Entanglement is a special phenomenon in quantum physics in which (to put it simply) two or more quantum objects are connected in such a way that the state of one quantum object is immediately linked to the state of the other, no matter how far apart they are.
This means that if you measure the state of one quantum object, you immediately know the state of the second, entangled quantum object—even if it is on the other side of the earth. Entanglement may sound as if two quantum objects can communicate with each other instantly, but in fact it cannot be used to transmit information faster than light. The reason for this lies in the rules of quantum physics:
- Random results: When measuring the state of an entangled quantum object, the result is random. Only by comparing the measurement results with those of the other quantum object can one see that they are correlated.
- Classical communication required: To exchange the measurement results and determine the correlation, classical signals must be used, which travel at most at the speed of light.
- No controllability: It is not possible to arbitrarily influence the result of a measurement in order to send a specific message. Entanglement only ensures that the results are connected to each other, not that they can be specifically controlled.
In short: entanglement creates instantaneous correlations, but no controllable communication. Information therefore still cannot be transmitted faster than light – the theory of relativity remains valid.
The phenomenon of entanglement contradicts our everyday experience, in which things normally only have a local effect, and was described by Albert Einstein as “spooky action at a distance.”
Entangled particles are very important for quantum computers and quantum communication because they enable information to be transmitted or processed in a completely new, extremely secure way.