Quantum entanglement, also called quantum correlation, is a quantum phenomenon for which under certain conditions two or more physical systems represent subsystems of a larger system, whose quantum state cannot be described individually, but only as a superposition of several states. From this it follows that the measurement of an observable of a system (subsystem) simultaneously determines the value for the others as well.
Since it is possible from the experimental point of view that these subsystems are spatially separated, entanglement implies in a counterintuitive way the presence of distance correlations without any limit , between their physical quantities, determining the non-local character of the theory.
The term entanglement was introduced by Erwin Schrödinger in a review of the famous article on the EPR paradox , which in 1935 revealed the phenomenon on a theoretical level .
Entanglement is one of the properties of quantum mechanics that led Einstein and other scholars to question its principles. In fact, in 1935 Einstein, Boris Podolsky and Nathan Rosen formulated the “ EPR paradox ” (from the initials of the three scientists), which highlighted the phenomenon of entanglement as paradoxical . It was born from the assumption of 3 hypotheses:
– principle of reality; – principle of locality; – principle of completeness of quantum mechanics .
In order for the paradox to be resolved, one of the three hypotheses had to be dropped, but considering the first two certainly true, as evident, the authors came to the conclusion that quantum mechanics was incomplete (that is, it contains hidden variables). In reality there was a fundamental error, highlighted in 1964 by John Stewart Bell with the demonstration, in the context of a theory of hidden variables that reproduce the predictions of quantum mechanics, of the incompatibility between the principles of locality and reality. The most widely shared interpretation of quantum mechanics ( Copenhagen interpretation) contemplates local (quantum field theory) and non-local (such as entanglement) aspects, rejecting the reality principle, while, for example, David Bohm’s interpretation, which is a typical theory of hidden variables, affirms the principle of reality, excluding that of locality.
In any case, quantum mechanics has proved capable of producing correct experimental predictions up to a precision never achieved before and the correlations associated with the phenomenon of quantum entanglement have actually been observed. In fact, at the beginning of the 1980s, Alain Aspect and others carried out a series of particularly accurate experiments which proved that the measured correlations follow the predictions of quantum mechanics . Subsequently, in ’98 Zeilinger and other scholars have improved these experiments confirming results in accordance with theoretical predictions.
Quantum entanglement underpins emerging technologies such as quantum computers and quantum cryptography . It also allowed experiments related to quantum teleportation . Although information cannot be transmitted through entanglement alone, the use of a classical communication channel in conjunction with an entangled state allows the teleportation of a quantum state, which would otherwise be impossible as it would require an infinite amount of information to be determined. In practice, as a consequence of the quantum no-cloning theorem , this rich information cannot however be read in its entirety, but it can nevertheless be used in calculations.
Quantum entanglement constitutes a difficulty for quantum theory from an epistemological point of view (a branch of philosophy that deals with the conditions under which scientific knowledge can be obtained and the methods for achieving such knowledge ), as it is apparently incompatible with the principle obvious and realistic of the locality , for which the passage of information between different elements of a system can only take place through successive causal interactions, which act spatially from beginning to end.
For example: according to the principle of locality, one person’s fist can strike another’s nose only if one is close enough or if one is able to set in motion mechanisms that, step by step, reach up to the nose.
Different interpretations of the entanglement phenomenon lead to different interpretations of quantum mechanics.
Another example could be a pair of gloves. If you found a right glove by yourself in your drawer, you can rest assured that the missing glove would fit in your left hand. The two gloves could be described as entangled , entangled , as knowing something about one would tell you something important about the other which is not a random feature.
In fashion, this concept isn’t all that strange. But the concept poses a problem for quantum mechanics.
Even if their understanding is still far away, the bizarre phenomena of quantum mechanics, including the Entanglement, are the basis of many technologies used daily, from computers to lasers, from solar cells to biomedical devices. They also force science to investigate new theories and possibilities, from superluminal interactions to the quantum “death” of the Universe …
Quantum mechanics is undoubtedly the most mysterious chapter in all of physics: anyone can easily realize its innumerable oddities, capable of so blatantly violating common sense.
Is it possible that the behavior of the physical system is somehow predetermined, independent of our ability to choose the experimental conditions at will, in providing the obtained result?
Or should we assume that the measurable quantum properties of particles are not “real”, but exist “simply” as a result of our perceptions (or more precisely of our measurements made on the physical system in question)?
If we are not willing to assume, as it is reasonable to assume, that the reality we experience is created solely by our interaction with the surrounding world at the act of perception or measurement, then we must accept the possibility that quantum interaction at a distance between intertwined particles is transmitted at a speed greater than that of light in vacuum.
Even placed at a distance of billions of light years, if something happens to one particle, that something will directly and immediately affect the other, so much so that it will be possible to know what happened to the first particle even if it is impossible, given the distance. get in touch with it.
Entanglement is usually observed on two photons, ie on the elementary particles at the base of the light.
The quantum teleportation is a technique in computer science quantum that allows, under certain restrictions, to transfer a quantum state in an arbitrarily distant point. Mainly, the effect involved is quantum entanglement.
Through a publication in Nature Physics, a group of scientists revealed to the world that they were able for the first time to teleport information between two chips that are not physically or electronically connected.
The researchers are part of the University of Bristol and the Technical University of Denmark and have managed to carry out this “teleportation” using entanglement, thanks to which two particles are linked together regardless of distance. By changing the properties of one particle, the other will also change instantly, no matter how much space you separate them.
In their study, the team of scientists generated pairs of entangled photons on the chips and then took a quantum measurement on one of them. This observation changed the state of the photon and the change occurred instantaneously in the partner photon present in the other chip as well.