“I simply accept quantum non-locality”

What exactly did Einstein object to?
Aspect: He thought that there was an underlying theory in which you could say that this photon will be plus one and that photon will give minus one. This was their disagreement. It was in a sense epistemological. Philosophical. And what Bell discovered 30 years later is that, in fact, if you follow the point of view of Einstein, that there is some property allowing you to know that for this system, this is what you will obtain – then if you follow Einstein's point of view, at some point there is a disagreement with quantum physics. It is remarkable that situations where disagreement happened are extremely rare. And so, in fact, in the 60s, in spite of a half a century of results, there was no available data to decide if Einstein was right or not. This is how John Clauser started the first experiment in 1972. And then there was our series of experiments at Institute d’Optique Paris Saclay in 81 and 82 which were more refined than those of Clauser. I mean, closer to the ideal scheme on which theorists discussed.

But in 1960 the laser was just invented. Wasn´t it a bit too early to start this kind of experiment with this immature photonic hardware?
Aspect: The first experiments of Clauser were without lasers. There was an experiment by Ed Fry in Texas in 1976. He used one laser. Later in my series of experiments, I was using several lasers to make precise excitation of the atom. As result my source was order of magnitude better than those of Clauser and Fry. It is clear, however, that their experiments were fantastic. Clauser was the first to address this topic experimentally. I could build on his approach to go farther. For that, I needed to develop a better source of entangled photons.

Entanglement. Einstein said it's kind of spooky…
Aspect: …No, not exactly. He used that word, but people are making mistakes about that. Einstein accepted entanglement, but he said, if you don't introduce supplementary theories, or if you don't complete quantum mechanics, then it looks like there is spooky action at a distance. So, it was his reasoning. He said: ‘I cannot believe in a spooky action at a distance, therefore I have to complete the theory.’

…ah, and with your experiments you tried to prove that he was right?
Aspect: After our experiments we know that you cannot complete it according to that. So, we must admit some spooky action at a distance. But I prefer to name it quantum non-locality.

At the beginning of your experiments, were you more on Team Einstein or Team Bohr?
Aspect: No, no. As an experimentalist, you must be fully open. I did the experiment because I was convinced by Einstein reasoning. On the other hand, I trusted quantum physics. I wanted to settle the contradiction. You shouldn’t have prejudice when starting an experiment. People that were convinced of Bohrs reasoning would have had no reason for doing these experiments. I had one, fore I knew that quantum physics was efficient, but I also followed Einstein reasoning. I couldn’t see how he could be wrong. And in fact, he was right in putting his finger on the fact that it is extraordinary.

We are speaking of the extraordinary character of entanglement…
Aspect: When people say that I am the guy who proved Einstein wrong, I reply that I demonstrated that Einstein put his finger on an extraordinary thing.

How did you proceed in your Nobel Prize-winning experiments in the early 1980s?
Aspect: At the end of the day, the only important thing is that you produce two photons and make measurements on them. And if you demonstrate that the correlations are so strong that you violate Bell's inequality, then you know that Einstein’s model is not right. But of course, building the black box, producing the pair of photons was my challenge and duty. I did it by calculating. I knew that I had to excite a certain level in calcium – well, in fact, Clauser identified that level, but he could not excite it as well as I was able to do ten years later. I used two photon excitation, an effect just demonstrated by a group in Paris simultaniously with others.

Weren't you constantly coming up against limitations with the beam sources and measurement technology available in the 1980s?
Aspect: Yes and no. The beauty of using an atom is that if you can excite it, it delivers a pair of entangled photons. But the drawback is that the photons are emitted in all directions in space. So, you must collect them with big lenses and other optics. That is complicated. Then in the mid 80s, people began to suggest using a different phenomenon: nonlinear optics. Rather than exciting a level with two lasers you do the opposite. You have an ultraviolet laser and split one ultraviolet photon into two visible photons. The big advantage is that the two photons are emitted in well-defined directions. So, you can inject it into optical fibers. But to give you an idea, people began to work on that in 1985, and they had a good source at the end of the 90s. It took them more than a decade. Because you need a certain quality of crystal which you must cut along the right direction, etc.

How did you overcome the limitations?
Aspect: I was using atoms. Atoms are quantum by construction. I could excite the atom at this level and therefore there was no further discussion that the two photons were entangled. While using a crystal, you depend on the quality of the crystal, on how you use it, and so on. This is why it took them 10 years to have a source as good as mine. Then it was better for the reason I explained: the photons are emitted in a well-defined direction and can be injected into a fiber.

But wasn't it that you had tons of technological limitations in exciting these atoms?
Aspect: Yes. But that is the job of an experimentalist (laughs).

Did you collaborate with photonic manufacturers to push limits?
I did. My photomultipliers were produced in France and so I could discuss with the manufacturer´s team and they selected the best ones for me. It´s a big advantage to have an industry close to you. Of course, you order and give exact specifications. But then you can discuss with the people and explain what you are working on, that you will test their devices and provide them with a test sheet. Both sides benefited. This was an advantage as well as the fact that all my photon counting electronics were borrowed. A very long-term loan from the electronics department of the Nuclear Centre in Paris Saclay. They were building these devices by dozens. I could borrow two that they didn’t miss. So, I could borrow all sorts of equipment, and once again, the key was that I was testing their systems and giving them some suggestions on how to improve them. They were happy with that and lent me a lot of equipment. You are right. I could count upon many people.

Is it true that you conducted these experiments as part of your PhD thesis—in other words, you experimented your way to a doctorate and a Nobel Prize?
Aspect: Absolutely. I defended my PhD in 1983. The title: Three experimental tests of Bell´s theorem of inequality. The first of them was analogous to Clauser’s experiment but with my better source. The second one was original because I could use two channel polarizer for the first time. At that time, no such polarizers were available off the shelf. I was lucky that a team from Philips produced custom made polarizers with two output channels for me, calculated and realized for my wavelengths. When I asked them for the price, they answered ‘too much for you’. So, they did it for free. But once again: I measured the system, told them my results, and both sides benefited. But let’s go back to my three experiments. The second one was done with these two-channel polarizers, giving the plus and minus one result, which was a big progress both for theory and experiment. The third experiment was the first implementation of changing the direction of measurement while the photons were in flight. For this, I had to build the system switch by myself. No one could provide it. But in fact, the trick was simple. You just had to think about it.

Would you please explain it?
The reason why no manufacturer could provide this kind of switch was that I needed a large system. There were just crystal-based devices available at the time. At the size that I needed, these crystals were breaking when putting the transducer on them. I searched literature for acousto-optics interaction and found that water is a good acousto-optic medium. They don't use it in industry, but in fact, it is a good medium. I put two transducers in water and there was no longer a problem of breaking because you cannot break water. This was a crucial step. My first prototype of the water-based switch is to be found in the Stockholm Museum that collects one crucial piece of equipment from every Nobel Prize Winner.

What role does teaching as a professor play in your quest for scientific knowledge?
Aspect: Teaching was always crucial for me. When I started, quantum optics was not yet a well-settled domain. There were still tons of discussions in the scientific community. Some said that you must quantize light while others stated that all phenomena we know can be described with classical electromagnetic waves. The crucial experiment of Leonard Mandel, Mario Dagenais, and Jeff Kimble, showing you need to quantize light, was in 1979. Throughout my life, the subject of quantum optics evolved. Teaching it meant thinking deeply about what you are going to tell the students. It helped me to clarify my point of view on quantum optics. My introductory book to Lasers and Quantum Optics that I wrote with two colleagues, published at Cambridge University Press, still sells well. It contains my vision of quantum optics that matured while teaching. In fact, the idea of the switch for light had its origin in teaching students who were preparing for a difficult exam in experimental physics. They had a big debate on whether there is a difference between using a standing acoustic wave or a running acoustic wave in the interaction of acoustic waves and light. I had to settle the debate, because at the end of the day I was the professor. While thinking about it, I had the initial idea of using acousto-optic interaction with a standing wave to realize the switch. By teaching you learn and clarify your thoughts because you must put them into spoken words.

I am curious if the knowledge of what you call quantum non-locality influenced your beliefs, or the way you make decisions? I mean, obviously there are influences that go beyond our understanding of space and time.
Aspect: I don't have much to say about philosophy. I just accept quantum non-locality. I can fully understand that some people don't like it. But the title of my book is ‘If Einstein Had Known’. We know that we have to choose between two fundamental elements of Einstein’s point of view, on the one hand that an object has a physical reality and on the other hand, that this physical reality cannot be affected faster than light. I think that he would have kept physical reality and accepted the faster than light influence between physical realities.

Why?
Aspect: You can’t utilize quantum non-locality in order to send utilizable information faster than light. You can’t push a button and lit instantaneously a lamp at a distance. So, there is no contradiction with operational relativity. Einstein was strong in his opinion that objects have a physical reality. Once, when one of his friends was advocating Bohrs argument that physical reality depends on how you observe an object, he pointed to the moon and asked: ‘Do you really think that the moon isn’t there if we don't look at it?’ I am also convinced of that. But we must accept a certain non-locality. But this has had no influence on my decision-making in everyday life.

So, after all, are you pleased when engineers take up your research and translate it into products?
Aspect: Of course, I am happy. When doing fundamental research, you work on creating knowledge. That is satisfactory in itself. If someone uses this knowledge to realize new products and applications, that's wonderful. I am happy that people try to develop quantum technologies and that engineers work hard to improve the systems. I happen to be one of the co-founders of a start-up called PASQAL. All my life I have told my collaborators, if you have a good idea, please go and create a start-up. Then some of my former students decided to launch PASQAL. They came to me and reminded me of my words. I agreed to get involved.