Agenda

Département
Langue
Date
Thématique
2026

février

  • 10:00
    13:00

    L'énumération des chemins dans des réseaux est un sujet central en combinatoire énumérative. En particulier, l'étude des chemins prenant des pas dans un ensemble admissible fini, et contraints dans une certaine région a fait l'objet d'un effort soutenu les vingt dernières années, prolongeant l'exemple historique des chemins de Dyck. Notamment, l'usage systématique des équations aux variables catalytiques pour traiter ce problème permet d'appliquer de nombreuses techniques provenant de combinatoire énumérative, de calcul formel, de théorie des probabilités, d’analyse ou de théorie de Galois aux différences, afin de classifier les séries génératrices de tels chemins selon une hiérarchie algébro-différentielle. Cette collaboration a mené à une première classification remarquable des séries génératrices de chemins issus de modèles à petits pas, initiée par les travaux de Fayolle, Iasnogorodski et Malyshev, et d'autre part de Bousquet-Mélou et Mishna, et complétée par de nombreux autres auteurs issus des disciplines susnommées. Depuis, de nombreux efforts ont été menés pour étendre les outils de cette première classification afin de traiter des cas plus complexes. C’est dans ce contexte que s’inscrit la thèse, construite autour de deux extensions différentes, la première concernant les modèles à grands pas, la seconde concernant les chemins à bords interactifs. 

    Amphi LaBRI
  • 10:45
    11:45

    Tensor networks (TNs) offer powerful algorithms for simulating quantum systems, but struggle to represent states with high entanglement. In contrast, quantum computers naturally accommodate entanglement, though developing efficient quantum algorithms remains an active area of research. Integrating TNs with quantum computing has emerged as a promising strategy to overcome limitations inherent to both classical and quantum approaches. In this talk, we will cover two examples of hybrid TN/quantum algorithms. First, we demonstrate how TNs can enhance the simulation of quantum dynamics on noisy quantum devices. In particular, we use a TN algorithm to compress quantum circuits and show that this approach significantly reduces noise requirements to reach a practical advantage on noisy hardware. Second, we explore how TNs can assist in preparing approximate ground states on quantum computers by optimizing parameterized quantum circuits. We show that carefully selecting TN algorithms enables scaling to large qubit systems and that pre-optimizing circuits offers a promising strategy to avoid barren plateaus by providing warm-start initialization. Finally, we analyze the classical simulation costs of this approach and identify scenarios where quantum computers exhibit favorable scaling.

    https://combalgo.labri.fr/pmwiki.php/Groupe/Info-Quantique

    Français
    Room 178
  • 14:30
    15:30

    We could expect the first large-scale fault-tolerant quantum computers to be few in number and only accessible to the masses via some sort of cloud service, where users would send computing tasks to remote quantum computers. Under the common belief that quantum computers will be more powerful than classical ones (i.e., that BPP is strictly contained in BQP), this raises the following question : if a quantum computer can efficiently perform a computation that a classical computer cannot, can the classical computer at least efficiently verify its result? Over the years, many techniques have been developed to make delegated quantum computations verifiable classically, using interaction and various assumptions. Notable examples include the protocols of Aharonov, Ben-Or, Eban and Mahadev ( [ https://arxiv.org/abs/1704.04487 | https://arxiv.org/abs/1704.04487 ] ), Broadbent, Fitzsimons and Kashefi ( [ https://arxiv.org/abs/0807.4154 | https://arxiv.org/abs/0807.4154 ] ), and Mahadev ( [ https://arxiv.org/abs/1804.01082 | https://arxiv.org/abs/1804.01082 ] ). However, none of these techniques are known to relativize to oracle problems, some of which provably separate BPP and BQP and aren't trivially verifiable by a classical agent.

    In this talk, I will introduce the first ever protocols for some of these oracle problems, namely Simon's problem and the Forrelation problem, by adapting known verification techniques in novel ways. I will also discuss the possibility of the existence of an oracle problem that is efficiently solvable by a quantum computer, but for which there couldn't exist a verification protocol between a quantum prover and a classical verifier. This talk is based on results from my PhD thesis, which are yet unpublished.

    https://combalgo.labri.fr/pmwiki.php/Groupe/Info-Quantique

    Français
    On Zoom
  • 09:00
    10:00

    Most of the quantum algorithms have been developed using a finite set of universal quantum gates, which allows for an easy decomposition of any unitary transformation. However, this set of gates includes entangling two-body gates. In the current context of faulty devices, the near-term noisy intermediate-scale quantum era, implementing these gates is deemed costly, both because the time requirement to implement these operations is much larger than for the single-qubit gates, and because they are more susceptible to errors. In contrast to the digital paradigm, analog quantum computers do not allow for arbitrary operations. But in turn of its lack of flexibility, analog devices excel in their robustness and resilience against noise.

    In this context, digital-analog quantum computing (DAQC) was proposed as a way of taking advantage of the robustness of analog quantum computers while maintaining the flexibility of digital quantum computing. For this, this paradigm employs the natural interaction Hamiltonian of the devices as a resource for the entangling operations. Then, universality can be reached by the application of arbitrary single qubit gates. This allows us to implement any desired operation without the need of employing expensive and noisy two-qubit gates.

    Here I present the contributions to the state of the art made during my PhD. In particular, I will describe the solution to one of the main challenges of DAQC, that was achieving universality employing efficient constructive methods. While optimal DAQC protocols require tackling an NP-Hard problem, we have proposed suboptimal protocols which only require polynomial classical resources to be calculated. In addition to this, we further studied its noise resilience, and introduced error mitigation techniques specifically designed for this paradigm. We further continued on the implementation of various algorithms and simulation tasks, showing the possibility that DAQC offers a valid alternative which in some cases might provide advantages in their run-time or fidelity.

    https://combalgo.labri.fr/pmwiki.php/Groupe/Info-Quantique

    Français
    LaBRI

mars