled states, where certain properties (e.g. polarization) are correlated between different photons, even though these properties acquire definite values only when a measurement is performed. As measurements on the different photons can occur at different places, this seemed to imply the possibility of superluminal transmission of information (Einstein–Podolsky–Rosen paradox), but a closer inspection shows that in reality this is not the case.

Of course, quantum theory can be applied to any kind of electromagnetic wave phenomena, not only to visible light. However, quantum effects are not as important e.g. in the field of radio technology, as in optics and laser technology. This is because the photon energy of radio waves is very tiny compared with the thermal energy k_B T at room temperature, whereas the opposite is true for optical phenomena.

In laser physics, a frequently considered case is that of photons propagating in media, e.g. in transparent crystals or glasses, including laser gain media. Strictly, the term photon is then no longer appropriate, as electromagnetic waves interact with such media, and what propagates are quasi-particles, sometimes called polaritons, which resemble coupled excitations of the electromagnetic field and a polarizable medium.

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