Entangled States—from Theory to Technology

Alain Aspect, John Clauser, and Anton Zeilinger conducted groundbreaking experiments using entangled quantum states. In such states, two particles behave as a single unit even when separated. Their results cleared the way for new technologies based on quantum information.

The once-ineffable effects of quantum mechanics are beginning to find practical applications. They are now the subject of a major research field that includes quantum computers, quantum networks, and secure quantum-encrypted communication.

A key to this progress is the way quantum mechanics allows two or more particles to exist in what is known as an entangled state. What happens to one particle in an entangled pair determines what happens to the other, even when the two are far apart.

For a long time, scientists debated whether this correlation arose because the particles in an entangled pair contained hidden variables—instructions telling them which result to produce in an experiment. In the 1960s, John Stewart Bell formulated the mathematical inequality that bears his name. It showed that if hidden variables existed, the correlations among the results of many measurements could never exceed a certain value. Quantum mechanics, however, predicted that a particular kind of experiment would violate Bell’s inequality and produce stronger correlations than would otherwise be possible.

John Clauser developed John Bell’s ideas into a practical experiment. His measurements clearly violated Bell’s inequality, supporting quantum mechanics. This meant that quantum mechanics could not be replaced by a theory based on hidden variables.

Some loopholes remained after John Clauser’s experiment. Alain Aspect refined the setup to close an important one. He could switch the measurement settings after an entangled pair had left its source, so the settings in place when the particles were emitted could not affect the outcome.

Using sophisticated instruments and a series of experiments, Anton Zeilinger began working with entangled quantum states. His group also demonstrated a phenomenon known as quantum teleportation, which makes it possible to transfer a quantum state from one particle to another across a distance.

“It has become increasingly clear that a new kind of quantum technology is emerging. We can see that the laureates’ work with entangled states is of great importance, even beyond the fundamental questions about the interpretation of quantum mechanics,” said Anders Irbäck, Chair of the Nobel Committee for Physics.

Translation: DeepL
Proofreader: Shiguang
Original: https://www.nobelprize.org/prizes/medicine/2022/press-release/