It was announced yesterday that Alain Aspect, John Clauser and Anton Zeilinger have been jointly awarded the Nobel prize in physics for their experiments to test the effects of quantum entanglement. These experiments are both extremely important and extraordinarily difficult.
The story can be said to begin with the famous Einstein-Podolsky-Rosen paper of 1935 where they seemed to suggest that the theory of quantum mechanics, by then already hailed as a massive success, had to be incomplete because there were elements of reality that were not represented in the theory. In their paper, they argued that in the standard Copenhagen interpretation of quantum mechanics that was embraced by most physicists, one could have situations where a measurement made on one particle could instantaneously influence the outcomes of a measurement on another particle however far away it was. Einstein felt that was ‘spooky’.
For a long time people thought that one could not distinguish experimentally between the so-called local reality model of nature (preferred by Einstein and spelled out in that paper) and the non-local, ‘action at a distance’ model of quantum mechanics of the Copenhagen interpretation, But John Bell showed in 1964 that such a test could be devised but the difficult experiments involved only started producing results the mid-1970s, and tended to support the Copenhagen model. So it took about 40 years to get even the first experiments done to test the EPR model. It took several more decades of careful experiments to create the current consensus that the Copenhagen interpretation, despite its unsatisfactory aspects, is the one that works best.
“It’s so weird,” Aspect said of entanglement in a telephone call with the Nobel committee. “I am accepting in my mental images something which is totally crazy.”
Yet the trio’s experiments showed it happens in real life.
“Why this happens I haven’t the foggiest,” Clauser told The Associated Press during a Zoom interview in which he got the official call from the Swedish Academy several hours after friends and media informed him of his award. “I have no understanding of how it works but entanglement appears to be very real.”
His fellow winners also said they can’t explain the how and why behind this effect. But each did ever more intricate experiments that prove it just is.
The press release announcing the award goes into some detail about what each of these experimenters did.
One key factor in this development is how quantum mechanics allows two or more particles to exist in what is called an entangled state. What happens to one of the particles in an entangled pair determines what happens to the other particle, even if they are far apart.
For a long time, the question was whether the correlation was because the particles in an entangled pair contained hidden variables, instructions that tell them which result they should give in an experiment. In the 1960s, John Stewart Bell developed the mathematical inequality that is named after him. This states that if there are hidden variables, the correlation between the results of a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment will violate Bell’s inequality, thus resulting in a stronger correlation than would otherwise be possible.
John Clauser developed John Bell’s ideas, leading to a practical experiment. When he took the measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.
Some loopholes remained after John Clauser’s experiment. Alain Aspect developed the setup, using it in a way that closed an important loophole. He was able to switch the measurement settings after an entangled pair had left its source, so the setting that existed when they were emitted could not affect the result.
Using refined tools and long series of experiments, Anton Zeilinger started to use entangled quantum states. Among other things, his research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.
It is rumored that the Nobel prize committee in physics avoids giving the prize to work that has the potential to be later proven wrong and hence there is often a long period between a piece of work and its recognition. This award means that the physics establishment has decided that the results supporting non-locality are solid and unlikely to be reversed.
I often have very little knowledge of the work of Nobel prize winners even in physics and hence cannot make a judgment as to the merits of their work. But this prize seems to me to be definitely well-deserved. The experimental work that needed to be done to get the results required great ingenuity, patience, and painstaking attention to detail.