The quantum Schur transform has become a foundational quantum algorithm, yet even after two decades since the seminal 2004 paper by Bacon, Chuang, and Harrow (BCH), some aspects of the transform remain insufficiently understood. Moreover, an alternative approach proposed by Krovi in 2018 was recently found to contain an error. In this talk, I present a corrected version of Krovi’s algorithm along with a detailed treatment of the high-dimensional version of the BCH Schur transform, based on Clebsch-Gordan transforms of the unitary group. This high-dimensional focus makes the two versions of the transform practical for regimes where the number of qudits $n$ is smaller than the local dimension $d$, with Krovi’s algorithm scaling as $\tilde{O}(n^{3.5})$ and BCH as $\tilde{O}(\min(n^5,nd^4))$. As an application, I will show how high-dimensional Clebsch-Gordan transforms can efficiently simulate Haar random unitaries. Speaker: Piotr Kopszak Title: Entanglement recycling in port-based teleportation Date and time: Thursday 20 November ~ 11:00 Room: 26-00/534 Abstract: We study entangled resource state recycling after one round of probabilistic port-based teleportation. We analytically characterize its degradation and, for the case of the resource state consisting of N EPR pairs, we demonstrate the possibility of reusing it for a subsequent round of teleportation in the N→∞ limit. For the case of an optimized resource state, we compare the protocol’s performance to multi-port-based teleportation, indicating that the resource state reuse is possible. An analogous comparison is made in the case of the deterministic scheme. All the best, Marco

Piotr Kopszak - High-dimensional quantum Schur transforms Date and time: Thursday 20 November at 10:00 Room

Thu, 20 Nov 2025 00:00:00 +0000

High-dimensional quantum Schur transforms Date and time: Thursday 20 November at 10:00 Room

Ce séminaire, donné par Piotr Kopszak, aura lieu le 20 November 2025, à 9:0. Il aura lieu en salle Not specified.

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Résumé

Bo Yang - Verifiable blind observable estimation

CHSH violation with optimal resources and partial information

Ce séminaire, donné par Flavien Hirsch, aura lieu le 09 September 2025, à 13:0. Il aura lieu en salle 26-00/534.

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Résumé

Entanglement is necessary for Bell nonlocality. In the case of the CHSH inequality/game, Bell pairs i.e. two-qubit maximally entangled states are known to be optimal to reach the Tsirelson bound. Nevertheless, from an informational perspective, one might wonder if Alice and Bob both need to exactly know the state they share to play CHSH optimally, or if some degree of epistemic ignorance is allowed. In this talk, we consider a variation of the standard Bell bipartite scenario, where Alice and Bob play the CHSH with optimal resources i.e. Bell pairs, but imperfect information about them, in order to answer the question How ignorant can Alice and Bob be about the optimal resources they share and still reach the Tsirelson bound, or at least violate the Bell-local bound?

All the best, Marco

Iria W. Wang - Designing Fault-Tolerant Blind Quantum Computation

Ce séminaire, donné par Adriano Innocenzi, aura lieu le 20 June 2025, à 14:0. Il aura lieu en salle Not specified.

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Résumé

Secret sharing is a fundamental primitive in cryptography, and it can be achieved even with perfect security. However, the distribution of shares requires computational assumptions, which can compromise the overall security of the protocol. While traditional Quantum Key Distribution (QKD) can maintain security, its widespread deployment in general networks would incur prohibitive costs. In this work, we present a quantum protocol for distributing additive secret sharing of 0, which we prove to be composably secure within the

Lucas Hanouz - Security of a secret sharing protocol on the Qline

Fri, 20 Jun 2025 00:00:00 +0000

Ce séminaire, donné par Lucas Hanouz, aura lieu le 20 June 2025, à 14:0. Il aura lieu en salle Not specified.

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In the current stage of noisy intermediate-scale quantum devices (NISQ), the leading eigenvalue algorithms typically rely on variational techniques familiar to many-body practitioners. These methods often involve evolving or optimising one state at a time, falling under the single-reference category.

We propose a novel algorithm, also inspired from many-body physics, that projects the matrix to be diagonalised into a low-energy subspace, resulting in a generalised eigenvalue problem of small dimension. Our approach stands out as a multi-reference technique due to the simultaneous use of multiple trial states. A key advantage of our method is that it does not require the preparation of high-quality initial states. Instead, it efficiently estimates eigenvalues using simple parametric ansatzes. This flexibility sets our algorithm apart from single-reference methods, since the preparation of good trial states is optional rather than mandatory.

During this talk, I will outline the benefits of multi-reference formulations and present an implementation of our algorithm based on coherent states of SU(2). These states form an overcomplete basis of the full Hilbert space, ensuring exactness of the method if a sufficient number of trial states is employed. Additionally, their straightforward algebraic structure allows us to factorise the expectation values of the matrices to be diagonalised, making it possible to retrieve the necessary matrix elements using only sequences of one-qubit expectation values. To demonstrate the power and limitations of the method, I will showcase its application to a toy example involving interacting fermions.

Vilasini Venkatesh R - Compatibility between quantum information-theoretic and spatio-temporal causal structures: no-go results for indefinite causality

Thu, 05 Oct 2023 00:00:00 +0000

Compatibility between quantum information-theoretic and spatio-temporal causal structures: no-go results for indefinite causality

Ce séminaire, donné par Vilasini Venkatesh R, aura lieu le 05 October 2023, à 14:0. Il aura lieu en salle 405 24/25 .

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Résumé

Causality can be defined in terms of a spatio-temporal structure or based on information-theoretic concepts such as agents’ interventions and observable correlations. The process matrix framework describes quantum indefinite causal structures in the information-theoretic sense, but the physicality of such processes remains an open question. At the same time, there are several experiments that claim to implement such processes in Minkowski space-time, which presents an apparent paradox: how can an indefinite information-theoretic causal structure be compatible with relativistic causality in a definite spacetime causal structure? We develop a general theoretical framework for addressing such issues, which formulates the compatibility between these two causality notions, even in scenarios where quantum systems are not necessarily localised in spacetime. First, we characterise information-theoretic causality purely operationally in terms of the signalling structure which agents can infer by performing interventions on a (possibly cyclic) quantum network. By embedding these in a background space-time, we formulate relativistic causality principles such as the impossibility of superluminal signalling. This framework leads to several no-go theorems for the realisations of process matrices in a space-time, showing in particular that indefiniteness of causal order in the process framework translates to the non-localisation of information in the space-time. Moreover, we resolve the apparent paradox by introducing the concept of fine-graining and showing that any realisation of an indefinite causal order process satisfying relativistic causality in a background space-time will ultimately admit a fine-grained description in terms of a definite and acyclic information-theoretic causal structure that is consistent with the light-cone structure of the spacetime. This approach enables a characterisation of the set of quantum processes realisable in Minkowski spacetime, which indicates that causal inequality violating processes cannot be physically realised in a fixed spacetime in a non-trivial manner. Related references: V. Vilasini and R. Renner https://arxiv.org/abs/2203.11245 V. Vilasini, L. del Rio, R. Renner (in preparation) partly based on QPL 2020 talk: https://wdi.centralesupelec.fr/users/valiron/qplmfps/papers/qs01t3.pdf M. Salzger and V. Vilasini (in preparation) based on MS’s master’s thesis https://arxiv.org/abs/2304.06735

Daniel Mills - Distributing circuits over heterogeneous, modular quantum computing network architectures

Wed, 27 Sep 2023 00:00:00 +0000

Distributing circuits over heterogeneous, modular quantum computing network architectures

Ce séminaire, donné par Daniel Mills, aura lieu le 27 September 2023, à 12:0. Il aura lieu en salle 105 Corridor 25-26.

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Résumé

This talk considers the compilation of quantum circuits to heterogeneous networks of quantum computing modules, sparsely connected via Bell states. A circuit too large to be implemented on any one module alone requires the insertion of operations, typically gate teleportation or qubit teleportation, consuming Bell states. These operations constitute a computational bottleneck and are likely to add more noise to the computation than operations performed within a module. We introduce several network architecture aware compilation techniques for transforming, through gate teleportation, a given quantum circuit to one implementable on a network of the aforementioned type, minimising the number of Bell states required. To do so we firstly extend the hypergraph approach of [Andres-Martinez & Heunen. 2019] to arbitrary network topologies. We introduce the use of Steiner trees to find efficient realisations of the entanglement sharing between modules, reusing already established connections as often as possible. Secondly, we extend the embedding techniques of [Wu, \textit{et al.} 2022], which allow for further entanglement reuse, to networks with more than two modules. We discuss how the seemingly incompatible approaches of embedding and of entanglement distribution with Steiner trees can be made to cooperate. Our proposals are implemented and benchmarked; the results confirming that, when orchestrated, the two approaches complement each other’s weaknesses.

Beatrice Polacchi - Certification and privacy for quantum networks through hybrid photonics platforms

Fri, 22 Sep 2023 00:00:00 +0000

Certification and privacy for quantum networks through hybrid photonics platforms

Ce séminaire, donné par Beatrice Polacchi, aura lieu le 22 September 2023, à 12:0. Il aura lieu en salle 24-25/405.

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Résumé

Quantum networks are at the core of a flurry of quantum information tasks, such as quantum communication, quantum key distribution, and quantum computation, as well as the pillar of the future quantum internet. The most promising way to implement near-term quantum networks resides in using hybrid quantum technologies and taking advantage of different existing approaches. These can be represented, for instance, by different kinds of quantum state sources, various physical systems, and the employment of mixtures of classical and quantum hardware to achieve given tasks. However, most importantly, a user of future quantum networks should be equipped with protocols for the certification of their quantum devices, as well as with privacy when interacting with untrusted nodes.In this talk, I will report on three results related to these topics. First, I will show a concrete example of a hybrid building block platform for quantum network applications, represented by a quantum dot-based plug-and-play polarization-entangled photon source. Second, I will illustrate a machine learning-based approach for the device-independent certification of arbitrarily large and complex quantum networks. Such an approach can be used not only to optimize arbitrary functions over supersets of quantum correlations but also to lower-bound violations of Bell-like inequalities compatibly with experimentally observed sets of correlations. Finally, quantum networks are also pivotal to private collaborative delegated quantum computing tasks. A well-established example is blind quantum computing, which allows a user with minimal quantum resources to delegate a quantum computation to a remote quantum server while keeping private input, output, and details of the computation. In this context, I will report on an experimental proof-of-concept multi-client blind quantum computing protocol, implemented on an adaptive and modular linear photonic network. Such findings may represent significant building blocks for the realization of secure and efficient quantum networks, as well as contribute to establishing new milestones on the roadmap towards the quantum internet.

Lorenzo Catani - Not specified

Wed, 29 Mar 2023 00:00:00 +0000

Not specified

Ce séminaire, donné par Lorenzo Catani, aura lieu le 29 March 2023, à 9:0. Il aura lieu en salle Not specified.

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Résumé

Not specified

Jef Pauwels - Not specified

Wed, 08 Mar 2023 00:00:00 +0000

Not specified

Ce séminaire, donné par Jef Pauwels, aura lieu le 08 March 2023, à 10:0. Il aura lieu en salle 25-26/105.

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Résumé

Not specified