# Alain Aspect

Alain Aspect (born June 15, 1947) is a French physicist who performed the crucial "Bell test experiments" that showed that Albert Einstein, Boris Podolsky and Nathan Rosen's "spooky action at a distance", did in fact appear to be realised when two particles were separated by an arbitrarily large distance. A correlation between their wave functions remained, as they were once part of the same wave-function that was not disturbed before one of the child particles was measured.

## Quotes

* Now comes the Einstein–Podolsky–Rosen entangled state. Now I see faces, people saying, "Oh..?" Don't worry! When you go to the concert, you don't need to be able to read the music, to enjoy the music. ...So here... [are] equations. It's a pleasure for my colleague physicists. If you can't read the equation, listen to me. I'm not going to sing, but... listen to the words... the words are... a way of describing the equations, and you don't need to know the mathematics...
** "2017 Andrew Carnegie Lecture: Professor Alain Aspect" (May 4, 2017) 16:14-16:55 @Official University of Glasgow Alumni YouTube channel.
* We pretend that if I get result +1 here, immediately the photon there is in the state |x>, but if I find -1, immediately the other photon assume[s] another state of polarization... [T]his image is not acceptable for Einstein because it seems as [though] something is going faster-than-light. ...It is by this ...reasoning that Einstein said... "If you want to make sense of this correlation at a distance, you have to accept that before they arrive at the measuring apparatus, the particles have already a property determining the outcome." ...Bohr disagreed immediately. ...I don't know anybody who finds Bohr's reply understandable. It's not a joke, what I'm going to say, although it sounds [like] a joke. Bohr is so cautious in his wording that he makes it almost impossible to understand... Bohr insisted on... complimentarity, and at one point he declared... that "Clarity and truth are complimentary," and he made all efforts to be as true as possible.
** "The future of quantum technologies: the Second quantum revolution" (Feb 13, 2019) 30:30-33:38. @Nokia Bell Labs YouTube channel.
* La principale difficulté pour vulgariser la physique quantique, c'est qu'on ne sait pas très bien comment en fabriquer des images dans notre monde. C'est en ce sens qu'elle est vraiment contre-intuitive.
** The main difficulty in popularizing quantum physics is that we do not really know how to make images of it in our world. In this sense it is really counterintuitive.
** Interview on the occasion of the CNRS Gold Medal Award Ceremony in December 2005.

### "Introduction: John Bell and the second quantum revolution" (2004)

"Introduction: John Bell and the second quantum revolution", in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics (2nd ed, 2004)
* The most remarkable feature of Bell's work was undoubtedly the possibility it offered to determine experimentally whether or not Einstein's ideas could be kept. The experimental tests of Bell inequalities gave an unambiguous answer: entanglement cannot be understood as usual correlations, whose interpretation relies on the existence of common properties, originating in a common preparation, and remaining attached to each individual object after separation, as components of their physical reality.
* I think it is not an exaggeration to say that the realization of the importance of entanglement and the clarification of the quantum description of single objects have been at the root of a second quantum revolution, and that John Bell was its prophet. And it may well be that this once purely intellectual pursuit will also lead to a new technological revolution.
* Certainly we do not need quantum mechanics for macroscopic objects, which are well described by classical physics – this is the reason why quantum mechanics seems so foreign to our everyday existence.
* As a witness of that period, I am also deeply convinced that John Bell indirectly played a crucial role in the progress of the application of quantum mechanics to individual objects, microscopic and mesoscopic. The example of his intellectual freedom, that had led to the recognition of the importance of entanglement, was no doubt an encouragement to those who were contemplating the possibility of developing new approaches, beyond the so-efficient paradigm developed decades earlier. His example opened the gate for new quantum explorations.
* John Bell devoted most of his efforts to conceptual and theoretical questions. Would he have liked that I also stress the importance of the technological revolutions that were, and will be, enabled by the conceptual revolutions? I cannot tell, but we know that he started his career in accelerator design, and that he always showed a profound respect for technological achievements. I like to think that he would have loved quantum-jumps-based atomic clocks, as well as entangled qubits.

### Closing the Door on Einstein and Bohr’s Quantum Debate (Dec 16, 2015)

    Alain Aspect, Physics Vol. 8, 123, American Physical Society.
* In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR) wrote a now famous paper questioning the completeness of the formalism of quantum mechanics. Rejecting the idea that a measurement on one particle in an entangled pair could affect the state of the other—distant—particle, they concluded that one must complete the quantum formalism in order to get a reasonable, "local realist," description of the world. This view says a particle carries... locally, all the properties determining the results of any measurement... (The ensemble of these properties constitutes the particle’s physical reality.)
* [In] 1964... John Stewart Bell... discovered inequalities that allow an experimental test of the predictions of local realism against those of standard quantum physics.
* In the ensuing decades, experimentalists performed increasingly sophisticated tests of Bell’s inequalities. But these tests have always had at least one “loophole,”...
* [B]y closing the two main loopholes at the same time, three teams have independently confirmed that we must definitely renounce local realism... Although their findings are... no surprise, they crown decades of experimental effort. The results... place several fundamental quantum information schemes... as device-independent quantum cryptography and quantum networks, on firmer ground.

## Quotes about Aspect
### Solving the Einstein-Podolsky-Rosen puzzle (2012)

    : the origin of non-locality in Aspect-type experiments (Jun 7, 2012) by Werner A. Hofer, arxiv-1109.6750v3
* One of the most puzzling results in modern physics, based originally on... EPR... is the apparent non-locality of correlation measurements in quantum optics. The experiments performed on pairs of entangled photons, beginning with the experiments by Alain Aspect in 1982, seem to prove... that the two measurements are not independent. The measurements are usually interpreted in terms of the Bell inequalities which assert that their violation, corresponding to the experimental results and... predictions of quantum optics, amounts to a nonlocal connection... There has been... debate on whether such... implies superluminal effects... for example Maudlin's book. The present consensus is that... no information travels faster than light...
** Ref: Tim Maudlin, Quantum Non-Locality and Relativity (1994)
* [W]e... approach... from a new angle. ...[W]e start from the assumption that quantum optics contains, in its mathematical formalism, the answer.... [T]he conceptual difficulty... has been due to the lack of transparency in its mathematical framework. We can formulate this hypothesis in two distinct statements: 1. Quantum optics is complete. 2. The connection between the two photons is due to their common source.
* The second statement relates... that a common source for both photons of an entangled pair is a common feature in all Aspect-type experiments. ...[T]hese ...insights are sufficient to develop a model of photon entanglement accounting for all features in the experiments.
* Experiments with time-like separation... were only possible about twenty years after the first experiments by Aspect.
* [W]e obtain the same correlations of polarizations as in Aspect's first experiments.

## External links

* Alain Aspect @NobelPrize.org
* Alain Aspect videos @YouTube
* Closing the Door on Einstein and Bohr’s Quantum Debate by Alain Aspect, Physics (Dec 16 2015) 8, 123, American Physical Society. Alt URL.