A photon is a particle of light that can't be divided into pieces. But if you put a shutter in its path, you don't get half a photon—you get something strange: a mixture of different numbers of photons that on the left looks like a single photon, and on the right, like emptiness. It's as if light is 'smeared' between existence and nothingness. How is that possible?
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.