Ghostly Confusion

In 1935, Einstein, together with Boris Podolsky and Nathan Rosen, published a famous polemical article in the Physical Review in which he analyzed a hypothetical situation - possible from the point of view of quantum theory, but completely unacceptable to classical physics and common sense.

The simplest way to illustrate it is with the example of a pair of particles, e.g. photons, connected by an incomprehensible quantum bond that physicists call an entangled state.

According to quantum mechanics, when making any measurement, for example, the direction of a photon's polarization, we cannot be certain of the result. We only know the probability of obtaining one polarization or another. The situation is different in the case of entangled photons. When we manage to measure the polarization of one of them, the measurement result for the other photon in the pair becomes... certain. We know its polarization without using any instruments, although just a moment ago, "the old woman was telling the truth"—the measurement could have yielded one or another value. This second photon, in some incomprehensible way, "knows" that we have checked its companion. And it doesn't matter how far away it is at the moment of measurement. One could be on Earth, the other on the Moon, Mars, or even further. The incomprehensible bond connecting the two photons acts instantaneously, without any delay, at any distance, and without any material intermediary in the form of particles, waves, or fields. Einstein called this phenomenon "spooky action at a distance."

In the 1980s, French physicist Alain Aspect conducted experiments that proved that such a strange bond actually exists. Recently, this has ceased to be just a curiosity. Numerous applications for entangled photons have been devised, including teleportation, quantum communication, and quantum computing