Correlations and intertwined states

Physics experts Alain Aspect, John Clauser and Anton Zeilinger have carried out groundbreaking experiments using quantum entangled states in which two particles behave identically even when they are separated. As a simplification we can imagine that we have two 'entangled' coins. This means that, if we flip one of the coins and it comes up heads, when we flip the other coin it will also come up heads, even if we flip it later. And this would happen regardless of whether or not the two coins were in the same room, or if one was on Earth and the other on Saturn.

Logically, this "strong connection" would not be possible for classical objects such as coins, but it is possible for quantum-mechanical objects such as photons.

The correlation between two systems indicates the Degree relationship that exists between them. And in quantum states, what happens to one of the individual particles that compose them determines what happens to the others. For example, there is a strong correlation between the height and weight of people. Taller people tend to weigh more than shorter people, but there is a negligible correlation between people's height and their taste for ice cream.

For a long time, scientists wondered whether classical physics could explain this correlation subject . Thus, in the 1960s, John Stewart Bell developed a mathematical inequality, Bell's inequality, which states that, if this correlation could be explained by classical physics, the observed value of this correlation between particles after a large issue of measurements could never exceed a certain value.

 Aspect, Clauser and Zeilinger constructed a photon-based experiment in which it was observed that Bell's inequality was violated: the value that establishes whether the system can be explained by classical physics was exceeded.

Thus they demonstrated that there are quantum effects that could not be explained by the traditional methods of physics.

Thanks to the fact that we can obtain systems that are more 'strongly' related than the classical ones, new horizons have opened up in quantum mechanics-based technologies, such as quantum computers or quantum networks.

The famous quantum algorithms that have the potential to revolutionize modern computing or the quantum cryptography protocols that promise to be invulnerable, even if the hacker possesses all the computing power in the universe, would not be conceivable without the phenomenon that the 2022 Nobel laureates have demonstrated in their laboratories. 

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Pedro Crespo and Josu Etxezarreta, researchers at department Biomedical Engineering and Sciences of Tecnun.