麻豆传媒

In the 1920s, Albert Einstein thought he had found a fundamental flaw in quantum physics. This set off a chain of investigations that, over several decades, showed he had instead discovered a聽crucial聽feature of quantum theory 鈥� and one of its oddest.

This property, now called Bell non-locality, which involves quantum objects maintaining coordinated behaviours even across cosmically large distances, has been unkind to our intuition. Yet embracing it in the 21^st聽century has turned out to be a fantastic idea.

The issue can be set out with the help of two hypothetical experimenters, Alice and Bob, who each have one of a pair of 鈥渆ntangled鈥� particles. Entanglement allows the particles to exhibit correlations even if they are so far apart that no signal could ever pass between them quickly enough to make a difference. Yet, for those correlations to become obvious, each experimenter must interact with their particle. Do the particles 鈥渒now鈥� they are correlated before Alice or Bob interacts with them, or is there something spooky going on between them?

Einstein, working with Nathan Rosen and Boris Podolsky, rejected spookiness. He proposed that there must be 鈥渓ocal hidden variables鈥� that researchers could measure to work out how the particles were always in the know. This would make quantum physics more like our daily experience, where objects only influence each other when they are nearby.

In the 1960s, physicist John Stewart Bell outlined a way to test the trio鈥檚 idea. After decades of attempts, in 2015, several experiments turned Bell鈥檚 test into reality in an unprecedentedly rigorous way, earning three of the聽physicists聽involved聽the Nobel聽prize聽in 2022. 鈥淭hat was the final nail to the coffin of all those ideas,鈥� says聽 at the University of Gda艅sk in Poland.聽Hidden variables聽couldn鈥檛聽save locality in quantum physics, says聽聽at Harvard University. 鈥淵ou can鈥檛 escape non-locality.鈥�

And there are real benefits聽if we聽stop聽trying to escape聽non-locality聽and embrace it聽instead. For聽聽at Delft University of Technology in the Netherlands, who led one of the experiments, the issue was never about spookiness. Rather, he conceived of the experiment as a feat of聽鈥渜uantum advantage鈥澛犫�撀爏omething聽beyond the聽abilities of聽any聽conventional聽computer. His intuition bore out: some of the machinery necessary for 鈥淏ell tests鈥� became a foundation for unprecedentedly secure quantum cryptography.

Hanson now builds quantum communication networks, leveraging entangled particles to develop a nearly unhackable future internet. Quantum computing researchers similarly use entangled particles to make computations more effective. Physicists haven鈥檛 yet fully unravelled the meaning of entanglement and are continuing to examine the assumptions that underlie Bell鈥檚 work, but entangling quantum objects reliably has become a technological resource, a stunning second act for a key player in the debate about our world鈥檚 quantumness.