Antonio Acin

Device-independent self-testing refers to the certification of quantum states based entirely on the correlations exhibited by measurements on separate subsystems. The fact that such a certification is possible at all is remarkable in its own right, and is intimately connected to the violation Bell’s inequalities by entangled quantum systems. In the bipartite case, self-testing of states has been completely characterized, up to local isometries, as there exist protocols that self-test arbitrary pure states of any local dimension. Despite the growing interest in device-independent certification protocols, an analogous result in the general multipartite case has remained elusive. In this work, we give a complete characterization of the qubit case, showing that any multipartite entangled state of qubits can be self-tested.

Device-independent quantum key distribution aims to provide key distribution schemes whose security is based on the laws of quantum physics but which does not require any assumptions about the internal working of the quantum devices used in the protocol. We provide a general security proof valid for a large class of device-independent quantum key distribution protocols in a model in which the raw key elements are generated by causally independent measurement processes. The validity of this independence condition may be justifiable in a variety of implementations and is necessarily satisfied in a physical realization where the raw key is generated by N separate pairs of devices.

Continuous variable quantum key distribution via discrete modulations (DM CVQKD) ensures information-theoretic security using standard telecom technologies, providing affordable and scalable quantum communications with simplified classical postprocessing. However, existing security proofs against general attacks often rely on restrictive assumptions, such as a bounded dimension for coherent states, or require impractically large block sizes. In this work, we develop a finite-size security analysis that removes these limitations while incorporating realistic experimental features. Our approach combines the dimension reduction technique, a security proof based on the marginal-constrained entropy accumulation, and a trusted detector model accounting for the receiver imperfections. We report positive key rates in the finite-size regime for relevant block sizes of the order of 10^8. These results contribute to narrowing the gap between theoretical security proofs and practical implementations of DM CVQKD systems.

In a previous work [Senno et al., Phys. Rev. Lett. 131, 130202 (2023)], we provided a framework to quantify the amount of intrinsic randomness produced by characterized but untrusted prepare-and-measure (P\&M) setups. While the cases of pure state preparations or extremal measurements were shown to be defined by semidefinite programs (SDPs), up until now we were missing computational methods for the general scenario of mixed states and nonextremal measurements. In this work, we present a hierarchy of SDP relaxations to lower bound the device-dependent conditional min-entropy. Benchmarking against known special cases, we find that the first level of the hierarchy already attains the optimal value. We then provide two applications. First, for setups affected by global depolarizing noise, we compute a matching lower bound to the analytical attack derived in [Curran et al., arXiv:2506.22294 (2025)], thus showing its optimality. Finally, we show that restricting the correlations between the P\&M boxes to be classical strictly decreases an adversary's predictive power, already in the most elementary setup of a qubit binary measurement.

Determining the Hamiltonian of a quantum system is essential for understanding its dynamics and validating its behavior. Hamiltonian learning provides a data-driven approach to reconstruct the generator of the dynamics from measurements on the evolved system. Among its applications, it is particularly important for benchmarking and characterizing quantum hardware, such as quantum computers and simulators. However, as these devices grow in size and complexity, this task becomes increasingly challenging. To address this, we propose a scalable and experimentally friendly Hamiltonian learning protocol for Hamiltonian operators made of local interactions. It leverages the quantum Zeno effect as a reshaping tool to localize the system's dynamics and then applies quantum process tomography to learn the coefficients of a local subset of the Hamiltonian acting on selected qubits. Unlike existing approaches, our method does not require complex state preparations and uses experimentally accessible, coherence-preserving operations. We derive theoretical performance guarantees and demonstrate the feasibility of our protocol both with numerical simulations and through an experimental implementation on IBM’s superconducting quantum hardware, successfully learning the coefficients of a 109-qubit Hamiltonian.

Quantum information tasks are often analyzed under varying trust assumptions about the devices involved. The semi-device-independent (SDI) framework offers a balance between needed assumptions and experimental feasibility. In this work, we study the energy-constrained SDI scenario, where the only assumption in a prepare-and-measure setup is an upper bound on the energy of the prepared quantum states. In contrast to previous studies that restricted the preparation and measurement devices to be classically correlated, we show that allowing entanglement strictly enlarges the set of achievable correlations. We identify two operational consequences of this result. The first concerns randomness certification, where we show that allowing the adversary to employ entangled strategies may significantly reduce the amount of certifiable randomness. This includes situations where the amount of randomness drops to zero in the presence of entanglement, while it remains positive when entanglement is excluded. Second, for the task of distinguishing an arbitrary quantum channel from the identity, we show that the known dimension-independent bound on the advantage conferred by entanglement is violated under an energy constraint.

Device-independent quantum key distribution (DIQKD) provides the strongest form of quantum security, as it allows two honest users to establish secure communication channels even when using fully uncharacterized quantum devices. The security proof of DIQKD is derived from the violation of a Bell inequality, mitigating side-channel attacks by asserting the presence of nonlocality. This enhanced security comes at the cost of a challenging implementation, especially over long distances, as losses make Bell tests difficult to conduct successfully. Here, we propose a photonic realization of DIQKD, utilizing a heralded preparation of a single-photon path entangled state between the honest users. Being based on single-photon interference effects, the obtained secret key rate scales with the square root of the quantum channel transmittance. This leads to positive key rates over distances of up to hundreds of kilometers, making the proposed setup a promising candidate for securing long-distance communication in quantum networks.

Continuous variable quantum key distribution (CVQKD) with discrete modulation combines advantages of CVQKD, such as the implementability using readily available technologies, with advantages of discrete variable quantum key distribution, such as easier error correction procedures. In this work we consider a phase-shift keying protocol using four coherent states (4-PSK protocol) and coarse-grained heterodyne measurements. We provide a security proof against coherent attacks and compute the achievable key rate in a finite size setting, i.e. with a finite number of rounds. To this end, we employ the generalized entropy accumulation theorem, as well conic optimisation, providing us with improved key rates compared to previous works. At metropolitan distances our method can provide positive key rates for the order of $10^9$ rounds. We also provide a theoretical method to overcome the assumption of a finite photon number cutoff made in previous works.

In this work, we study the consequences of entanglement-assistance (EA) for randomness generation in the semi-DI framework based on energy constraints introduced in (van Himbeeck et al., Quantum 1, 33 (2017)). We show that, given an energy bound $\omega$, the minimum min-entropy that non-EA honest devices can certify decreases when entanglement between the prepare and measurement boxes (of the devices prepared by Eve) is allowed.

Device-independent quantum key distribution is a secure quantum cryptographic paradigm that allows two honest users to establish a secret key, while putting minimal trust in their devices. Most of the existing protocols have the following structure: First, a bipartite nonlocal quantum state is distributed between the honest users, who perform local projective measurements to establish nonlocal correlations. Then, they announce the implemented measurements and extract a secure key by post-processing their measurement outcomes. We show that no protocol of this form allows for establishing a secret key when implemented on certain entangled nonlocal states, namely on a range of entangled two-qubit Werner states. To prove this result, we introduce a technique for upper-bounding the asymptotic key rate of device-independent quantum key distribution protocols, based on a simple eavesdropping attack. Our results imply that either different tools---such as different reconciliation techniques or non-projective measurements---are needed for device-independent quantum key distribution in the large-noise regime, or Bell nonlocality is not sufficient for this task.