Our group is concerned with research in quantum theory, information theory and the interplay between these fields. We explore applied and fundamental questions related to the certification, the quantification and development of resources for quantum technologies. More on the research section.
We are based in the Institut de Physique Théorique (CEA/CNRS/Univeristé Paris-Saclay).
For related research, visit our colleagues at https://quantum.paris.
Open Position
If you are interested in joining our group as a master’s student, PhD student, or postdoctoral scholar, do not hesitate to contact Jean-Daniel Bancal. Positions open up from time to time on all of our projects.
We welcome people from all background and particularly encourage people from minorities in science to apply.
Research Events
05 Jun 2025
Spin-squeezed states enable measurements with precision beyond classical limits and are central to quantum technologies such as atomic clocks and sensors. Their detection typically relies on the Wineland parameter, estimated from measured spin variances and means. However, standard methods based on error bars fail to account for the possibility that similar statistics could arise from non-squeezed states, especially when measurements are limited. In a paper published in Physical Review Letter, we develop a statistical framework that recasts spin-squeezing detection as a hypothesis test, quantifying the likelihood that observed data could arise from a non-squeezed state. Our analysis reveals that many existing experiments, particularly those involving large spin ensembles (≥ 10³), do not collect enough data to draw statistically significant conclusions. Our method offers a rigorous and practical tool to certify spin squeezing under finite statistics and sets clear benchmarks for future experiments aiming to demonstrate quantum advantage.
26 Mar 2025
Entangled particles is at the heart of the second quantum revolution. When such particles are measured, their entanglement manifests itself in the form of unique outcome statistics. These quantum statistics are key to estimate relevant physical quantities and can sometimes even let us completely identify the physical model describing the entangled objects, a feature known as “self-testing”. For these reasons, the set of all possible quantum correlations has attracted a lot of attention. However, despite decades of effort, this set has always resisted a direct analysis. In a new result published in Nature Physics, we provide the first exact and complete description of a set of quantum statistics. This allows identifying limits of quantum theory itself, as it bounds the extent of experimental results one can expect to observe provided that nature abides to the rules of quantum physics. It also paves the way for exhaustive test procedures for quantum devices. This result has been highlighted by CEA, received a News and Views, and has been covered by several media including the NewScientist.
02 Jul 2024
A dual-based approach of quantum correlations uncovers extreme Bell-like inequalities, providing geometrical insights on the set of quantum statistics. In 1969, Clauser, Horne, Shimony and Holt (CHSH) devised a linear inequality allowing to test J. S. Bell’s prediction that the results of measurements on a pair of entangled particles don’t admit a local model. Since then, remarkable Bell experiments have confirmed this prediction, resulting in numerous applications in quantum information such as secure cryptographic protocols. Yet, the exact set of experimental statistics that are achievable by quantum theory in such experiments remains poorly understood, even in simple scenarios. In this work, we use a novel approach based on duality, a mathematical tool that allows investigating a problem from two equivalent but very different perspectives, to shed new light on the geometry of the quantum set. Concretely, we show that the renown CHSH expression can be decomposed in terms of eight, more fundamental, expressions, each leading to a tight constraint on the set of quantum correlations. One implication of our findings is that an interesting part of the quantum set takes the form of an octagonal-based pyramid. Our result was published in Physical Review Letters.
11 May 2023
Superconducting circuits constitute one of the most promising platform for future quantum computers. Storz and his colleagues from the Wallraff group at ETH Zurich showed that they are also suited to perform a loophole-free Bell test. By achieving a single-qubit readout of less than 100 nanoseconds, the ETHZ group was able to reduce the required qubit separation to around 30 meters to close the locality loophole. Then, they developed a low-loss cryogenic waveguide of this size to connect two fridges containing each a superconducting qubit to reach a high-fidelity two-qubit connected system. As a result, Storz and colleagues’ set-up violates Bell’s inequality by a higher margin than previous photon-based experiments, with a higher rate of data production than that obtained in previous matter-based experiments. This expands the superconducting circuit toolbox and shows that this platform is also promising for realizing device-independent quantum information tasks. This impressive experiment, to which the quantum information theory group had the opportunity to contribute, was published in Nature. The results have been highlighted by ETHZ and CEA, received a Nature News and Views and have been covered by several media including NewScientist and Arstechnica.
27 Jul 2022
A method known as device-independent quantum key distribution has long held the promise of communication security unattainable by other means. Together with an international team of scientists, we provided the theoretical groundwork needed to demonstrate experimentally, for the first time, an approach to quantum key distribution that uses high-quality quantum entanglement to provide much broader security guarantees than previous schemes. The results has been published in Nature, received news & views both in Nature and in APS, and was highlighted by CEA [1,2] as well as other major Europeans institutions [ETH Zürich, Oxford University, EPFL]. Our work was also cited by the Nobel committee in the ‘Scientific background‘ supporting their decision.
11 Nov 2021
We would like to invite you to the Quantum Computer and Simulator 2021 Conference. With talks given by renowned experts in the field of quantum information, this conference will address and answer broad questions about quantum computing and simulation, from the theoretical status of these fields to concrete applications. QCS 2021 is held online and is co-organized by Nicolas Sangouard, Jean-Daniel Bancal and Pierfrancesco Urbani (CEA).
30 Jul 2021
Symmetries play an important role in many areas of physics and often lead to drastic simplifications. Together with collaborators at the University of Geneva, the University of Cologne and the Perimeter Institute, we developed RepLAB, a toolbox to study finite representations of compact groups and decompose them efficiently. The toolbox’s built-in interface with convex optimization solvers makes it particularly suitable to solve concrete problems with symmetries.
11 Jun 2020
Hackers in possession of quantum computers represent a serious threat to today’s cryptosystems. Researchers are therefore working on new encryption methods based on the principles of quantum mechanics. However, current encryption protocols assume that the communicating devices are known, trustworthy entities. But what if this is not the case and the devices leave a back door open for eavesdropping attacks? In collaboration with the group of Professor Renato Renner of ETH Zurich, we made a step towards fully secure encryption with untrusted devices. Our results have been published in Physical Review Letters and highlighted by the university of Basel.
13 May 2020
The no-signalling principle imposes constraints that are expected to be satisfied by any “reasonable” theory: a choice of measurement setting made in one place should not affect the statistics observed at a distance. This principle has proven particularly fruitful in studying generalized probabilistic models in Bell scenarios. With the recent
generalization of Bell nonlocality to networks, we set out to examine with colleagues from Geneva, Bristol, Brussels and Zurich how the principle of no-signalling could be complemented by the independence assumption of distinct source inherent to networks. In a paper published in Nature Communications we show that the resulting condition of non-signaling and independence (NSI) gives rise to highly nontrivial constraints even in absence of measurement settings. This suggest that generalized probabilistic theories on networks have a rich structure.
12 Feb 2019
Quantum entanglement and Bell nonlocality are two fundamental properties of quantum systems whose relation still remains poorly understood today. A remarkable link between them has been known for a long time, however, namely that every pure bipartite states violates some Bell inequality. In other words, the entanglement of a pure bipartite state can be demonstrated device-independently, a result sometimes referred to as Gisin’s theorem. Almost three decades later, we show with colleagues from the university of Innsbruck in Austria that this statement can be generalized to multipartite systems in the following way: the genuine entanglement of every pure multipartite state can be demonstrated device-independently. The algorithmic construction of Svetlichny-type inequalities that we propose to reach this conclusion is economical in resources and provides a natural level of resistance to noise. Our results has been published in PRL.
21 Dec 2018
Bell tests provide certification techniques that are device-independent, that is, they do not rely on a detailed description of the hardware. In a recent article published in Physical Review Letter, we showed how to certify the quality of specific type of joint measurements, namely Bell-state measurements, device independently. Our certificate is noise-tolerant, hence opening a way for an experimental realisation. These results might play an important role for certifying the building blocks of future quantum networks.
05 Nov 2018
Please join us in congratulating Roman Schmied, Jean-Daniel Bancal, Baptiste Allard, Matteo Fadel, Valerio Scarani, Philipp Treutlein and Nicolas Sangouard for winning the Paul Ehrenfest Best Paper Award for Quantum Foundations 2017. The committee awarded the prize to Schmied et al. “for the certification of Bell nonlocality in a many-body quantum system composed of hundreds of atoms, thereby confirming the presence of quantum effects at the mesoscopic scale”. Some of the authors were present to receive the award in Vienna, as shown in the photograph. The prize-winning work can be found here.
02 Nov 2018
The field of quantum computing has made great strides in the past decade or so. As the field continues to proceed towards performing quantum-enabled computation at increasing scale and precision, certification of the constituent ingredients of a quantum computer becomes increasingly crucial. In this work we provide a method based on Bell’s theorem to certify coherent operations for the storage, processing and transfer of quantum information. This completes the set of tools needed to certify all the building blocks of a quantum computer. These results have been published in Physical Review Letters, and have been highlighted by the SNSF and also in the Department website.
