13/03/2024

Folha: The strange phenomenon of entanglement

Reproduction of Marcelo Viana's column in Folha de S. Paulo.

The famous EPR paper, published in 1935 by Einstein, Podolsky, and Rosen, pointed out that, according to quantum mechanics, particles that interact at some point become linked ("entangled") in such a way that experiments performed with any one of them determine the properties of the others, no matter how far apart they are.

This conclusion repulsed Einstein, for whom reality is necessarily local. In his opinion, the flaw lay in quantum mechanics itself: there would be "hidden variables," disregarded by this theory, that would explain experimental facts without needing to resort to "spooky actions at a distance."

But Niels Bohr's response to the EPR article, published in the same year, convinced most experts that everything was fine. Among those convinced was the American David Bohm (1917–1992).

A member of the Communist Party, Bohm was investigated by the House Un-American Activities Committee and was even arrested. Although he was acquitted, Princeton University did not renew his contract, and even Einstein was unable to get him accepted into the Institute for Advanced Study.

Unable to find work in his country, Bohm came to Brazil, where he was welcomed by the Physics Institute at USP (University of São Paulo), and even acquired Brazilian citizenship. Later, he moved to Israel and then to the United Kingdom.

In 1957, he no longer found Bohr's explanation satisfactory. With his student Yakir Aharonov , he published another version of the EPR paradox that had a better chance of being experimentally verified. The idea was to focus on properties such as spin, which take on a finite number of values.

An electron and a positron are created from a photon of light and soon move towards two distant observers, Alice and Bob. The spin of each has two possible values, +1/2 or -1/2, both with a 50% probability. But, since the positron is the antiparticle of the electron, the spins along any direction must be symmetrical. Therefore, if Alice verifies that the spin of the electron along a given direction is +1/2, this is enough to know that the spin of the positron along that direction is -1/2, so Bob doesn't need to measure anything. The information seems to be "transmitted" instantaneously from one particle to the other, even though they are millions of light-years apart.

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