Light consists of portions — photons. Max Planck was the first to understand that light is emitted in quanta. Today, we can guide single photons. When such a photon encounters a fast shutter, it cannot be sliced — half a photon doesn't exist. Instead, a quantum mixture of states with different photon numbers emerges: zero, one, two, three... without limit. It's like a drop hitting a blade and scattering into a fountain of splashes, where each splash is the probability of a particular outcome.
Paradoxically, this complex mixture looks simple: to the left of the shutter (the very same photon with its original energy, to the right) complete darkness, and between them — a razor-thin transition zone. A surprising detail: if the shutter acts faster than the pulse duration, the detector may register two or three photons, even though only one was sent. This is a consequence of quantum smearing.
This behavior of light paves the way to better photometers (instruments for measuring brightness) and quantum cryptography methods, where every photon counts.
With a sufficiently fast shutter, a single photon can multiply: the detector sees two or three, though only one was sent. It's not an illusion, but a direct consequence of light's quantum nature.