Cutting a Photon Unveils a Cloud of Zero-to-Infinity States

Physicists simulated slicing a single photon with a shutter and found that the global quantum state becomes a complex mixture spanning zero to infinitely many photons. Locally, measurements on one side can reveal a single-photon state while the other side appears as vacuum, illustrating a paradoxical, nonlocal distribution and raising questions about the nature of particles and causality. The work, reported in Physical Review Letters, could lead to a clearer way to describe particle interactions and extend to other quantum particles like electrons; in realistic setups the infinite case requires an idealized infinitely fast shutter.