The exploitation of quantum coherence at the level of propagation represents a powerful paradigm for quantum communication networks. In this work, we show that the coherent superposition of spatially distinct communication links enables entanglement generation inherently during distribution. Specifically, separable quantum states can be deterministically transformed into entangled states, when the noisy communication links they traverse are coherently superposed. Contrary to the conventional view of noise as a detrimental effect, we demonstrate that quantum noise itself can be transformed into a constructive resource for entanglement generation for both bipartite and multipartite entanglement. Given the practical feasibility of implementing spatial superposition in interferometric setups, our approach provides a feasible method for distributed entanglement engineering, opening new directions for quantum communication and networked quantum technologies.
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This work has been funded by the European Union under Horizon Europe ERC-CoG grant QNattyNet ("Quantum-Native Communication Networks: from Quantum Message to Quantum Functioning"), n.101169850. Details at https://qnattynet.quantuminternet.it
The exploitation of quantum coherence at the level of propagation represents a powerful paradigm for quantum communication networks. In this work, we show that the coherent superposition of spatially distinct communication links enables entanglement generation inherently during distribution. Specifically, separable quantum states can be deterministically transformed into entangled states, when the noisy communication links they traverse are coherently superposed. Contrary to the conventional view of noise as a detrimental effect, we demonstrate that quantum noise itself can be transformed into a constructive resource for entanglement generation for both bipartite and multipartite entanglement. Given the practical feasibility of implementing spatial superposition in interferometric setups, our approach provides a feasible method for distributed entanglement engineering, opening new directions for quantum communication and networked quantum technologies.
Entanglement generation and distribution is a core functionality of quantum networks. Traditional approaches to this functionality rely on chains of quantum repeaters or on manipulation of multipartite states. In this work, we explore an alternative paradigm that explores quantum coherence at propagation level, by enabling genuine quantum instantiations of communication channels, existing in superposed configurations. We demonstrate that this mechanism can be harnessed to generate both bipartite and multipartite entangled states from separable inputs. Specifically, we show that Bell states can be obtained from initially separable two-qubit states, through the superposition of two unitary quantum channels. The proposed approach is further extended to multipartite systems, by generating n-qubit GHZ and W states and thereby revealing the constructive role of superposed quantum operations in entanglement creation. Moreover, we analyze the robustness of the proposed scheme against noise, showing that the resulting states retain high fidelity and concurrence even under realistic imperfections. Our results suggest that, under appropriate quantum control, noisy superposed paths can be effectively harnessed to engineer entanglement generation and distribution in quantum networks.
The controlled generation of quantum entanglement from separable states remains a central challenge in quantum information science. Here, we investigate entanglement generation using two related control paradigms: coherent path superposition of local unitary operations and stochastic implementations of Pauli channels under coherent control. We show that entangled states belonging to the Bell, GHZ and W classes, can be deterministically generated from fully separable inputs by coherently superposing alternative sets of local unitary transformations. Conditions on the local operators for entanglement generation are derived, and the resulting states are shown to be locally unitary equivalent to standard multipartite entangled states. We further extend the analysis to noisy scenarios, where separable mixed states evolve through pairs of Pauli channels arranged in path-superposition and indefinite causal order configurations. Closed-form expressions for the output states are obtained, and entanglement is quantified using concurrence. By exploring representative channel families across their parameter space, we identify regimes where stochastic entanglement emerges, determine the associated success probabilities, and characterize trade-offs between entanglement and purity.
Entanglement generation and distribution is a core functionality of quantum networks. Traditional approaches to this functionality rely on chains of quantum repeaters or on manipulation of multipartite states. In this work, we explore an alternative paradigm that explores quantum coherence at propagation level, by enabling genuine quantum instantiations of communication channels, existing in superposed configurations. We demonstrate that this mechanism can be harnessed to generate both bipartite and multipartite entangled states from separable inputs. Specifically, we show that Bell states can be obtained from initially separable two-qubit states, through the superposition of two unitary quantum channels. The proposed approach is further extended to multipartite systems, by generating n-qubit GHZ and W states and thereby revealing the constructive role of superposed quantum operations in entanglement creation. Moreover, we analyze the robustness of the proposed scheme against noise, showing that the resulting states retain high fidelity and concurrence even under realistic imperfections. Our results suggest that, under appropriate quantum control, noisy superposed paths can be effectively harnessed to engineer entanglement generation and distribution in quantum networks.
A quantum internet aims at harnessing networked quantum technologies, namely by distributing bipartite entanglement between distant nodes. However, multipartite entanglement between the nodes may empower the quantum internet for additional or better applications for communications, sensing, and computation. In this work, we present an algorithm for generating multipartite entanglement between different nodes of a quantum network with noisy quantum repeaters and imperfect quantum memories, where the links are entangled pairs. Our algorithm is optimal for GHZ states with 3 qubits, maximising simultaneously the final state fidelity and the rate of entanglement distribution. Furthermore, we determine the conditions yielding this simultaneous optimality for GHZ states with a higher number of qubits, and for other types of multipartite entanglement. Our algorithm is general also in the sense that it can optimise simultaneously arbitrary parameters. This work opens the way to optimally generate multipartite quantum correlations over noisy quantum networks, an important resource for distributed quantum technologies.
Transmission of quantum entanglement will play a crucial role in future networks and long-distance quantum communications. Quantum Key Distribution, the working mechanism of quantum repeaters and the various quantum communication protocols are all based on quantum entanglement. On the other hand, quantum entanglement is extremely fragile and sensitive to the noise of the communication channel over which it has been transmitted. To share entanglement between distant points, high fidelity quantum channels are needed. In practice, these communication links are noisy, which makes it impossible or extremely difficult and expensive to distribute entanglement. In this work we first show that quantum entanglement can be generated by a new idea, exploiting the most natural effect of the communication channels: the noise itself of the link. We prove that the noise transformation of quantum channels that are not able to transmit quantum entanglement can be used to generate distillable (useable) entanglement from classically correlated input. We call this new phenomenon the Correlation Conversion property (CC-property) of quantum channels. The proposed solution does not require any non-local operation or local measurement by the parties, only the use of standard quantum channels. Our results have implications and consequences for the future of quantum communications, and for global-scale quantum communication networks. The discovery also revealed that entanglement generation by local operations is possible.
Spatial diversity is a key mechanism in classical Multi-Input Multi-Output (MIMO) communications, where multiple propagation alternatives provide redundant observations that improve reliability in noisy environments. A direct translation of this principle to quantum communications is forbidden by the no-cloning theorem, since an unknown quantum state cannot be copied and transmitted through independent links. In this Perspective article, we argue that spatial path superposition provides a no-cloning-compatible analogue of the spatial-diversity principle of MIMO. Rather than duplicating the transmitted quantum state, a quantum carrier is coherently delocalized over multiple communication links, so that alternative spatial evolutions are experienced in quantum superposition. In this framework, classical redundancy is replaced by spatial coherence, and the communication link itself becomes part of the quantum dynamics. Indeed, spatial coherence can enhance the fidelity of the received quantum state, providing the quantum counterpart of the classical diversity gain, and can be converted into bipartite and multipartite entanglement during propagation. This implies that, under suitable coherent path control, noisy channel evolutions can contribute constructively to entanglement generation. This Perspective identifies spatial path superposition as a quantum-native primitive for the Quantum Internet, linking no-cloning-compatible spatial diversity, fidelity enhancement, entanglement generation, and coherent path control