Entangled: from mathematical space to physical intuition

Interview with Alain Aspect, recipient of the Nobel Prize in Physics 2022 for his experiments with entangled photons, that confirmed that quantum mechanics gives a correct description of entanglement, and paved the way for quantum computers, quantum networks and quantum encrypted communication

“If Einstein had known”[1] about your experiments on entanglement, would he have accepted quantum mechanics as a theory capable of describing the world?
Oh, yes, there is no doubt, because our experiments show that quantum mechanics describes the world even in the most surprising situations. Had he known, Einstein would have had to admit that, within his epistemological vision of the world, that we call “local realism”, there is something that must be abandoned. Following our experimental tests, it is no longer possible to uphold both locality and realism at the same time, one must give up at least one of these two assumptions. In the final chapter of my book, I try to answer precisely the question of which of the two it is more reasonable to abandon, but I cannot reveal it here, otherwise I would deprive you of the pleasure of reading it!

[1] If Einstein had known is Aspect’s third popular science book, published in 2025 by Editions Odile Jacob.

How would you explain the experiment that earned you the Nobel Prize to a non-specialist audience?
The experiment is based on a situation where two photons are emitted in opposite directions. They start from the same source and are entangled, meaning that their quantum states have something in common. Then, when they are at a distance – in our case, ten meters apart – a property of each photon is measured, one on one side and one on the other. And the outcome of each individual measurement appears completely random: it may be +1 or -1 with no obvious pattern. But when the two results are compared, the one on one photon and the one on the other photon, it turns out that they are identical: if you obtain +1 on one side, then you obtain +1 on the other side as well; and if you obtain -1 on one side, then you obtain -1 on the other side too. Einstein believed that this phenomenon could only be understood by accepting that, from the outset, the two photons possessed a property stating, for instance, “this time it will be +1”. But Clauser’s[2] experiments and my own demonstrate that this intuitive description does not work. The outcomes are not predetermined, they are established at the moment of measurement: until then, the result can be either +1 or -1. My experiment, specifically, makes it possible to modify the configuration of the apparatus at the very last moment and, thanks to this feature, we were able to observe that the outcome genuinely depends on the conditions present at the moment of measurement, it is not determined in advance.

[2] He was also awarded the Nobel Prize in Physics 2022.