2022 Nobel Prize in Physics goes to groundbreaking tests of haunted remote action

Physicist N. David Mermin of Cornell University once described quantum entanglement as “the closest thing to magic” because it means that disturbances in one part of the universe can immediately affect distant other parts of the universe, with one or the other. way around the cosmic speed of the universe. light limit. Albert Einstein memorably called it “ghostly action at a distance.” Today, the Royal Swedish Academy of Sciences honored three physicists with the 2022 Nobel Prize in Physics for their work on entanglement. Alain Aspect, John F. Clauser, and Anton Zeilinger were recognized “for experiments with entangled photons, establishing the violation of Bell inequality, and pioneering quantum information science.”

When subatomic particles interact, they can become invisibly joined together, even though they are physically separated. So knowledge about one partner can immediately reveal knowledge about his twin brother. If you measure the state of one particle, you know the state of the other without having to do a second measurement, because the first measurement also determines the properties of the other particle.

There are many different ways in which particles can become entangled, but in each case both particles must arise from a single “mother” process. For example, if you pass a single photon through a special kind of crystal, that photon can split into two new “daughter” particles. We call them “green” and “red” (short for more abstract particle properties such as spin or velocity). Those particles will get entangled. Energy must be conserved, so both daughter particles have a lower frequency and energy than the original mother particle, but the total energy between them is equal to the energy of the mother. We don’t know which is the green and which is the red. We only know that each daughter photon has a 50-50 chance of being one color or the other. But if we happen to see one of the particles and notice that it is red, we can immediately conclude that the other must be green.

Much of this was laid out in a seminal 1935 paper by Einstein, Boris Podolsky and Nathan Rosen, introduced as a thought experiment to show that quantum mechanics was not a complete theory of physics. If the result of a measurement on one particle of an entangled quantum system can have an immediate effect on another particle, regardless of the distance of the two parts, it is called ‘non-local behaviour’. But this seems to violate one of the central principles of relativity: information cannot be transmitted faster than the speed of light, as this would violate causality.

Einstein and his collaborators had the rough idea that hidden variables could amplify conventional quantum mechanics: hitherto unknown local properties of the system that serve as hidden instructions that “tell” particles what results to determine in a given experiment. Einstein, Podolsky and Rosen argued that this should explain the discrepancy so that no immediate ghostly action would be needed. But they didn’t have a specific model to propose. And the physics community became convinced that hidden variables were impossible.

However, John Bell questioned that rejection of hidden variables after reading the heated debates about the philosophical implications of quantum mechanics in the 1920s and 1930s. “I hesitated to think it was wrong,” he once said, “but I… knew it was rottenBell was inspired by David Bohm’s construction of a theory of hidden variables that seemed to work perfectly, but it came at a price: the violation of locality.

Bell devised a way to distinguish between theories that matched the experimental predictions of quantum mechanics and those that couldn’t, proving that local theories would never get the job done. Astrophysicist and philosopher Adam Becker gave a brief summary of the significance of Bell’s work earlier this year during a Pioneer Works broadcast on the subject:

In the EPR thought experiment, there was a perfect correlation between the two electrons, but only if their spins were measured along the same axis. If their spins were measured along different axes — say one along the vertical axis and the other along an axis halfway between vertical and horizontal — quantum mechanics predicted an imperfect correlation between the two. And for certain angles between those axes, the correlation was greater than could be explained without an immediate, long-range connection between them.

In short, Bell had shown that EPR was only half right: the choice was not between ghostly action or incomplete quantum mechanics. The choice was between ghostly action and quantum mechanics incorrect. Quantum mechanics predicted immediate long-range correlations. Could the prediction be confirmed in the lab?