Alain Aspect

2013 Balzan Prize for Quantum Information Processing and Communication

For his pioneering experiments which led to a striking confirmation of quantum mechanics as opposed to local hidden-variable theories. His work has opened the way to the experimental control of entangled quantum states, the essential element of quantum information processing.

Alain Aspect is widely recognized for his experimental test of Bell’s inequalities, carried out in 1981-1982 at the Institut d’Optique in Orsay, France. This work is considered to be part of the foundations on which Quantum Information Science has been built, and is known to a general audience as “the Aspect experiments”.
Bell’s inequalities, proposed by the late John Bell in 1964, offer a way to distinguish the predictions of quantum mechanics from those of a large set of alternative theories, known as “local hidden-variable theories”, an example of which is classical (non-quantum) physics. Such tests require measurements to be made on photons moving apart in opposite directions to look for correlations between some of their physical properties, for instance their polarizations. The fact that the measurements are made some distance apart from each other is essential, because to derive Bell’s inequalities one needs to assume “locality”, combined with the fact that information cannot move at faster than the speed of light.

Aspect’s major contribution was to propose a workable experimental scheme to make these measurements by rapidly changing the orientation of polarizers while photons moved through the apparatus, and even more important, to implement it successfully in his experiments.
The photons measured in Aspect’s experiment originate from a single atom and form what, in Quantum Mechanics, is called an “entangled” state. The observation of the state of one photon allows one to predict the outcome of a measurement on the state of the second photon, which in itself is an example of a quantum bit (qubit), a quantum system which may exist in two states, corresponding to the two polarization states. The qubit is the basic element of the “quantum computer”, a concept developed at the same time as the Aspect experiments.

Aspect’s experiments have attracted enormous attention and triggered an avalanche of theoretical and experimental work on quantum entanglement. As a result, new avenues have been explored to implement algorithms for quantum computing and to generate in the laboratory entangled states of photons, cold atoms, cold trapped ions and, later, solid-state systems. The early experiments by Alain Aspect marked the very beginning of Quantum Information Science.

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