Jacqueline Bloch, magician of light

You are here Jacqueline Bloch, light magician In the palm of her hand, she exhibits as a fabulous treasure a half-reflective semicircle, as fine as a razor blade. "This is typically a sample of microcavities that we make right here, at the Center for Nanosciences and Nanotechnologies (C2N)1!" The magician of the place, Jacqueline Bloch, 59 years old, with a smile that is almost constantly luminous, joyful voice, uses this miniature laboratory to mix light and matter and thus observe the quantum world. Today, a research director at C2N, a member of the Academy of Sciences, she has just been awarded the 2026 gold medal of the CNRS. How does one become a light magician? "My father, Claude Bloch, was a great physicist, and I lost him when I was 4 years old," she recounts with a first emotional smile. "My uncle was also a scientist. So I grew up in the admiration of this brilliant father whom I did not know, and more generally in a kind of aura of physicists, which showed me the light." Little Jacqueline thus begins her existence with the desire to become a research physicist. The revelation of polaritons After high school and a year of preparatory classes, she opts for the École supérieure de physique et de chimie industrielles (ESPCI). "I loved the teaching, especially the practical work, during which we had complete freedom to set up a scientific experiment. It was exciting! I met two extraordinary professors there: Claude Boccara, in optics, and Jacques Lewiner, in solid-state physics." Inspired by their teachings, she ventures to the intersection of these two disciplines. After a DEA in condensed matter physics, Jacqueline Bloch pursues a thesis at the Laboratory of Microstructures and Microelectronics (L2M), on the behavior of electrons in nanostructures. Two years later, her true revelation: polaritons, which are born, in particular, in microcavities similar to the one the researcher presented in her hand. Most often micrometer in size2, these cavities are composed of very thin layers of semiconductors sandwiched between two mirrors facing each other. When they are trapped in this trap, photons continuously bounce between the two mirrors. Passing through the semiconductor layers, they can transfer their energy to the electrons present in the semiconductors, catapulting them to a higher energy level. This excitation of the material forms what physicists call an "exciton". This exciton can, in turn, restore its energy by emitting a photon. Thus, within the microcavity, energy passes back and forth – and very quickly – from light to matter, and vice versa. So it is no longer possible to distinguish the photon from the exciton. The duo formed a new quantum object half-light half-matter excitation: the polariton. "The light part of the polaritons allows us to observe them. Indeed, since the mirrors are not perfect, some light escapes from the cavity and can therefore be measured by our detectors. By studying this light, we are able to reconstruct the behavior of the polaritons inside the system," the researcher specifies. Their matter part, in turn, gives them the possibility to interact with each other, whereas photons almost never collide. It becomes possible to transform light into a kind of malleable fluid. Hybrid entities Throughout her career, Jacqueline Bloch has never stopped playing with these extraordinary hybrid objects, unique laboratories to observe, but also to reproduce and manipulate physical phenomena. Just after her thesis, obtained in 1994, she is recruited to the CNRS as a research officer at L2M. "I immediately wanted to study the properties of these polaritons in depth," she recounts. "Following this thread, I had no idea where it would lead me." Straight away, quite far. From 1997, she signs several publications that arouse great attention in the community. She notably shows that it is possible to sculpt the microcavity into the shape of a pillar. This is, in a way, to add walls to the sides of the sandwich and thus, by immobilizing the photons in a tiny area, to better control the states they can occupy. Playing grounds In 1998, she reunites with her partner, a postdoctoral researcher in the United States, and joins the prestigious Bell laboratories as an invited researcher. There, in this temple of innovation, she trains in ultrafast spectroscopy techniques, which allow to track the evolution of polaritons in real time. And it is also there, in America, that she gives birth to her first child. Her second will be born in France, three years later. Today aged 24 and 27, her two sons are both aspiring scientists, one a physicist, the other a chemist. Besides science, the family shares another passion: the mountains. "We continue to hike together summer as winter!" A proud smile. Returning to France in the year 2000, Jacqueline Bloch re-enters L2M. There she happily finds not only her favorite playground – the microcavities from which she brings forth hybrid entities – but also her teammates. "A wonderful collective adventure" "To advance my research, I have greatly benefited from the incomparable 'homemade' know-how in sample fabrication. In particular, Isabelle Sagnes and Aristide Lemaître knew how to shape high-quality microcavities. And, throughout my career, I have had the chance to work in a very stimulating environment, with PhD students and postdocs of multiple nationalities, with two young brilliant and talented research colleagues, Alberto Amo and Sylvain Ravets, and within many collaborations at national and international levels. I live a wonderful collective adventure that allows exploring multiple horizons!" In the following years, she moves towards nonlinear optics. In a system called "linear", when you add twice as much light, you get a signal twice as intense on the detector, without its nature changing. But, by injecting a massive amount of light into a particularly confined space, Jacqueline Bloch manages to considerably increase the number of polaritons, and thus their interactions. From her cavities, collective behaviors totally unprecedented can emerge: the light can change into a kind of fluid, flow, draw swirls. Magician, you say. Scientific rigor With such systems, called "nonlinear", Jacqueline Bloch makes her hybrid entities dance in perfect synchronization until they form just one object that physicists call a "Bose-Einstein condensate3" for polaritons. In 2010, she makes one of the great breakthroughs of her career. She uses her cavities, sculpted in different shapes, to manipulate the movement of the polariton condensates. But, at that time, the interpretations of these experiments are debated: is it really a condensate or just a laser? It is true that, in the measurements, the signal difference is subtle… The researcher then strives to clarify the distinction between the two, with strong explanations that serve as a reference today. With a smile full of humility, she confirms: "I have been recognized for my scientific rigor." And for her breakthroughs, which will follow. Simulating the event horizon of a black hole Never without imagination, Jacqueline Bloch always sculpts more shapes with her microcavities to impose a precise path and rules of movement on the polaritons, and thus perfect their exploration of matter. She shapes pillars, which can then be assembled like Lego pieces, but also microcircuits or fractal structures with fascinating mathematical properties. In 2015, she simulates, with her marvelous light fluids, nothing less than the event horizon of a black hole. "In our experiment, we make a fluid of polaritons flow and, along its path, we place a defect of a few micrometers, explains the researcher. It is a bit technical, so remember that this defect has the effect of accelerating the fluid as a whole, but slowing down the small density waves that travel within it. As a result, upstream of the defect, the fluid moves slower than the small waves and, like fish in a river, they can still go upstream. But, downstream, the fluid is faster: even those that try to go back are swept away. A boundary then appears, mathematically similar to that of the event horizon of a black hole, from which nothing, not even light, can escape." One of the objectives of this miniature cosmic monster is to shed light on Hawking radiation, predicted by the physicist Stephen Hawking in the 1970s. "To this day, we have not yet touched it, but research continues actively, particularly at the Kastler Brossel4 Laboratory, with whom we collaborate." In the intimacy of matter In parallel, Jacqueline Bloch has focused on another kind of magic, this time with the goal of penetrating the intimacy of matter. How does the architecture of a material dictate the behavior of its particles and thus give it its properties? What plays out at the atomic scale is invisible… To access it, the physicist undertakes to build sorts of models of these materials on a larger scale – and this, thanks to her microcavities sculpted like Lego pieces. Thus, in 2014, the team assembles a honeycomb structure within which the polaritons reproduce certain properties of graphene (known for its remarkable conduction properties). A first. Thanks to these models, Jacqueline Bloch and her colleagues can freely change certain parameters to explore the full potential of the material or to verify theoretical predictions. But they can also modify the architectures and thus explore materials that do not exist in nature. Helping young female talents In 2016, L2M becomes C2N5, but Jacqueline Bloch remains Jacqueline Bloch. She collects awards and remarkable publications, shines for her results as well as for the enthusiasm and clarity with which she presents them in conferences. A talent that has always pushed her to become a teacher: "I really like this other part of my activity. For example, I like doing tutorials in small groups, being close to the students." And perhaps some years later, she will also commit herself as a mentor within a mentoring program in partnership with the Women & Sciences association6. "It is to support young female researchers throughout their doctoral studies, not in their research, but in the surrounding aspects: organizing work, managing interactions with the thesis advisor. At a time when the number of women in science is decreasing each year, let us try to help young female talents who have chosen the adventure of research!" Disruptive technologies As a teacher and now mentor, Jacqueline Bloch has never turned away from research, far from it. In 2022, her team shows – yet another great achievement – that the fluctuations of a polariton condensate seem to obey the same universal laws that dictate the growth of frost on a windowpane, or the progression of fire on a sheet of paper. And, in 2025, she demonstrates in her experiments the same phenomenon "not just in one, but in two dimensions, which required even more sophisticated analyses". Fruitful, the research of Jacqueline Bloch and her colleagues could one day lead to disruptive technologies. "It is true that they raise the hope of being able to, for example, process and transmit information only with light (with, in essence, the possibility of greatly reducing the energy footprint of digital technologies, Ndlr). Our research sometimes leads to proofs of concept. But what I like most, she concludes in one of her luminous smiles, is fundamental research! There is still so much to discover… " See you at the next magic show. Also see Author Science journalist with a particular appetite for astronomy and field reports, Émilie Martin is the section chief of the magazine Ciel & Espace, author of documentaries for television, and regularly collaborates on scientific museum projects.