Entangled States: From Theory to Technology

Alain Aspect, John Clauser, and Anton Zeilinger conducted pioneering experiments with entangled quantum states. In these states, two particles behave as a single unit even when separated. Their results opened the way to new technologies based on quantum information.

Effects of quantum mechanics that once seemed impossible to describe are now finding practical uses. They have become the focus of a major research field encompassing quantum computers, quantum networks, and secure communication through quantum encryption.

One key to this progress is a feature of quantum mechanics that allows two or more particles to share what is known as an entangled state. What happens to one particle in an entangled pair determines what happens to the other, even when they are far apart.

For a long time, scientists debated whether this correlation arose because the particles in an entangled pair carried hidden variables: instructions telling them which result to produce in an experiment. In the 1960s, John Stewart Bell formulated the mathematical inequality that now bears his name. It showed that if hidden variables existed, 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 hidden-variable theories allowed.

John Clauser turned John Bell’s ideas into a practical experiment. His measurements clearly violated Bell’s inequality, supporting quantum mechanics. The result showed 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 and closed an important one. He could switch the measurement settings after an entangled pair had left its source, ensuring that the settings in place when the particles were emitted could not affect the outcome.

Using precise instruments in a series of experiments, Anton Zeilinger began putting entangled quantum states to use. His group also demonstrated a phenomenon known as quantum teleportation, which makes it possible to transfer a quantum state from one particle to another over 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/