Aspect's experiment was the first quantum mechanics experiment to demonstrate the violation of Bell's inequalities with photons using distant detectors. Its 1982 result allowed for further validation of the quantum entanglement and locality principles. It also offered an experimental answer to Albert Einstein, Boris Podolsky, and Nathan Rosen's paradox which had been proposed about fifty years earlier.
It was the first experiment to remove the locality loophole, as it was able to modify the angle of the polarizers while the photons were in flight, faster than what light would take to reach the other polarizer, removing the possibility of communications between detectors.
The experiment was led by French physicist Alain Aspect at the Institut d'optique théorique et appliquée in Orsay between 1980 and 1982. Its importance was immediately recognized by the scientific community. Although the methodology carried out by Aspect presents a potential flaw, the detection loophole, his result is considered decisive and led to numerous other experiments (the so-called Bell tests) which confirmed Aspect's original experiment.
For his work on this topic, Aspect was awarded part of the 2022 Nobel Prize in Physics.
History Entanglement and the EPR paradox
The Einstein–Podolsky–Rosen (EPR) paradox is a thought experiment proposed by physicists Albert Einstein, Boris Podolsky and Nathan Rosen which argues that the description of physical reality provided by quantum mechanics is incomplete. In the 1935 EPR paper titled "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?", they argued for the existence of "elements of reality" that were not part of quantum theory, and speculated that it should be possible to construct a theory containing these hidden variables. Resolutions of the paradox have important implications for the interpretation of quantum mechanics.
The thought experiment involves a pair of particles prepared in what would later become known as an entangled state. Einstein, Podolsky, and Rosen pointed out that, in this state, if the position of the first particle were measured, the result of measuring the position of the second particle could be predicted. If instead the momentum of the first particle were measured, then the result of measuring the momentum of the second particle could be predicted. They argued that no action taken on the first particle could instantaneously affect the other, since this would involve information being transmitted faster than light, which is forbidden by the theory of relativity. They invoked a principle, later known as the "EPR criterion of reality", positing that: "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity." From this, they inferred that the second particle must have a definite value of both position and of momentum prior to either quantity being measured. But quantum mechanics considers these two observables incompatible and thus does not associate simultaneous values for both to any system. Einstein, Podolsky, and Rosen therefore concluded that quantum theory does not provide a complete description of reality.
Bell's inequalities In 1964, Irish physicist John Stewart Bell carried the analysis of quantum entanglement much further. He deduced that if measurements are performed independently on the two separated particles of an entangled pair, then the assumption that the outcomes depend upon hidden variables within each half implies a mathematical constraint on how the outcomes on the two measurements are correlated. This constraint would later be named the Bell inequalities. Bell then showed that quantum physics predicts correlations that violate this inequality. Consequently, the only way that hidden variables could explain the predictions of quantum physics is if they are "nonlocal", which is to say that somehow the two particles are able to influence one another instantaneously no matter how widely they ever become separated.
In 1969, John Clauser and Michael Horne, along with Horne's doctoral student Abner Shimony, and Francis Pinki's doctoral student Richard Holt, came up with the CHSH inequality, a reformulation of Bell inequality that could be better tested with experiments.
Early experiments in the United States The first rudimentary experiment designed to test Bell's theorem was performed in 1972 by Clauser and Stuart Freedman at University of California, Berkeley. In 1973, at Harvard University, Pipkin and Holt's experiments suggested the opposite conclusion, negating that quantum mechanics violates the Bell inequalities. Edward S. Fry and Randall C. Thompson of Texas A&M University reattempted the experiment in 1973 and agreed with Clauser. These experiments were only a limited test, because the choice of detector settings was made before the photons had left the source.
Advised by John Bell, Alain Aspect worked to develop an experiment to remove this limitation.
In France Alain Aspect completed his doctoral thesis in 1971 working on holography and then went abroad to teach in the École Normale in Cameroon. He returned to France in 1974 and joined the Institut d'optique in Orsay working for his habilitation thesis. Physicist handled him various papers from Bell and Aspect worked for five year in the construction and preliminary tests for his experiment. He published his first experimental results in 1981, and completed his habilitation in 1983 with the final results of his experiment. The referees included André Maréchal and Christian Imbert from the Institut d'optique, Franck Laloë, Bernard d'Espagnat, Claude Cohen-Tannoudji, and John Bell.
Theoretical scheme
The illustration above represents the principle scheme from which John Bell demonstrated his inequalities: a source S of entangled photons simultaneously emits two photons, and , whose polarization is prepared so that both photons' state vector is:
This formula simply means that the photons are in a superposed state: they are in a linear combination of both photons vertically polarized plus both photons horizontally polarized, with an equal probability. These two photons are then measured using two polarizers P1 and P2, each with a configurable measuring angle: α and β. The result of each polarizer's measurement can be (+) or (−) according to whether the measured polarization is parallel or perpendicular to the polarizer's angle of measurement.
One noteworthy aspect is that the polarizers imagined for this ideal experiment give a measurable result both in the (−) and (+) situations. Not all real polarizers are able to do this: some detect the (+) situation for example, but are unable to detect anything in the (−) situation (the photon never leaves the polarizer). Early experiments used the latter sort of polarizer. Alain Aspect's polarizers were better able to detect both scenarios and therefore much closer to the ideal experiment.
Given the apparatus and the initial state of polarization given to the photons, quantum mechanics is able to predict the probabilities of measuring (+,+), (−,−), (+,−) and (−,+) on the polarizers (P1,P2), oriented on the (α,β) angles:
The quantity of interest is a correlation function given by
with
where (α',β') are a set of different angles. According to the CHSH inequality,
a type of Bell inequality. Quantum mechanics predicts a maximal violation of this inequality for |α − β| = |α' − β| = |α' − β'| = 22.5° and |α − β''' | = 67.5°.
Proposal In 1975, since a decisive experiment based on the violation of Bell's inequalities and verifying the veracity of quantum entanglement was still missing, Alain Aspect proposed in an article, an experiment meticulous enough to be irrefutable.
Alain Aspect specified his experiment so that it would be as decisive as possible. Namely:
Its source of entangled particles must be excellent to shorten the duration of the experiment and to provide as clear a violation of Bell's inequalities as possible. It must show correlations in measurements, but also demonstrate that these correlations are indeed the result of a quantum effect (and consequently of an instantaneous influence) and not of a classical slower-than-light effect between the two particles. The experimental scheme must match John Bell's as closely as possible to demonstrate his inequalities so that the agreement between the measured and predicted results be as significant as possible.
Experiments
Alain Aspect carried out a three-round series of increasingly complex experiments from 1980 to 1981. The first round of experiments reproduced the experimental tests of Clauser, Holt and Fry. In the second round of experiments, Aspect added two-channel polarizers, which improved the efficiency of the detections. These two rounds of experiments were conducted with the help of research engineer Gérard Roger and physicist , an undergraduate student at the time.
The third round of experiments took place in 1982 and were carried out in collaboration with Roger and physicist Jean Dalibard, a young student at the time. This last round, the closest to the initial specifications, will be described here.
Photon source The first experiments testing Bell's inequalities possessed low-intensity photon sources and necessitated a continuous week to complete. One of Aspect's first improvements consisted in using a photon source several orders of magnitude more efficient. This source allowed a detection rate of 100 photons per second, thus shortening the length of the experiment to 100 seconds.
The source used is a calcium radiative cascade, excited with a krypton laser.
Polarizers with an adjustable orientation variable and on a remote position One of the main points of this experiment was to make sure that the correlation between the measurements P1 and P2 had not been the result of "classical" effects, especially experimental artefacts.
As an example, when P1 and P2 are prepared with fixed angles α and β, it can be surmised that this state generates parasitic correlations through current or mass loops, or some other effects. As a matter of fact, both polarizers belong to the same setup and could influence one another through the various circuits of the experimental device, and generate correlations upon measurement.
One can then imagine that the fixed orientation of the polarizers somehow impacts the state the photon couple is emitted with. In such a case, the correlations between the measurement results could be explained by local hidden variables within the photons, upon their emission. Alain Aspect had mentioned these observations to John Bell himself.
One way of ruling out these kinds of effects is to determine the (α,β) orientation of the polarizers at the last moment—after the photons have been emitted, and before their detection—and to keep them far enough from each other to prevent any signal from reaching any one of them.
This method assures that the orientation of the polarizers during the emission has no bearing on the result (since the orientation is yet undetermined during emission). It also assures that the polarizers do not influence each other, being too distant from one another.
As a consequence, Aspect's experimental setup has polarizers P1 and P2 set 6 metres apart from the source, and 12 metres apart from one another. With this setup, only 20 nanoseconds elapse between the emission of the photons and their detection. During this extremely short period of time, the experimenter has to decide on the polarizers' orientation and to then orient them.