arXiv subjects

Marwan Haddara

Publications and source records attributed to Marwan Haddara.

5 recordsLinked to original sources

arXiv2026

Detection-loophole-free nonlocality in the simplest scenario

Loophole-free quantum nonlocality often demands experiments with high complexity (defined by all parties' settings and outcomes) and multiple efficient detectors. Here, we identify the fundamental efficiency and complexity thresholds for quantum steering using two-qubit entangled states. Remarkably, it requires only one photon detector on the untrusted side, with efficiency $ε> 1/X$, where $X \geq 2$ is the number of settings on that side. This threshold applies to all pure entangled states, in contrast to analogous Bell-nonlocality tests, which require almost unentangled states to be loss-tolerant. We confirm these predictions in a minimal-complexity ($X = 2$ for the untrusted party and a single three-outcome measurement for the trusted party), detection-loophole-free photonic experiment with $ε= (51.6 \pm 0.4)\% $.

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arXiv2025

Properties and Applications of Partially Deterministic Polytopes

The assumption of a deterministic local hidden variable model constrains the experimentally accessible statistics in a Bell experiment to be contained in the Bell-local polytope. But what if the outputs for only a subset of the measurements at each site are predetermined by the model? In this work, we thoroughly explore this concept of `partial determinism', allowing for arbitrary numbers of parties, inputs and outputs per site. The resulting objects form new classes of convex polytopes which recover the Bell and the no-signalling polytopes as special cases. Nontrivial equivalence classes of partially deterministic models arise, which we classify completely. In particular, the Bell polytope for any scenario can be expressed in multiple different ways in terms of local partially deterministic models. This allows us to generalise Fine's theorem, recovering the original formulation as a special case, but finding new constraints otherwise. We discuss scenarios with different physical motivations, which do not require the causal structure of the Bell scenario, and where classes of partially deterministic polytopes are relevant. Our example applications include device-independent quantum state inseparability witnesses, classes of broadcast-local polytopes, and Local Friendliness scenarios in quantum foundations. We also point out instances in previous literature where classes of related objects have been studied. In the case of correlations compatible with the Local Friendliness assumptions, we find a one-to-one correspondence between partially deterministic polytopes and sequential extended Wigner's friend scenarios so that every partially deterministic polytope has physical relevance. We discuss how the framework captures a broad class of non-classicality notions, and identify an even broader notion of `composable sets', of which partially deterministic polytopes are special cases.

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arXiv2024

Local Friendliness Polytopes In Multipartite Scenarios

Recently the Local Friendliness (LF) no-go theorem has gained a lot of attention, owing to its deep foundational implications. This no-go theorem applies to scenarios which combine Bell experiments with Wigner's friend-type set ups, containing space-like separated superobservers who are assumed to be capable of performing quantum operations on a local observer, also known as their "friend". Analogously to the hypothesis of local hidden variables in Bell scenarios, a set of assumptions termed "Local Friendliness" constrains the space of probabilistic behaviours accessible to the superobservers to be a particular subset of the no-signalling polytope in such scenarios. It has additionally been shown, that there are scenarios where the set of behaviours compatible with Local Friendliness is strictly larger than the Bell-local polytope, while in some scenarios those sets are equal. In this work, we complete the picture by identifying all the canonical Local Friendliness scenarios, with arbitrary but finite numbers of superobservers, friends, measurements and outcomes, where the set of LF correlations admits a local hidden variable model, and where they do not. Our proof is constructive in the sense that we also demonstrate how a local hidden variable model can be constructed, given a behaviour compatible with LF in the appropriate scenarios. While our principal motivation is the foundational question of better understanding the constraints from Local Friendliness, the same inequalities constraining LF polytopes have been shown to arise in a priori unrelated contexts of device-independent information processing. Our results may thus find use in those research areas as well.

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arXiv2022

A possibilistic no-go theorem on the Wigner's friend paradox

In a recent work, Bong et al. [Nature Physics 16, 1199 (2020)] proved a no-go theorem demonstrating a contradiction between a set of assumptions called "Local Friendliness" (LF) and certain quantum phenomena on an extended version of the "Wigner's friend" paradox. The LF assumptions can be understood as the conjunction of two independent assumptions: Absoluteness of Observed Events (AOE) requires that events observed by any observer have absolute, rather than relative, values; Local Agency (LA) encodes the assumption that an intervention cannot influence events outside its future light cone. The proof of the LF no-go theorem, however, implicitly assumes the validity of standard probability theory. Here we present a probability-free version of the Local Friendliness theorem, building upon Hardy's no-go theorem for local hidden variables. The argument is phrased in the language of possibilities, which we make formal by using a modal logical approach. It relies on a weaker version of Local Agency, which we call "Possibilistic Local Agency": the assumption that an intervention cannot influence the possibilities of events outside its future light cone.

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arXiv2018

Entanglement Protection via Periodic Environment Resetting in Continuous Time Quantum Dynamical Processes

The temporal evolution of entanglement between a noisy system and an ancillary system is analyzed in the context of continuous time open quantum system dynamics. Focusing on a couple of analytically solvable models for qubit systems, we study how Markovian and non-Markovian characteristics influence the problem, discussing in particular their associated entanglement-breaking regimes. These performances are compared with those one could achieve when the environment of the system is forced to return to its input configuration via periodic instantaneous resetting procedures.

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