What are the “entangled” photons that have won the Nobel Prize in Physics 2022

What are the “entangled” photons that have won the Nobel Prize in Physics 2022

Entanglement. A term difficult to translate in Italian (some dare to intertwine or entangle) that describes one of the most bizarre and counterintuitive phenomena of quantum mechanics, the branch of physics that describes the behavior of particles on a microscopic scale. Whatever you prefer to call it, entanglement is the basis of many technological applications of the present and of the future, including information technology, telecommunications and quantum cryptography. So much so that - and this is the news - the committee of the Nobel Prize for Physics today decided to award Alain Aspect, John Clauser and Antoon Zeilinger "for having demonstrated the potential to investigate and control particles that are in entangled states [...] The development of these experimental tools laid the foundations for a new era of quantum technology. ”

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Let's try to understand something more. In the subatomic world, the one regulated by the laws of quantum mechanics, a particle can be in two different conditions, or states, at the same time. For example (simplifying a bit) a particle can "rotate" in one direction or the other - the so-called spin - but also in both directions at the same time. This multitude of states, also called quantum superposition, persists until an observer measures the spin: at that moment it collapses on only one of the two states. It is an absolutely bizarre phenomenon, but experimentally verified, which has opened the door to philosophical speculations on the role of the observer and on the very existence of an objective reality. Leaving this aspect aside for a moment, things get even more complicated: two (or more) quantum particles can be intrinsically connected to each other (entangled, in fact) in such a way that both have the same superposition of states at the same time. same time. If you perform a measurement on the first particle, causing it to collapse, for example, in the spin-up state, the second will instantly collapse, even if it is distant, in the spin-down state. The fate of the particles, their nature, is in short indissolubly linked, and both undergo the same alteration during the measurement process, even if this is performed on only one of them.

The existence of entanglement - phenomenon repeatedly verified experimentally at ever greater distances - it made Albert Einstein, among others, very uneasy, who poorly digested this concept of "disturbing action at a distance": how is it possible, the German scientist wondered, that something is influenced by an event that happens elsewhere, without sending any kind of signal? And, above all, how is it possible that this phenomenon occurs instantaneously, when instead it is impossible to transfer information at speeds higher than the speed of light? One of the possible answers to this objection lies (goes) in the fact that perhaps quantum mechanics does not describe nature "completely", but is part of a broader - unknown - theory: and perhaps, again, quantum particles also incorporate some other type of information that we cannot measure but whose existence justifies the phenomenon of entanglement. This is the so-called hypothesis of hidden variables, which physicist John Stewart Bell worked on in the 1960s, discovering that there is a type of experiment capable of determining whether the world is purely quantum-mechanical or whether there can be another description of it in terms of hidden variables. If this experiment is repeated many times, all theories with hidden variables must show a correlation between the results that must be less than or equal to a specific value: it is the so-called Bell inequality.

And this is where Clauser's work comes into play, who developed Bell's ideas and conducted several experiments that showed that quantum mechanics actually violates Bell's inequality, meaning that it predicts correlation values among the results higher than those admissible with the theory of hidden variables. In other words, quantum mechanics cannot simply be "replaced" by a theory that uses hidden variables. That is, to be even clearer: quantum mechanics is correct and there is no hidden variable. The work of Clauser was then followed by those of Aspect and Zeilinger, who refined their understanding of the problem and managed to create states of quantum entanglement at a distance - the so-called quantum teleportation. These (and other) experiments have laid the foundations for research in the field of quantum information: being able to control and manipulate the quantum states of entangled particles, in fact, gives us (and will give) the possibility to transfer and store information in an enormous way. faster, more efficient and safer. All current multi-particle entanglement systems currently in use in quantum processors are due to the experiments of Clauser, Aspect and Zeilinger. Nobel more than deserved, in short.

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