Program
Workshop schedule, titles, and abstracts
Wednesday, December 2
Opening session
- Welcome remarks from the organizers.
- Introduction to the JFLI by its director, Philippe Codognet.
- Introduction to NII’s international internship programs and partnerships with French institutions by Emmanuel Planas, deputy director of NII’s Global Liaison Office.
Quantum Learning with Higher-Order Quantum Computation
Efficiently learning the properties of quantum objects is one of the most promising applications of quantum computers. In this talk, I will introduce higher-order quantum algorithms for learning unknown quantum channels. These algorithms are based on higher-order quantum computation, in which a black-box quantum channel is transformed into another quantum channel or a numerical quantity characterizing its properties. In this framework, a quantum computer acquires “quantum data” through black-box queries and processes it coherently, without reconstructing a full classical description of the channel. Quantum learning in this framework aims not only to extract classical information about unknown quantum channels but also to transform them into other quantum channels that can be used for subsequent quantum information processing. After an overview of higher-order quantum learning algorithms developed by our group, I will discuss some of our recent results on quantum algorithms for learning unknown quantum channels in more detail.
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Recent Developments in Quantum Internet Applications
We will present our findings regarding the protocols and applications we have recently been developing toward the creation of a showcase for quantum internet applications. Furthermore, at Keio University’s Faculty of Science and Technology, the percentage of students with a basic understanding of quantum teleportation is expected to reach a majority by next academic year. We will also introduce these initiatives aimed at developing quantum natives.
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Thursday, December 3
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Quantum circuits via classical functions
It is well-known that every n-qubit unitary can be decomposed into a product of controlled single-qubit unitaries. We show that the control conditions in such a decomposition naturally define a classical function that we term a quasi-process function, motivated by its role in the study of indefinite causal order. Here quasi-process functions yield a representation of every n-qubit unitary in a canonical form, namely, as a quasi-process function unitary (qPFU). We use this representation to show how quantum nonlocality without entanglement (QNLWE) can be understood in terms of unitaries that encode the trade-off between causal order and locality noted in R. Kunjwal and Ä. Baumeler, arXiv:2202.00440. Specifically, we obtain two sufficient (but not necessary) criteria—mutual exclusivity of control conditions (MECC) and Unambiguity—for a unitary to be a process function unitary (PFU), i.e., it admits a qPFU representation where the quasi-process function is a process function, an object that can model paradox-free classical causal loops. We also show the inequivalence of these criteria, neither implying the other. We then show that the circuit depth of MECC unitaries admits a characterization in terms of clique-partitioning problems on Hamming graphs and use this correspondence to provide bounds on the circuit depth of n-qubit MECC unitaries. Finally, we show how MECC unitaries provide a recipe for constructing a unitary purification of the associated process function, thereby providing a large class of unitary processes of interest in quantum causality. The generality of this causality-inspired representation of unitaries also makes it a versatile tool for analyzing the structure of quantum circuits in other domains. This talk is based on joint work with Nasra D. Ahmed, arXiv:2610.00579 (2026).
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Calibration Method For Reduced-Bandwidth Light-Matter Simulations
Photons are natural carriers of quantum information, making photonic technologies essential building blocks for quantum communication and distributed quantum computing. Understanding how photons propagate and interact with matter at the quantum level is therefore important for modeling and designing quantum-network components. Hamiltonian-based microscopic models provide a natural framework for describing these interactions, but their analytical treatment is often challenging, making numerical simulations a valuable tool. However, such simulations can become computationally demanding due to the large number of photonic modes involved. In this talk, I will present our recent work on improving the computational efficiency of light–matter simulations while preserving their physical accuracy. We show that restricting the spectral range of photonic modes requires a consistent calibration of model parameters to avoid altering simulation results. We demonstrate this calibration strategy through single- and two-photon scattering simulations across different geometries, taking steps towards efficient microscopic simulations of light-matter systems, with the long-term goal of modeling and emulating optical components for quantum networks.
Friday, December 4
Recent progress on the PPT^2 conjecture
I will review the PPT^2 conjecture and its recent resolution by OpenAI models. I will give the main idea behind the counterexample and present future research directions.
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Computational Monogamy of Entanglement and Non-Interactive Quantum Key Distribution
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Entanglement in quantum channel discrimination: sometimes less is more
Entanglement is known to be a powerful resource that improves performance in various quantum information and computational tasks. A standard example that demonstrates the usefulness of entanglement is the possibility of perfectly discriminating between all four Pauli operations in a single shot via the superdense coding protocol. While entanglement often improves quantum channel discrimination, this is not necessarily the case. In this work, we identify scenarios in which a maximally entangled state is a bad choice of input state and, more generally, we show that excessive entanglement can reduce channel discriminability dramatically. For instance, we present an explicit pair of unitary channels that are perfectly discriminable without entanglement, but for which any strategy with a maximally entangled input state is ϵ-close to a blind uniform guessing strategy. We introduce the concepts of Maximal Entanglement Best Case (MEBC) and Maximal Entanglement Worst Case (MEWC) pairs of channels, characterising channel discrimination tasks where a maximally entangled input state is optimal or as suboptimal as it can be, respectively. For both of these cases, we present conditions and explicit examples. Furthermore, we show that the optimal input states for discriminating MEWC pairs of channels are necessarily separable, and provide non-trivial examples of measurement channels for which any entanglement necessarily reduces the maximum probability of discrimination.
Open Problems in Large-Scale, Fault-Tolerant Quantum Computer Architecture
Are there any open problems in quantum computer architecture, or have we reached the “Shut up and build it” stage? This talk will first assert that we know everything we need to know…then perhaps walk that back and suggest where academic research can contribute to the design and implementation of truly large-scale quantum computing systems.
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Random dilation superchannel: Go and no-go results
We present a quantum circuit that implements the random dilation superchannel, transforming parallel queries of an unknown quantum channel into the same number of parallel queries of a randomly chosen dilation isometry of the input channel. This is a natural generalization of the random purification channel, which transforms copies of an unknown mixed state into copies of a randomly chosen purification state. The circuit complexity of our construction is O(poly(n, log d_I, log d_O)), where n is the number of queries, and d_I and d_O are the input and output dimensions of the input channel, respectively. This random dilation superchannel is extended to the sequential queries approximately, by transforming the parallel random dilation isometry into sequential random dilation unitaries with O(poly(d_I)) overhead in the number of queries. We also show that our results can be further extended to random dilation of quantum combs. On the other hand, we show a no-go theorem on the exact random dilation of sequential queries with o(poly(min{d_I,d_O})) query overhead, showcasing a fundamental difference between the parallel and sequential cases.
Purification as an Operational Resource
Purifications provide a powerful way of representing quantum states and channels, but which purifications can actually be obtained when the underlying resource is only available through black-box uses?
We will first discuss the possibility of constructing purifications of unknown quantum states and channels, showing that no finite number of uses can produce a fixed unknown purification exactly. One may then ask whether this obstruction can be overcome by averaging over the unknown purifying degrees of freedom. While such an averaged notion captures the relevant information in parallel settings, we show that it also fails in the sequential setting. This is witnessed operationally through channel discrimination: we exhibit perfect discrimination using two sequential queries to the purified channels and a query to the bare channel that cannot be achieved by any finite number of queries only to the bare channel, as well as strict gaps in two-query discrimination. These results reveal a fundamental distinction between bare and purified access to quantum channels.