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arXiv:2601.04636

Hardy nonlocality for entangled pairs in a four-particle system

Duc Manh Doan, Hung Q. Nguyen

quant-phSubmitted Jan 8, 2026• Announced Jan 9, 2026• 0 citations
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Abstract

Nonlocality can be studied through different approaches, such as Bell's inequalities, and it can be found in numerous quantum states, including GHZ states or graph states. Hardy's paradox, or Hardy-type nonlocality, provides a way to investigate nonlocality for entangled states of particles without using inequalities. Previous studies of Hardy's nonlocality have mostly focused on the fully entangled systems, while other entanglement configurations remain less explored. In this work, the system under investigation consists of four particles arranged in a cyclic entanglement configuration, where each particle forms entangled pairs with two neighbors, while non-neighboring particles remain unentangled. We found that this entanglement structure offers a larger set of conditions that lead to the contradiction with the LHV model, compared to the fully entangled systems. This enhancement can be attributed to the presence of multiple excluded states and correlations, in which the measurement result of a particle only influences the result of its paired partners. We implement quantum circuits compatible with the cyclic entanglement structure, and through simulation, the correlation patterns and the states of interest are identified. We further execute the proposed circuits on IBM Brisbane, a practical backend; however, the results show considerable deviations from the simulation counterparts.

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Nonlocality can be studied through different approaches, such as Bell's inequalities, and it can be found in numerous quantum states, including GHZ states or graph states. Hardy's paradox, or Hardy-type nonlocality, provides a way to investigate nonlocality for entangled states of particles without using inequalities. Previous studies of Hardy's nonlocality have mostly focused on the fully entangled systems, while other entanglement configurations remain less explored. In this work, the system under investigation consists of four particles arranged in a cyclic entanglement configuration, where each particle forms entangled pairs with two neighbors, while non-neighboring particles remain unentangled. We found that this entanglement structure offers a larger set of conditions that lead to the contradiction with the LHV model, compared to the fully entangled systems. This enhancement can be attributed to the presence of multiple excluded states and correlations, in which the measurement result of a particle only influences the result of its paired partners. We implement quantum circuits compatible with the cyclic entanglement structure, and through simulation, the correlation patterns and the states of interest are identified. We further execute the proposed circuits on IBM Brisbane, a practical backend; however, the results show considerable deviations from the simulation counterparts.
Hardy’s non-locality provides a proof of the incompatibility between quantum mechanics and local realism without using Bell inequalities. While this argument has been extensively studied for two- and three-qubit systems, a detailed analysis of the four-qubit case is still lacking. In this work, we investigate Hardy’s non-locality for a four-qubit system within the standard two-setting framework. We explicitly construct the entangled state satisfying the Hardy conditions and determine the measurement settings that maximize the success probability. • We present an explicit analytical construction of a four-qubit Hardy state together with the corresponding projective measurement settings. • The maximum Hardy success probability for four qubits is found to be 0.143, exceeding the values obtained in the two- and three-qubit cases. • The resulting state is non-maximally entangled and exhibits genuine multipartite quantum correlations.
Local realistic models cannot completely describe all predictions of quantum mechanics. This is known as Bell's theorem that can be revealed either by violations of Bell inequality, or all-versus-nothing proof of nonlocality. Hardy's paradox is an important all-versus-nothing proof and is considered as ``the simplest form of Bell's theorem.'' In this work, we advance the study of Hardy's paradox based on Bell's inequality. Our formalism is essentially equivalent to the ``logical Bell inequality'' formalism developed by Abramsky and Hardy, but ours is more easily applied to experimental tests. Remarkably, we find that not only for a two-qubit two-setting case but also for a two-qubit four-setting case, we can construct stronger Hardy-type paradoxes based on Bell's inequality, whose successful probabilities can attain four times and seven times larger than the original one, respectively, thus providing more friendly tests for experiment. Meanwhile, we experimentally test the stronger Hardy-type paradoxes in a two-qubit system. Within the experimental errors, the experimental results coincide with the theoretical predictions.
Bell inequalities were formulated by John Bell to test the possible violation of local realistic theories by quantum mechanical systems. It was shown that entangled quantu-m states of multiple particles violate various Bell’s inequalities. This proved that quan-tum mechanics allows correlations between spatially separated systems that have no classical analogue. The main focus of this work is to investigate genuine multiqubit non-locality in families of entangled 3 and 4-qubit pure states by studying a Bell-type inequality that is violated only if all qubits are non-locally correlated. We numerically study the relationship between entanglement and violation of the Svetlichny Bell-type inequality. We analyze non-local correlations in 3-qubit generalized Greenberger-Hor-ne-Zeilinger (GHZ) states, maximal slice (MS) states, and W states. Our studies show that the correlations exhibited by three particles cannot in general be described by hid-den variable theories with at most two-particle non-locality. However, some 3-qubit entangled states do not violate the Svetlichny’s inequality. We then extend our analysis to 4-qubit generalized Greenberger-Horne-Zeilinger (GHZ) states, maximal slice (MS) states, and W states. The results are similar to the 3-qubit case for GHZ and MS states. The range of parameters for which we see a violation is the same for the 3 and 4-qubit GHZ states. However, the 4-qubit W states do not violate Bell-type inequality, unlike the 3-qubit W states. Our results show the complex nature of multiqubit entang-lement and non-locality and provide tools for designing useful quantum communica-tion tasks.
We test Hardy's paradox of non-locality experimentally on the IBM five-qubit quantum computer for the first time. The quantum circuit is constructed on superconducting qubits corresponding to the original Hardy's test of non-locality. Results confirmed the theory that any non-maximally entangled state of two qubits violates Hardy's Equations, whereas any maximally entangled state and product state of two qubits do not exhibit Hardy's non-locality. We also point out the difficulties associated with the practical implementation of any Hardy's paradox based quantum protocol and propose three performance measures for any two qubits of any quantum computer.
Multipartite quantum states may exhibit different types of quantum entanglement in that they cannot be converted into each other by local quantum operations only, and fully understanding mathematical structures of different types of multipartite entanglement is a very challenging task. In this paper, from the viewpoint of Hardy's nonlocality, we compare W and GHZ (Greenberger-Horne-Zeilinger) states and show a couple of crucial different behaviors between them. Particularly, by developing a geometric model for Hardy's nonlocality problem of W states, we derive an upper bound for its maximal violation probability, which turns out to be strictly smaller than the corresponding probability of GHZ states. This gives us a new comparison between these two quantum states, and the result is also consistent with our intuition that GHZ states are more entangled. Furthermore, we generalize our approach to obtain an asymptotic characterization for general NNN-qubit W states, revealing that when NNN goes up, the speed that the maximum violation probabilities decay is exponentially slower than that of general NNN-qubit GHZ states. We provide some numerical simulations to verify our theoretical results.
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.