If you try to 'cut off' a piece of a photon with an optical shutter, you don’t get another photon, nor a mixture of a photon and vacuum. Instead, a complex quantum state emerges—a superposition (overlay) and mixture of states with any number of photons, from zero to infinity. Amazingly, locally, to the left and right of a narrow transition region, this state is equivalent to a single photon or vacuum, respectively. That is, on one side we see a photon, and on the other—emptiness. This work shows that even a simple attempt to block light can generate an infinite variety of quantum states from a single photon.
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.