Jean-François Roch: when diamond defects transform matter
Jean-François Roch is a physics professor at ENS Paris-Saclay and a researcher at the Light, Material and Interfaces Laboratory (LuMin - Univ. Paris-Saclay/ENS Paris-Saclay/National Centre for Scientific Research, CNRS/CentraleSupélec). He is also Co-Director of the coordinated research laboratory on quantum sensors between LuMIn and Thales R&T. For over twenty years, the physicist has focused his research on the crystalline defects in diamonds for the joint development of quantum sensors and quantum light sources.
In 1984, Jean-François Roch entered the École normale supérieure de l'enseignement technique (ENSET), which became ENS Cachan and is now ENS Paris-Saclay. He has "extraordinary" memories of his years as a student in the physics department. "The School's message was both demanding and caring: love science!" he recalls. After passing his teaching qualification (agrégation) in physics in 1987, he was assigned to the French Alternative Energies and Atomic Energy Commission (CEA) in Saclay as a contingent scientist to carry out his military service. There, working with experienced engineers, he perfected his handling of complex instruments. On graduating from ENS Cachan in 1988, Jean-François Roch began a thesis at the Institut d'Optique Graduate School under the supervision of Philippe Grangier.
The discovery of QND measurements
Jean-François Roch's thesis focused on quantum non-demolition measurements (QND) in optics. The experiment involved using an atomic beam to couple two laser beams passing through it. One of these beams carries the signal, while the other is used to obtain the measurement. One day, he and Philippe Grangier wondered whether the information obtained in this way was any better than a simple glass slide that picks up part of the signal beam's intensity. This comparison led them to define quantitative criteria that separate quantum non-demolition measurements from conventional measurements. Jean-François Roch defended his thesis in 1992, then joined the Institut d'Optique Graduate School as a CNRS research fellow. Philippe Grangier suggested replacing the atomic beam with a cold atom trap, the principle of which had just been discovered. "Despite the complexity of the experiment, the team achieved record QND measurement efficiency in 1996, a performance that remains unequalled to this day!" says Jean-François Roch.
Demonstrating the wave-particle nature of the photon
In 1996, inspired by Michel Orrit's work on the detection of single molecules, Jean-François Roch turned to condensed matter physics. He joined Serge Haroche's team at the Laboratoire Kastler Brossel and studied gallery modes in silica microspheres, with the idea of coupling these modes to a single molecule. Bernard Decomps, Head of ENS Paris-Saclay, invited him to join the newly-created quantum physics laboratory. Jean-François Roch became a professor there in 1988 and focused on developing single-photon sources by controlling the emission from a single molecule. "For the first time, I became interested in the coloured centres of diamonds; these are atomic-scale defects in diamonds, the source of the diamond's colours, but they also behave like artificial atoms trapped in the lattice. More specifically, I was studying NV centers where a nitrogen atom (N) replaces a carbon atom and associates with a vacancy (V) in the crystal.
In 2005, Jean-François Roch wanted to revisit the single-photon interference experiment carried out by Philippe Grangier and Alain Aspect in 1986, incorporating "Wheeler's delayed choice". "The idea was to probe the nature of our single photon, wave or particle, as late as possible in the process." Overcoming a number of technical challenges and after a year and a half of collaborative work, the experiment was a success. "The results, published in 2007, demonstrated that wave-particle duality cannot be reduced to a binary choice, but depends on the experimental context."
Using crystalline defects
A visit to Stuttgart marked a turning point in Jean-François Roch's research. German physicist Jörg Wrachtrup showed him an experiment in which diamond NV centers can map magnetic fields on a nanometric scale. Impressed by this method, he perfected it in partnership with PhD candidate Vincent Jacques. Together, they successfully developed a unique technique now used by countless teams working on nanomagnetism and spintronics.
In 2009, Jean-François Roch became Deputy Director of ENS Paris-Saclay. In the early days of building Université Paris-Saclay, the institutional experience was exciting but time-consuming, to the point that research activities were put on hold. Choosing to return to his work as a physicist, he resigned in 2011. From 2012 to 2017, he was Director of the Aimé Cotton Laboratory (LAC - Univ. Paris-Saclay/CNRS). He worked closely with Thierry Debuisschert, an engineer at Thales R&T, to develop various NV center magnetometry techniques, which led to the creation of the joint laboratory.
Manufacturing diamond defects
In 2016, Jean-François Roch spent a year in Bochum, Germany, where he familiarised himself with atom implantation techniques to create and localise colour centres. On his return, he designed a device to produce them using a focused ion beam. This prototype, developed in partnership with the French company Orsay Physics, has since been marketed for sale.
2017 marked a new turning point. Jean-François Roch became interested in diamond anvil cells, an "extraordinary" tool capable of creating extreme pressures, comparable to those at the centre of the Earth. "These are two point-cut diamonds pressed together, which, by concentrating the applied force, allows for the generation of extremely high pressures between the two anvils. The device acts like a button that can completely change the physical properties of materials. We can thereby create superconductors that retain this quantum property up to very high temperatures, or make magnetic properties appear or disappear." Working very closely with Paul Loubeyre and his team at the CEA's Military applications division (CEA-DAM) in Bruyères-le-Châtel, he has shown that NV centers can be created on these anvils and used as magnetic sensors in direct contact with the sample.
In 2019, Jean-François Roch visited Cambridge as part of the French Fellows programme at Churchill College. In an environment conducive to reflection, he submitted the Equipex+ e-DIAMANT project in 2020, bringing together the academic partners LuMin, the Systems and Technology Integration Laboratory (List - Univ. Paris-Saclay, CEA) and the Centre for Nanoscience and Nanotechnology (C2N - Univ. Paris-Saclay/National Centre for Scientific Research, CNRS/Univ. Paris Cité), the Albert Fert Laboratory (LMF - Univ. Paris-Saclay/CNRS/Thales), the Group for the Study of Condensed Matter (GEMAC - Univ. Paris-Saclay/UVSQ/CNRS) and the manufacturer Thales. "This innovative project aims to make sure we have the tools needed to improve the quantum properties of diamond required for the development of quantum sensors."
Today, the researcher is helping to set up a high-pressure instrumentation laboratory at ENS Paris-Saclay. His Q-PRESSE (Quantum Probes for High-Pressure Superconductivity) project, supported by an ERC Advanced Grant, explores many types of superconductors at pressures in excess of one million atmospheres. He is also involved in education, creating the ARTeQ diploma in association with C2N's Pascale Senellart, which is designed to enhance students' skills in quantum technologies.
A senior member of the Institut Universitaire de France (University Institute of France) since 2021, in 2024, Jean-François Roch was honoured with both the Jaffé Prize from the Foundation and the Berthelot Medal from the Institut de France, awards which pay tribute to the impact and boldness of the research carried out by this physicist with an exceptional career.
