Matilde Baroni | LIP6 - Équipe QI

Non-locality in communication scenarios : assumptions beyond space-like separation

Mon, 30 Mar 2026 00:00:00 +0000

Félicitations Dr.Baroni !

Résumé

Standard quantum information and quantum foundations have historically focused on Bell non-locality in the simplest setting: bipartite, non-communicating systems. This thesis explores the boundaries of this standard Bell scenario by extending it along three main axes: restricted communication, multipartite settings, and operator-algebraic formulations. To preserve the gap between classical, quantum and ‘supra’-quantum correlations while allowing for information exchange, we investigate communication networks restricted by dimensional, computational (via homomorphic encryption), and operational constraints. In doing so, we develop rigorous analytical frameworks to handle the complex structure of multipartite correlations, focusing on sequential communication models and composable simultaneous purifications. To tackle these challenges, the thesis introduces novel mathematical tools from the operator-algebraic framework, such as a new chain rule for the Radon-Nikodym Theorem. By bridging physics, computer science, and mathematics, this work provides both practical certification techniques for quantum devices and deep structural insights into the nature of physical correlations.

Composable simultaneous purification: when all communication scenarios reduce to spatial correlations

Mon, 09 Mar 2026 00:00:00 +0000

Bell non-locality is a powerful framework to distinguish classical, quantum and post-quantum resources, which relies on non-communicating players. Under which restriction can we have the same separations, if we allow for communication? Non-signalling state assemblages, and the fact that they can always be simultaneously purified, turned out to be the key element to restrict the simplest bipartite communication scenario, the prepare-and-measure, to the standard bipartite Bell scenario. Yet, many distinctive features of quantum theory are genuinely multipartite and cannot be reduced to two-party behaviour. In this work we are interested in extending this simultaneous purification inspired result to all multipartite communication schemes. As a first step, we unify and extend the simultaneous purification result from states to instruments and super-instruments, which are composable structures, and open up the possibility to explore more complex communication scenarios. Our main contribution is to establish that arbitrary compositions of non-signalling assemblages cannot escape the standard spatial quantum Bell correlations set. As a consequence, any interactive quantum realization of correlations outside of this set must involve at least one signalling assemblage of quantum operations, even when the resulting correlations are non-signalling.

Quantitative quantum soundness for all multipartite compiled nonlocal games

Mon, 09 Mar 2026 00:00:00 +0000

Compiled nonlocal games transfer the power of Bell-type multi-prover tests into a single-device setting by replacing spatial separation with cryptography. Concretely, the KLVY compiler (STOC'23) maps any multi-prover game to an interactive single-prover protocol, using quantum homomorphic encryption. A crucial security property of such compilers is quantum soundness, which ensures that a dishonest quantum prover cannot exceed the original game’s quantum value. For practical cryptographic implementations, this soundness must be quantitative, providing concrete bounds, rather than merely asymptotic. While quantitative quantum soundness has been established for the KLVY compiler in the bipartite case, it has only been shown asymptotically for multipartite games. This is a significant gap, as multipartite nonlocality exhibits phenomena with no bipartite analogue, and the difficulty of enforcing space-like separation makes single-device compilation especially compelling. This work closes this gap by showing the quantitative quantum soundness of the KLVY compiler for all multipartite nonlocal games. On the way, we introduce an NPA-like hierarchy for quantum instruments and prove its completeness, thereby characterizing correlations from operationally-non-signaling sequential strategies. We further develop novel geometric arguments for the decomposition of sequential strategies into their signaling and non-signaling parts, which might be of independent interest.

Bounding the asymptotic quantum value of all multipartite compiled non-local games

Sun, 11 Jan 2026 00:00:00 +0000

Abstract Non-local games are a powerful tool to distinguish between correlations possible in classical and quantum worlds. Kalai et al. (STOC’23) proposed a compiler that converts multipartite non-local games into interactive protocols with a single prover, relying on cryptographic tools to remove the assumption of physical separation of the players. While quantum completeness and classical soundness of the construction have been established for all multipartite games, quantum soundness is known only in the special case of bipartite games. In this paper, we prove that the Kalai et al.’s compiler indeed achieves quantum soundness for all multipartite compiled non-local games, by showing that any correlations that can be generated in the asymptotic case correspond to quantum commuting strategies. Our proof uses techniques from the theory of operator algebras, and relies on a characterisation of sequential operationally no-signalling strategies as quantum commuting operator strategies in the multipartite case, thereby generalising several previous results. On the way, we construct universal C(^)-algebras of sequential PVMs and prove a new chain rule for Radon-Nikodym derivatives of completely positive maps on C(^)-algebras which may be of independent interest.

Experimental Quantum Electronic Voting

Tue, 09 Dec 2025 00:00:00 +0000

Quantum information protocols offer significant advantages in properties such as security, anonymity, and privacy for communication and computing tasks. An application where guaranteeing the highest possible security and privacy is critical for democratic societies is electronic voting. As computational power continues to evolve, classical voting schemes may become increasingly vulnerable to information leakage. In this work, we present the experimental demonstration of an information-theoretically secure and efficient electronic voting protocol that, crucially, does not rely on election authorities, leveraging the unique properties of quantum states. Our experiment is based on a high-performance source of Greenberger-Horne-Zeilinger (GHZ) states and realizes a proof-of-principle implementation of the protocol in two scenarios: a configuration with four voters and two candidates employing privacy enhancement techniques and an election scenario supporting up to eight voters and sixteen candidates. The latter is particularly well-suited for secure board-level elections within organizations or small-scale governmental contexts.

Translating Bell Non-Locality to Prepare-and-Measure Scenarios under Dimensional Constraints

Tue, 09 Dec 2025 00:00:00 +0000

Understanding the connections between different quantum information protocols has been proven fruitful for both theoretical insights and experimental applications. In this work, we explore the relationship between non-local and prepare-and-measure scenarios, proposing a systematic way to translate bipartite Bell inequalities into dimensionally-bounded prepare-and-measure tasks. We identify sufficient conditions under which the translation preserves the quantum bound and self-testing properties, enabling a wide range of certification protocols originally developed for the non-local setting to be adapted to the sequential framework of prepare-and-measure with a dimensional bound. While the dimensionality bound is not device-independent, it still is a practical and experimentally reasonable assumption in many cases of interest. In some instances, we find new experimentally-friendly certification protocols. In others, we demonstrate equivalences with already known prepare-and-measure protocols, where self-testing results were previously established using alternative mathematical methods. Our results unify different quantum correlation frameworks, and contribute to the ongoing research effort of studying the interplay between parallel and sequential protocols.

Matilde Baroni - Composable simultaneous purification: The signalling nature of non-signalling correlations

Wed, 29 Oct 2025 00:00:00 +0000

Composable simultaneous purification: The signalling nature of non-signalling correlations

Ce séminaire, donné par Matilde Baroni, aura lieu le 29 October 2025, à 13:0. Il aura lieu en salle 25-26/ 105.

Vous trouverez un plan du campus ici.

Résumé

Purification theorems play a crucial role in quantum information, showing that mathematical models of quantum states and operations relate to physically realisable circuits. A long standing line of research has been concerned with the problem of finding appropriate conditions for simultaneous purification, allowing assemblages of quantum states or operations to be realised by equivalent circuits, up to measurements of auxiliary registers. One such result, the celebrated S-G-HJW theorem, resolves this question for quantum states, while the situation remains much less clear in the more general case of quantum operations. In this paper, we propose an operational no-signalling condition that generalises known simultaneous purification results to the case of quantum instruments. We proceed to show that this generalisation is well-behaved under sequential composition, composition along directed acyclic graphs, and composition with indefinite causal order. A further generalisation to quantum super-instruments allows us to further treat communication scenarios involving loops and players with internal states, thus establishing the equivalence of quantum correlations arising in multipartite space-like separated scenarios with a large class of multipartite communication scenarios admitting operational no-signalling. As a consequence, it follows that any interactive quantum realisation of non-signalling correlations outside of the quantum set must involve signalling communication.

Quantum bounds for compiled XOR games and $d$-outcome CHSH games

Tue, 28 Oct 2025 00:00:00 +0000

Nonlocal games play a crucial role in quantum information theory and have numerous applications in certification and cryptographic protocols. Kalai et al. (STOC 2023) introduced a procedure to compile a nonlocal game into a single-prover interactive proof, using a quantum homomorphic encryption scheme, and showed that their compilation method preserves the classical bound of the game. Natarajan and Zhang (FOCS 2023) then showed that the quantum bound is preserved for the specific case of the CHSH game. Extending the proof techniques of Natarajan and Zhang, we show that the compilation procedure of Kalai et al. preserves the quantum bound for two classes of games: XOR games and d-outcome CHSH games. We also establish that, for any pair of qubit measurements, there exists an XOR game such that its optimal winning probability serves as a self-test for that particular pair of measurements.

Corrections to the Bethe lattice solution of Anderson localization

Fri, 31 May 2024 00:00:00 +0000

Matilde Baroni - Quantum bounds for compiled XOR games and d-outcome CHSH games

Wed, 15 May 2024 00:00:00 +0000

Quantum bounds for compiled XOR games and d-outcome CHSH games

Ce séminaire, donné par Matilde Baroni, aura lieu le 15 May 2024, à 9:45. Il aura lieu en salle Not specified.

Vous trouverez un plan du campus ici.

Résumé

Nonlocal games play a crucial role in quantum information theory and have numerous applications in certification and cryptographic protocols. Kalai et al. (STOC 2023) introduced a procedure to compile a nonlocal game into a single-prover interactive proof, using a quantum homomorphic encryption scheme, and showed that their compilation method preserves the classical bound of the game. Natarajan and Zhang (FOCS 2023) then showed that the quantum bound is preserved for the specific case of the CHSH game. Extending the proof techniques of Natarajan and Zhang, we show that the compilation procedure of Kalai et al. preserves the quantum bound for two classes of games: XOR games and d-outcome CHSH games. We also establish that, for any pair of qubit measurements, there exists a compiled XOR game such that its near-optimal winning probability serves as a robust self-test for that particular pair of measurements. Finally, we derive computational self-testing of three anticommuting qubit observables, based on the compilation of the nonlocal game corresponding to the so-called elegant Bell inequality.