The nonlocality of a single photon refers to a phenomenon in quantum mechanics known as entanglement. Entanglement is a property where two or more particles, such as photons, become correlated in such a way that the state of one particle is instantaneously connected to the state of the other, regardless of the distance between them. This means that measuring or manipulating one particle can instantly affect the state of the other particle, even if they are separated by large distances.
This instantaneous correlation, often referred to as "spooky action at a distance," is a fundamental aspect of quantum mechanics. When two photons are entangled, their states become inseparable, and any change or measurement performed on one photon will have an immediate effect on the other, regardless of the spatial separation between them. This nonlocal correlation has been experimentally verified in numerous studies.
It's important to note that the nonlocality of entangled photons does not allow for faster-than-light communication. Although the measurement outcomes are instantly correlated, this correlation cannot be exploited to transmit information faster than the speed of light. This is due to the fact that the specific outcomes of measurements are probabilistic and cannot be predetermined or controlled. Therefore, while entanglement exhibits nonlocal correlations, it does not violate the principles of causality or allow for superluminal communication.