Cornell University physicist N. David Mermin once described quantum entanglement as "the closest thing we have to magic," since it means that disturbances in one part of the universe can instantly affect distant other parts of the universe, somehow bypassing the cosmic speed-of-light limit. Albert Einstein memorably dubbed it "spooky 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 inequalities and pioneering quantum information science."
When subatomic particles interact, they can become invisibly connected even though they may be physically separated. So knowledge about one partner can instantly reveal knowledge about its twin. If you measure the state of one, you will know the state of the other without having to make a second measurement because the first measurement determines the properties of the other particle as well.
There are many different ways particles can become entangled, but in every case, both particles must arise from a single "mother" process. For instance, passing a single photon through a special kind of crystal can split that photon into two new "daughter" particles. We'll call them "green" and "red" (shorthand for more abstract particle properties like spin or velocity). Those particles will be 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 equals the mother's energy. We have no way of knowing which is the green one and which is the red. We just know that each daughter photon has a 50-50 chance of being one or the other color. But should we chance to see one of the particles and note that it is red, we can instantly 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 demonstrate that quantum mechanics was not a complete physical theory. If the result of a measurement on one particle of an entangled quantum system can have an instantaneous effect on another particle, regardless of the distance of the two parts, it's called "nonlocal behavior." But this appears to violate one of the central tenets of relativity: Information can't be transmitted faster than the speed of light because this would violate causality.
Einstein and his collaborators had the rough idea that hidden variables could augment conventional quantum mechanics: as-yet-unknown local properties of the system that serve as hidden instructions "telling" particles which result should be determined in a given experiment. Einstein, Podolsky, and Rosen argued that this should account for the discrepancy, so that no instantaneous spooky action would be necessary. But they didn't have a specific model to propose. And the physics community became convinced that hidden variables were impossible.
John Bell, however, questioned that rejection of hidden variables after reading up on the heated debates around 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 rotten." Bell was inspired by David Bohm's construction of a hidden variable theory that seemed to work perfectly well, but it came at a cost: the violation of locality.
Bell figured out a way to distinguish between theories that matched the experimental predictions of quantum mechanics and those that could not and proved that local theories would never be up to the task. 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 topic:
In the EPR thought experiment, there was 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 larger than could be explained without an instantaneous, long-distance connection between them.
In short, Bell had shown that EPR was only half-right: the choice wasn’t between spooky action and quantum mechanics being incomplete. The choice was between spooky action and quantum mechanics being incorrect. Quantum mechanics predicted instantaneous long-distance correlations. Could the prediction actually be upheld in the laboratory?
Enter John Clauser and Stuart Freedman, who set out to test that prediction in the University of California, Berkeley lab. (Freedman died in 2012.) Their experiment used calcium atoms that could emit entangled photons after being illuminated with a special light. They set up a filter on either side to measure the photons' polarization, rather than the spins of electrons, and made a series of measurements. They published the results of their test of Bell's theorem in 1972, which violated a Bell inequality, thereby ruling out local hidden variables. Many other experiments confirmed those results.
Still, there was a possible loophole. Alain Aspect closed that loophole with his own experimental tests of Bell's theorem. Essentially, he figured out a new way of exciting the atoms so they emitted entangled photons at a higher rate. Those experiments also allowed researchers to switch between different settings, so the system would not contain any advance information that could affect the results.
Anton Zeilinger's work testing Bell's theorem appeared in 1982. His group's approach involved creating entangled pairs of photons by shining a laser on a special crystal and using random numbers to shift between measurement settings. To ensure the signals could not affect one another, the team used signals from distant galaxies to control the filters. Zeilinger's team in 1997 also achieved the first experimental demonstration of quantum teleportation. The group "teleported" information about a single photon across a tabletop, recreating an exact copy on the other side. (The original photon was destroyed in the process.) By 2003, the technique had been sufficiently developed that scientists at the University of Geneva in Switzerland managed to teleport photons a distance of 1.2 miles through fiber-optic cable.
These are not easy experiments. Entanglement is fragile and can easily be disrupted. If part of the entangled system becomes entangled with the outside world—even just by a collision with a single molecule of air, for example—the system "decoheres" and the original entanglement is broken. So any experiment involving entangled particles must take great pains to isolate the entangled pairs.
None of this definitively ruled out alternative ideas to resolving this ongoing fundamental concern about quantum mechanics, but it demonstrates that hidden variables cannot be local. And the work of Bell, Clauser, Aspect, and Zeilinger, among others, led to what Aspect later called "the second quantum revolution," because quantum entanglement forms the basis of several cutting-edge technologies. Most notably, quantum computation uses entangled quantum states to significantly expand the set of possibilities that can be efficiently explored so that some calculations can be done much more quickly than what was possible using classical computers.
Quantum cryptography relies upon entangled particles to transmit signals that cannot be intercepted by an eavesdropper without leaving a trace. Not only is there no solid copy of transmitted "messages" (and thus no chance of them being intercepted), if someone tries to eavesdrop on the data stream, it constitutes an observation or measurement. This alters the photons' quantum states and alerts the two parties that their communication channel is compromised.
Entanglement also plays an essential role in various important theoretical concepts in physics, such as Hawking radiation; the ongoing (and often heated debates) over the decades-old black hole information paradox; and possibly even the emergence of spacetime via tensor networks: a series of interlinked nodes with individual morsels of quantum information fitted together like Legos, all held together by the "glue" of entanglement. It doesn't get more fundamental than that.
