How the 2022 Nobel Prize in Physics Finally Proved Einstein Wrong About Quantum Entanglement

Quantum entanglement describes a state in which two or more particles become so intimately connected that measuring one instantly determines the state of the other — regardless of the distance separating them. In 1935, Albert Einstein, together with Boris Podolsky and Nathan Rosen, published a paper (known as the EPR paradox) arguing that this phenomenon proves quantum mechanics is incomplete. Einstein used the famous term “spooky action at a distance” to describe something he considered impossible: that information could be transmitted instantaneously, violating the theory of relativity. He proposed that there must be “hidden variables” — unknown parameters that predetermine measurement outcomes.

In 1964, John Stewart Bell, a Northern Irish physicist working at CERN, published a paper that forever changed physics. Bell constructed a mathematical inequality — known as Bell's inequality — that sets an upper limit on correlations between measurements on two distant particles, if nature follows the rules of local realism. Local realism consists of two assumptions: first, that particles have definite properties independent of measurement (realism); and second, that no influence can travel faster than light (locality). Bell proved that if quantum mechanics is correct, then correlations between entangled particles violate this inequality. In other words, no model with local hidden variables can reproduce the predictions of quantum physics. The most well-known form of the inequality is the CHSH (Clauser–Horne–Shimony–Holt) version, which states that the value S must be ≤ 2 for local hidden variables, while quantum mechanics predicts values up to 2√2 ≈ 2.83 (known as the Tsirelson bound).

John Clauser, an American experimental physicist, was the first to decide to test Bell's theorem in the laboratory. Together with Stuart Freedman, he performed the first Bell test in 1972 at the University of California, Berkeley. They used pairs of entangled photons produced from calcium atomic cascades. The results were clear: the Bell inequality was violated, supporting the predictions of quantum mechanics. However, this pioneering experiment had serious practical limitations. The choice of detector settings was made before the photons left the source, leaving open the "locality loophole": theoretically, the detectors could have exchanged information without violating relativity.

Alain Aspect, a French physicist at the University of Paris-Sud in Orsay, designed a series of experiments that are considered milestones in physics. In the crucial 1982 experiment, Aspect introduced an innovation: fast-switching polarizers that randomly changed measurement settings while the photons were already in flight. This meant the “decision” about what to measure was made after the photons had already been emitted, eliminating the possibility of information exchange between the two sides. The results showed violation of the Bell inequality by 5 standard deviations, dramatically confirming quantum mechanics. Aspect essentially proved that nature does not operate with local hidden variables — entanglement is real. Pushing the question of “local realism or quantum mechanics?” further, Aspect himself later commented that the experiments “close the door on the debate between Einstein and Bohr.”

Anton Zeilinger, an Austrian physicist at the University of Vienna, brought entanglement from the domain of fundamental physics into the world of technology. In 1997, his team achieved the first successful quantum teleportation, transferring the quantum state of one photon to another photon without physical movement — using exclusively entanglement and a classical communication channel. Zeilinger also developed entanglement swapping techniques, through which two particles that never interacted with each other can become entangled. He implemented quantum cryptography with entangled photons (2000), opening the path toward the quantum internet. In 2000, his team experimentally implemented the GHZ (Greenberger–Horne–Zeilinger) test, a form of Bell's theorem without inequalities, using three entangled photons.

The early Bell experiments, although they showed violation of the inequality, left open technical “loopholes” that could theoretically explain the results without requiring quantum mechanics. The two main loopholes were: the detection loophole, arising from the low percentage of detected photons, and the locality loophole, concerning the spatial distance and temporal sequence of measurements. Until 2015, no experiment had closed both loopholes simultaneously. In October 2015, three independent teams managed to close both loopholes at the same time. The Hensen team at Delft used electronic spins in NV (nitrogen-vacancy) centers in diamonds, separated by 1.3 kilometres. The value S = 2.42 ± 0.20 clearly violated the classical bound of 2. Simultaneously, the Shalm (NIST) and Giustina (Vienna) teams performed analogous experiments with photons. As Aspect himself commented: "No experiment can be considered absolutely loophole-free, but these experiments remove the last doubts that we should abandon local hidden variables."

On October 4, 2022, the Royal Swedish Academy of Sciences announced that the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science." The award does not merely reward experimental confirmations of a theorem. It marks the birth of a new era. Quantum entanglement, once considered a philosophical paradox, today forms the foundation of quantum computing, quantum cryptography, quantum teleportation, and quantum networks. In 2017, Chinese researchers using the Micius satellite achieved Bell inequality violation at a distance of 1,203 kilometres, with a value of S = 2.37 ± 0.09 under strict Einstein locality conditions. The question “does God play dice?” has now transformed into “how do we harness the dice that nature throws?” — and the answer lies in the technology being built upon the work of these three Nobel laureates.