What is Entanglement? ⚛️

Einstein never called entanglement “spooky” in the paper that made the EPR argument famous — and the argument he started ultimately turned into an experiment that ruled out the kind of local hidden-variable explanation he favored. In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen argued that quantum mechanics had to be incomplete. Their argument focused on what they regarded as a conflict between quantum mechanics, locality, and their criterion for what could count as an element of physical reality. The famous phrase “spooky action at a distance” does not appear in their 1935 paper at all. It came later, from a private letter Einstein wrote to his friend Max Born on March 3, 1947. Back in 1935, Erwin Schrödinger responded to the EPR argument and introduced the term “entanglement” to describe the peculiar quantum relationship between separated systems. For nearly three decades, the dispute remained largely theoretical. Bohr defended quantum mechanics, while Einstein remained unconvinced. Then, in 1964, CERN theorist John Bell found a way to turn the disagreement into a strict mathematical test. Bell showed that any local hidden-variable theory satisfying the relevant assumptions would have to obey a mathematical inequality, while quantum mechanics predicted situations in which that inequality could be violated. Philosophy had become an experiment. The first experimental violation came in 1972, when John Clauser and Stuart Freedman tested Bell’s inequality using entangled photons and obtained results consistent with quantum mechanics rather than local hidden-variable theories. Alain Aspect and his colleagues at Orsay, near Paris, then carried out a series of increasingly sophisticated experiments in the early 1980s. One 1981 experiment produced a Bell-inequality violation exceeding 13 standard deviations. In their famous 1982 experiment, Aspect and his colleagues used time-varying analyzers and a calcium-atom source to generate correlated photon pairs. The researchers rapidly switched the analyzer orientations using acousto-optical switches placed 6 meters on either side of the source. This put the two switches about 12 meters apart, corresponding to a light-travel time of approximately 40 nanoseconds between them. Because the switching between the two channels occurred about every 10 nanoseconds, the analyzer settings could change during the photons’ flight, making a detection event on one side and the corresponding change of orientation on the other side spacelike separated. The experiment violated Bell’s inequality by 5 standard deviations, providing strong experimental evidence against local hidden-variable theories and addressing an important locality concern. Even that experiment was not completely loophole-free: the analyzer switching was quasiperiodic rather than genuinely random, leaving a remaining experimental limitation. Then, in 2015, three independent teams performed landmark experiments that simultaneously closed the two major experimental loopholes: the locality loophole and the detection loophole. In the most striking of these, at Delft University of Technology, researchers entangled electron spins associated with nitrogen-vacancy centers in diamonds located in laboratories about 1.3 kilometers apart. The measurement events were arranged to be spacelike separated, so no light-speed-or-slower influence from one measurement could reach the other in time. The result was a loophole-free violation of Bell’s inequality. Alain Aspect, John Clauser (who, with Stuart Freedman, carried out the first experimental violation of a Bell inequality in 1972), and Anton Zeilinger shared the 2022 Nobel Prize in Physics for their experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science. Einstein was right to see that something deeply puzzling was going on. He was wrong about the possibility of explaining those quantum correlations through a local hidden-variable theory of the kind Bell later made experimentally testable. Bell experiments have repeatedly shown that nature cannot be described by a theory in which distant systems simply carry independent, locally predetermined properties that determine the outcomes of every possible measurement. At the same time, quantum entanglement does not allow us to send controllable information faster than light.– Physics Archives 📚