arXiv subjects

Eleftherios-Ermis Tselentis

Publications and source records attributed to Eleftherios-Ermis Tselentis.

5 recordsLinked to original sources

arXiv2025

Graphical Tests of Causality

Bell inequalities limit the possible observations of non-communicating parties. Here, we present analogous inequalities for any number of communicating parties under the causal constraints of static causal order, definite causal order, and bi-causal order. All derived inequalities are remarkably simple. They correspond to upper bounds on the winning chance in graphical games: Given a specific directed graph over the parties, the parties are challenged to communicate along a randomly chosen arc. In the case of definite causal order, every game that we find is specified by a kefalopoda digraph. Based on this we define weakly causal correlations as those that satisfy all kefalopoda inequalities. We show that the problem of deciding whether some correlations are weakly causal is solvable in polynomial time in the number of parties.

quant-ph↗

arXiv2025

No quantum advantage for violating fixed-order inequalities?

In standard quantum theory, the causal relations between operations are fixed. One can relax this notion by allowing for dynamical arrangements, where operations may influence the causal relations of future operations, as certified by violation of fixed-order inequalities, e.g., the k-cycle inequalities. Another, non-causal, departure further relaxes these limitations, and is certified by violations of causal inequalities. In this paper, we explore the interplay between dynamic and indefinite causality. We study the k-cycle inequalities and show that the quantum switch violates these inequalities without exploiting its indefinite nature. We further introduce non-adaptive strategies, which effectively remove the dynamical aspect of any process, and show that the k-cycle inequalities become novel causal inequalities; violating k-cycle inequalities under the restriction of non-adaptive strategies requires non-causal setups. The quantum switch is known to be incapable of violating causal inequalities, and it is believed that a device-independent certification of its causal indefiniteness requires extended setups incorporating spacelike separation. This work reopens the possibility for a device-independent certification of the quantum switch in isolation via fixed-order inequalities instead of causal inequalities. The inequalities we study here, however, turn out to be unsuitable for such a device-independent certification. In this work, we initiate the question posed by the title. This question, however, remains unanswered.

quant-ph↗

arXiv2025

The Möbius game and other Bell tests for relativity

We derive multiparty games that, if the winning chance exceeds a certain limit, prove the incompatibility of the parties' causal relations with any partial order. This, in turn, means that the parties exert a back-action on the causal relations; the causal relations are dynamical. The games turn out to be representable by directed graphs, for instance by an orientation of the Möbius ladder. We discuss these games as device-independent tests of spacetime's dynamical nature in general relativity. To do so, we design relativistic settings where, in the Minkowski spacetime, the winning chance is bound to the limits. In contrast, we find otherwise tame processes with classical control of causal order that win the games deterministically. These suggest a violation of the bounds in gravitational implementations. We obtain these games by uncovering a "pairwise central symmetry" of the correlations in question. This symmetry allows us to recycle the facets of the acyclic subgraph polytope studied by Grötschel, Jünger, and Reinelt in the mid-80s for combinatorial optimization. In addition, we derive multiparty games in a scenario where the polytope dimension grows only linearly in the number of parties. Here, exceeding the limits not only proves the dynamical nature of the causal relations, but also that the correlations are incompatible with any global causal order.

gr-qc↗

arXiv2025

The Möbius Game: A Quantum-Inspired Test of General Relativity

We present a tight inequality to test the dynamical nature of spacetime. A general-relativistic violation of that inequality certifies change of curvature, in the same sense as a quantum-mechanical violation of a Bell inequality certifies a source of entanglement. The inequality arises from a minimal generalization of the Bell setup. It represents a limit on the winning chance of a collaborative multi-agent game played on the Möbius graph. A long version of this Letter including other games and how these games certify the dynamical character of the celebrated quantum switch is accessible as arXiv:2309.15752 [gr-qc].

gr-qc↗

arXiv2023

Admissible Causal Structures and Correlations

It is well-known that if one assumes quantum theory to hold locally, then processes with indefinite causal order and cyclic causal structures become feasible. Here, we study qualitative limitations on causal structures and correlations imposed by local quantum theory. For one, we find a necessary graph theoretic criterion--the "siblings-on-cycles" property--for a causal structure to be admissible: Only such causal structures admit a realization consistent with local quantum theory. We conjecture that this property is moreover sufficient. This conjecture is motivated by an explicit construction of quantum causal models, and supported by numerical calculations. We show that these causal models, in a restricted setting, are indeed consistent. For another, we identify two sets of causal structures that, in the classical-deterministic case, give rise to causal and non-causal correlations respectively.

quant-ph↗