arXiv subjects

Anna Steffinlongo

Publications and source records attributed to Anna Steffinlongo.

6 recordsLinked to original sources

arXiv2026

Almost all pure entangled states enable unbounded nonlocality sharing

We establish a connection between Hardy's paradox and nonlocality sharing in sequential bipartite scenarios, where each subsystem is measured in turn by a chain of observers. We show that any correlations exhibiting a Hardy paradox in the two-input two-output scenario enable sequential violations of the CHSH inequality between arbitrarily many pairs of observers, using only projective measurements and the assistance of a small local ancilla system. Since almost all pure entangled states, with the only exception of the maximally entangled one, admit Hardy correlations, our protocol applies generically: almost all pure entangled states, if assisted by a local ancilla, allow for nonlocality sharing between arbitrarily many observer pairs using only projective measurements.

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arXiv2026

Simulating Noncausality with Quantum Control of Causal Orders

Logical consistency with free local operations is compatible with non-trivial classical communications, where all parties can be both in each other's past and future-a phenomenon known as noncausality. Noncausal processes, such as the "Lugano (AF/BW) process", violate causal inequalities, yet their physical realizability remains an open question. In contrast, the quantum switch-a physically realizable process with indefinite causal order-can only generate causal correlations. Building on a recently established correspondence [Kunjwal & Baumeler, PRL 131, 120201 (2023)] between the SHIFT measurement, which exhibits nonlocality without entanglement, and the Lugano process, we demonstrate that the SHIFT measurement can be implemented using a quantum switch of classical communications in a scenario with quantum inputs. This shows that the structure of the Lugano process can be simulated by a quantum switch and that successful SHIFT discrimination witnesses causal nonseparability rather than noncausality. Finally, we identify a broad class of "superposition of classical communications" derived from classical processes without global past capable of realizing similar causally indefinite measurements. We examine these results in relation to the ongoing debate on implementations of indefinite causal orders.

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arXiv2025

Long-distance device-independent quantum key distribution using single-photon entanglement

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.

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arXiv2024

Projective measurements are sufficient for recycling nonlocality

Unsharp measurements are widely seen as the key resource for recycling the nonlocality of an entangled state shared between several sequential observers. Contrasting this, we here show that nonlocality can be recycled using only standard projective measurements, without using quantum ancillas. Focusing on the CHSH inequality, we determine the optimal trade-off in the Bell parameters for a maximally entangled two-qubit state in the presence of shared classical randomness. We then find that non-maximally entangled states make possible larger sequential violations, which contrasts the standard CHSH scenario. Furthermore, we show that nonlocality can be recycled even when only using projective measurements and local randomness. We discuss the implications of our results for experimental implementations of sequential nonlocality.

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arXiv2024

Device-Independent Quantum Key Distribution beyond qubits

Device-Independent Quantum Key Distribution (DIQKD) aims to generate secret keys between two parties without relying on trust in their employed devices, imposing strict noise constraints for key generation. This study explores the resilience of high-dimensional quantum systems in DIQKD, focusing on a comparison between qubits and qutrits. Lower bounds on achievable key rates are investigated through numerical optimization, while upper bounds are evaluated using the Convex-Combination attack, which has been further extended to account for arbitrary dimensions. The observed difference between these bounds provides insights into noise thresholds and potential enhancements in DIQKD scenarios, prompting debate on the merit of increased dimensions given the associated experimental efforts required.

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arXiv2022

Recycling nonlocality in a quantum network

It has recently been discovered that the nonlocality of an entangled qubit pair can be recycled for several Bell experiments. Here, we go beyond standard Bell scenarios and investigate the recycling of nonlocal resources in a quantum network. We realise a photonic quantum 3-branch star network in which three sources of entangled pairs independently connect three outer parties with a central node. After measuring, each outer party respectively relays their system to an independent secondary measuring party. We experimentally demonstrate that the outer parties can perform unsharp measurements that are strong enough to violate a network Bell inequality with the central party, but weak enough to maintain sufficient entanglement in the network to allow the three secondary parties to do the same. Moreover, the violations are strong enough to exclude any model based on standard projective measurements on the EPR pairs emitted in the network. Our experiment brings together the research program of recycling quantum resources with that of Bell nonlocality in networks.

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