Paul Méhaignerie, doctoral student in physics

How do you study the quantum interactions of matter ?

I work in a field called quantum simulation. Its aim is to study little-known quantum phenomena in the laboratory, using a " simulator ". Today, we know how to analyze a few interacting particles, but as soon as this is extended to many particles, it becomes more complex. This is due either to the lack of mathematical models for studying them, or to the lack of computing capacity to simulate them on computers. The principle of quantum simulation is to recreate them for real in the laboratory, to give people working on them new avenues of study.
These problems concern, for example, the superconducting properties of certain materials, the study of the shape of biological proteins, or the very fundamental interactions between elementary particles. These varied problems are first studied mathematically, until a momentary blockage necessitates experimentation. This is where the prototype quantum simulator  comes in: its aim is to provide an experimental system that is flexible and precise enough to study these different types of problem.

What does a quantum simulator  actually look like?

It's a complex machine in which several individual quantum elements are interacting over a fairly long period of time. So there are several challenges to overcome. First , you have to be able to set up these different quantum elements side by side. Secondly, we need to be able to make them interact in a parameterizable way to study different types of problem, and thirdly, we need to be able to make them live long enough. These are the three challenges of quantum simulation. There are several technological avenues for achieving this : with trapped ions, with a kind of mini quantum transistor, or with neutral atoms. That's what we're doing here.
Everything takes place in a vacuum chamber, into which we send a small amount of rubidium gas. We then trap a number of these atoms on an array of light beams. These are like light traps, allowing us to place them wherever we like. Each atom is a quantum element that we can make interact with the others to study a problem.

Could the simulator revolutionize quantum physics theories ?

The simulator is unlikely to completely revolutionize modern physics ; all the elementary laws of quantum physics have been well known for several decades.
Professor Haroche has also experimentally described the fundamentals of quantum physics, which have been well-established since the 1930s, by studying extremely elementary quantum systems in the laboratory. For example, he succeeded in studying the life and death of a photon, a single grain of light, and its interaction with an atom. These experiments have confirmed our very precise understanding of quantum physics. Even if we continue this long story, I don't think that what we're doing now will lead to any fundamental discoveries. Rather, it's a tool that will explore regions of quantum physics that we don't yet know very well, or explain certain phenomena.
It's part of the current craze for quantum physics. We hear a lot of talk about quantum computers and quantum cryptography. These achievements are referred to as the second quantum revolution, because they follow on from the first technological applications of the 1960s. Today, the goal of the quantum computer is to take advantage of all these properties of quantum physics to make calculations that are inaccessible to classical computers, but this remains uncertain.

How long have you been interested in quantum physics ?

It's more a matter of chance and curiosity. After passing my baccalauréat in 2012, I went on to study engineering at a preparatory school and then at Polytechnique. That's when I came across quantum physics, during lectures by Philippe Grangier and Alain Aspect at the Institut d'Optique. They're from the same generation as Professor Serge Haroche - one of those French physicists who worked on great quantum physics experiments. It was really exciting. They were very enthusiastic about all the applications of this second quantum revolution. They were very inspiring for many of us in my class.

What's a typical day like for you ?

When I arrive in the morning, I switch on all the equipment and check that everything is set up properly. That is, the lasers are at the right power and acting correctly on the atoms in the vacuum chamber. The rest of my day is spent behind the monitoring computers in the experiment room, taking all the measurements.
To manipulate the atoms, we have to build the computerized experimental sequences that control these elements. For example, when do we send the lasers ? Which ones ? In what order ? These are all settings that we modify every day. Sometimes we make major technical changes that require us to shut down everything, such as changing a laser. This can take anywhere from a week to a year, depending on the extent of the modification.

When you're working on unknown problems, how do you check that everything's working ?

To develop the quantum simulator, we work in known regimes where we know what we can achieve at each stage. We know exactly what state we're taking our atoms to, and how to do it. This means we can experimentally verify that it works, by following a margin of results.

The quantum simulator has to be very flexible to adapt to very different problems. Isn't this a technological challenge ?

The question of system flexibility is crucial. In practice, we work with very specific atomic states, known as circular Rydberg states. In concrete terms, for an atom in a circular Rydberg state, the electron is very far from the nucleus, and we bring it into a truly circular orbit around it. The electron revolves around its nucleus like a planet around the Sun. In this state, atoms have very special properties. They interact a lot with each other, are very sensitive to electric and magnetic fields, and are very stable. They live a very long time. It's thanks to these different properties that we should obtain a system that's highly malleable and flexible, but also very stable and precise. For example, by placing circular Rydberg atoms side by side and changing the electric field just a little, we should be able to change the interaction regime between these atoms.

Are you still fascinated by quantum physics ?

Maybe I don't have the same sense of wonder that I used to feel in lectures at Polytechnique, when I first discovered this science. It's true that practicing it on a daily basis desacralizes its magical side. However, it did introduce me to the world of research, which was still very abstract until my thesis.
Still, there's a sense of pride in working here, around a great experience like this. These are very advanced devices. It's like building a Formula 1 supercar . To build something like this, you have to take on some real technical challenges. For example, we recently succeeded in trapping the atoms side by side and arranging them in the arbitrary geometry we wanted. These are great successes. Quantum simulation is a field in which I believe. I think there will be many successes, and the idea of being part of them makes me very proud.

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Paul Méhaignerie works in the Cavity Quantum Electrodynamics team at the Laboratoire Kastler-Brossel, under the supervision of Dr. Michel Brune. He is also a doctoral student at Sorbonne University under the supervision of Dr. Jean-Michel Raimond. His thesis is entitled " Quantum simulation with circular Rydberg atoms ".

Photos © Patrick Imbert
Interview by Aurèle Méthivier