Rather than producing a photon on one side and a vacuum on the other, the shutter generates something far more strange and complex: a superposition of states containing infinitely many photons simultaneously.

This happens because, in quantum mechanics, empty space isn't truly empty—in reality, it seethes with fluctuations in the electromagnetic field. By rapidly switching the shutter, the team found that these fluctuations are disturbed—and in doing so, they spontaneously create new photons. Crucially though, if you were to look only at the region immediately either side of where the shutter operated, the state would appear deceptively normal: indistinguishable from a single photon on one side, and a simple vacuum on the other.