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).

## 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 Nicolas Sangouard or Jean-Daniel Bancal! Positions open up from time to time on all of our projects.

Currently, we are specifically looking for PhD students to investigate theoretically the potential of using rare-earth doped crystals to implement a novel architecture for fault-tolerant quantum computing. It will focus on designing a tailored error correction code together with its universal gate set, adapting known and developing new algorithms suitable for this architecture. Upon success, this PhD work will open a new road towards fault-tolerant quantum computing. Interested students are invited to send a CV and a motivation letter to Nicolas Sangouard and Eva González Ruiz.

We welcome people from all background and particularly encourage people from minorities in science to apply.

## Research Events

### 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.

### 08 Dec 2023

Physics World has acclaimed the efforts of the group around Ben Lanyon at the University of Innsbruck to which we modestly contributed for their groundbreaking quantum repeater node published in Physical Review Letters. The story starts in 2009 when we envisioned a quantum network architecture in which entanglement is generated remotely between nodes made with trapped ions and extended to long distances by deterministic manipulations of the ion’s internal states. To make this architecture efficient, we proposed to embed the ions in high-fitness cavities and to convert photons to the telecom band. Ben Lanyon’s group showed that the nodes of this network architecture can actually be implemented. This has been awarded by Physics World as one of the top 10 breakthroughs of the year and highlighted by CEA’s newsletter !

### 24 Jul 2023

While quantum computing can offer substantial speed-ups for solving specific problems, billions of operations are typically required for implementing large scale algorithms. This means that the convergence of quantum algorithms cannot realistically be ensured by requiring physical errors to occur with a probability smaller than the inverse of the number of required operations. Instead, the concept of fault-tolerant quantum computation is envisioned: if the rate of physical errors is below a certain threshold, quantum error correction schemes suppress the logical error rate to arbitrary low levels and make possible—at least in principle—arbitrary long sequences of operations. With its relatively high thresholds, the surface code is one of the most popular quantum error correction codes. As a 2D code, the number of physical qubits per logical qubit increases quadratically with the code distance. Their actual implementation hence comes at the price of a significant overhead in physical resources, with typically hundreds or even thousands of physical qubits per logical qubits to achieve the level of protection required for performing billions of noise-free operations. What would we gain in having a physical platform exhibiting a noise bias so that a simple 1D code would be enough to perform fault-tolerant operations? Answers can be found in our latest paper, which was recently published in Physical Review Letters.

### 22 May 2023

What could a quantum repeater look like? In a proposal from 2009, we proposed an architecture in which entanglement is generated remotely between nodes made with trapped ions and extended to long distances by means of deterministic operations on the ion internal states. We proposed to embed the ions in high-finess cavities to produce photonic entanglement with high efficiencies and frequency conversions was envisioned to bring the photon wavelength to the telecom band. A team from Innsbruck around B. Lanyon managed to implement a trapped ion node with all the properties envisioned almost 15 years ago. This experimental tour de force has been published in Physical Review Letters, received an Editor’s Suggestion and was featured in Physics and in Physics World.

### 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.

### 06 Feb 2023

Trapped ions are one of the leading systems to build quantum computers. To link multiple such quantum systems, interfaces are needed through which the quantum information can be transmitted. Until now, trapped ions were only entangled with each other over a few meters in the same laboratory. Researchers from the university of Innsbruck succeeded to entangle two trapped ions located in two labs separated by 230 meters. These efforts have been supported by a theoretical model of a single trapped ion including most of experimental noise sources. The model that we have developed, provided a guidance for the experimentalists, showing for example what to expect when increasing the distance or helping to identify the most detrimental noise sources. With the entanglement of far away ions, Austrian researchers show that trapped ions are a promising platform for future quantum networks that span cities and eventually continents. The results have been published in Physical Review Letters. They have been highlighted by CEA and received an Editors’ Suggestion and a Synopsis in Physics.

### 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).

### 1 Oct 2021

The standard approach for building a quantum computer consists in realising an enormous processor with millions of qubits. This raises significant engineering challenges in qubit integration and packaging, refrigerated space and cooling power. We have considered an alternative architecture where a small processor is combined with a memory. As in a classical computer, the basic idea is to store all the information in a quantum memory, release a few qubit states to process them in the processor, store them back to the memory and repeat the process until the computation is done. We have shown that this reduces the number of qubits in the processor by two orders of magnitudes for factoring 2048 bit RSA integers using Shor’s algorithm. One additional order of magnitude is even achievable by choosing an appropriate error correction code. Details have been published in Physical Review Letters. They received an editor’s suggestion, a synopsis from Physics and have been highlighted by 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.

### 19 Feb 2021

Together with teams from EPFL and MIT, we succeeded to leverage universal properties of spontaneous Raman scattering to demonstrate Bell correlations between light and a collective molecular vibration. By measuring the decay of these hybrid photon-phonon Bell correlations with sub-picosecond time resolution, we found that they survive over several hundred oscillations at ambient conditions. Our results pave the way for ultra-fast quantum technologies and promote a new technique to probe the role of phonon-mediated entanglement in chemistry or even biology. These results have been published in Science Advances, and highlighted by the CEA (french).

### 02 Oct 2020

Quantum entanglement is a crucial resource for quantum communication devices. Together with the universities of Geneva and Basel, we have successfully entangled the outputs of two optical fibers sharing a single photon at a distance of 2 km. This shows how a form of quantum entanglement that is simple to produce can be distributed and detected over long distances — one more step towards the construction of a secure internet. Our results have been published in Physical Review Letters and highlighted by the CEA.

### 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.

### 21 Oct 2019

Optical frequency photons play an important role in quantum communication, thanks to the ability of carrying quantum information over large distances. To create such photon and to process the information they carry, cavity QED has a great potential to exploit. In this scope, the Warburton group in Basel developed a gated semi-conductor quantum-dot strongly coupled to an optical micro-cavity. We developed a detailed theoretical model of such a hybrid system which allowed us to gain insight into the physics of this coherent exciton-photon interface and demonstrate its great potential for future quantum communications. These results have been published in the prestigious journal Nature and highlighted by the University of Basel. Optical frequency photons play an important role in quantum communication, thanks to the ability of carrying quantum information over large distances. To create such photon and to process the information they carry, cavity QED has a great potential to exploit. In this scope, the Warburton group in Basel developed a gated semi-conductor quantum-dot strongly coupled to an optical micro-cavity. We developed a detailed theoretical model of such a hybrid system which allowed us to gain insight into the physics of this coherent exciton-photon interface and demonstrate its great potential for future quantum communications. These results have been published in the prestigious journal Nature and highlighted by the University of Basel.

### 06 May 2019

Please join us in congratulating Alexey Melnikov for winning the Cozzarelli Prize in the physical and mathematical sciences category. This award acknowledges Melnikov et al. “Active learning machine learns to create new quantum experiments”, a paper published in PNAS, in which reinforcement learning is used to explore a space of possible quantum experiments in order to find one (or more) design fulfilling a specific aim. This price was delivered personally to Alexey and his coauthors during a ceremony in Washington D.C.. Alexey Melnikov appeared recently on national russian television news, in which he talks (Russian) about his prize.

### 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.