Coherence ultimately has to do with photons interfering with themselves. Films and beamsplitters can “cut” a photon in two pieces in the sense that the photon

Coherence ultimately has to do with photons interfering with themselves. Films and beamsplitters can “cut” a photon in two pieces in the sense that the photon takes both paths. Separately, the photon has an uncertainty in its energy which corresponds to a bandwidth and hence a coherence time. If the system conspires to bring the photon paths back together again, meaningful interference effects occur when the photon is destroyed (measured) provided the path difference is within the coherence time. This is relatively easy to do in a bulk optical instrument like a Michelson or Mach-Zender interferometer: just carefully adjust the two optical paths to be equal. Remarkably, if you sweep one path back and forth with respect to the other, while passing through the zero optical path difference point, and take the Fourier transform of the detected signal, an accurate depiction of the light source spectrum is produced.