Quantum entanglement: »Spooky long-distance effect« - simply explained
Entanglement is a quantum physical phenomenon. In this practical tip, we explain what quantum entanglement is and what is so special about it.
Quantum entanglement: a "spooky long-distance effect"?
Quantum entanglement is the phenomenon that two spatially separated particles can exchange information about their properties instantaneously (ie without a time delay). This contradicts all laws of classical physics. Even Einstein rejected this "haunted long-distance effect" throughout his life, since it would have to be able to transmit information faster than the speed of light.
- In order to understand the entanglement, the principle of superposition is required. Then a particle is in all possible states until a measurement is made. A particle on which no measurement has yet been made is in a state of superposition of all possible states. For example, the spin of an electron is not fixed before the measurement (Stern-Gerlach experiment). Only when it hits the screen does the electron get the spin "up" or "down" (directional quantization). The measurement resolves the superposition and the particle assumes one of the possible states. The superposition is also easy to understand on the basis of the double slit experiment. Without influencing the double slit, the typical interference pattern on the screen results, even if each individual particle hits a certain point on the screen and the distribution on the screen follows a probability function. This probability pattern is destroyed by measuring one of the gaps. The particle is only forced into a state by the measurement.
- Due to the superposition principle, the emitted particles remain in an unclear state until they are measured. In entanglement experiments, two electrons are generated at the same time. Your spin is measured in two different devices. It can be seen that the electrons each have an opposite spin. This is astonishing, since according to the principle of superposition, the state had not yet been determined until the measurement. It cannot be argued that electrons with an opposite spin were already present when they were formed. Only with the measurement does one particle decide for one state and at the same time the other particle for another state. Despite the spatial distance to each other, the two electrons are to be understood as a system that is in the superposition state before the measurement. Comparable experiments can also be carried out with entangled "twin photons" and other particles.
- A measurement on an entangled particle immediately determines the state of the other particle. Information transmission at the speed of light at most, as required by Einstein, only applies to individual, separate objects.
- So far, entanglement phenomena have only been observed at the particle level. Does the instantaneous transfer of information also exist in our world or even in the macrocosm? Albert Einstein, who is extremely skeptical about entanglement, has already described space-time curvature in the general theory of relativity and, in the beginning, has promised the possible existence of wormholes. Even in wormholes, which, as massless structures, result solely from the geometry of space-time, two distant places are linked to one another in such a way that space and time merge into one point. Matter and information that fly through this wormhole get from one place to another without delay.
- The instantaneous exchange of information would also be of enormous practical benefit in our highly technical everyday life. So-called quantum computers should take up the principle of entanglement. If information is manipulated on a computer, it would be immediately available on the recipient computer without any delay.
