Spooky Action

… at a distance called Albert Einstein the entanglement of particles, an effect that is laid down in the Schrödinger equation. This equation describes the behavior of subatomic particles and allows calculating their whereabouts on a micro geometric level. The so-called wave function gives the probability of a particle in space from here to infinity.

©Nobel Prize Committee

Now imagine a system of two photons (Alice and Bob) - one spinning in one, the other in the opposite direction - moving apart. Their wave function changes in shape - will be smeared out - but remains common to the system, i.e., the photons stay entangled. That means that if one photon changes its spin, the other has to follow simultaneously because the Pauli exclusion principle demands that the total spin of a system remains constant.

The late John Bell doing "chalk physics" at CERN (©CERN)

This entanglement aligns with Bell's theorem that quantum mechanics is incompatible with local hidden-variable theories. Entanglement shows that particle interactions are not only mediated by physical fields but can occur at speeds greater than the speed of light. No "hidden variables" dictate the "paradoxical" correlation between entangled particles; in other words, the experimental results of this year's Noble Prize winners show that quantum mechanics is complete.

In particular, Anton Zeilinger (University of Vienna, Austria) used entangled quantum states to demonstrate quantum teleportation, which allows a quantum state to be transferred from one particle to another at a distance. This earned him the nickname "Mr. Beam," referring to the "Star Trek" series.

My former institute, CERN, not only contributed a photo of the late John Bell, who had spent the last years of his career in Geneva but a general outlook too: These delicate, pioneering experiments not only confirmed quantum theory but established the basis for a new field of science and technology that has applications in computing, communication, sensing, and simulation.

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