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Register a new userEnhancement of Entanglement Percolation in Quantum Networks via Lattice Transformations
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We study strategies for establishing long-distance entanglement in quantum networks. Specifically, we consider networks consisting of regular lattices of nodes, in which the nearest neighbors share a pure, but non-maximally entangled pair of qubits. We look for strategies that use local operations and classical communication. We compare the classical entanglement percolation protocol, in which every network connection is converted with a certain probability to a singlet, with protocols in which classical entanglement percolation is preceded by measurements designed to transform the lattice structure in a way that enhances entanglement percolation. We analyze five examples of such comparisons between protocols and point out certain rules and regularities in their performance as a function of degree of entanglement and choice of operations.
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Establishing long-distance quantum entanglement, i.e., entanglement transmission, in quantum networks (QN) is a key and timely challenge for developing efficient quantum communication. Traditional comprehension based on classical percolation assumes a necessary condition for successful entanglement transmission between any two infinitely distant nodes: they must be connected by at least a path of perfectly entangled states (singlets). Here, we relax this condition by explicitly showing that one can focus not on optimally converting singlets but on establishing concurrence -- a key measure of bipartite entanglement. We thereby introduce a new statistical theory, concurrence percolation theory (ConPT), remotely analogous to classical percolation but fundamentally different, built by generalizing bond percolation in terms of sponge-crossing paths instead of clusters. Inspired by resistance network analysis, we determine the path connectivity by series/parallel rules and approximate higher-order rules via star-mesh transforms. Interestingly, we find that the entanglement transmission threshold predicted by ConPT is lower than the known classical-percolation-based results and is readily achievable on any series-parallel networks such as the Bethe lattice. ConPT promotes our understanding of how well quantum communication can be further systematically improved versus classical statistical predictions under the limitation of QN locality -- a quantum advantage that is more general and efficient than expected. ConPT also shows a percolation-like universal critical behavior derived by finite-size analysis on the Bethe lattice and regular two-dimensional lattices, offering new perspectives for a theory of criticality in entanglement statistics.
Multi-photon entangled states of light are key to advancing quantum communication, computation, and metrology. Current methods for building such states are based on stitching together photons from probabilistic sources. The probability of $N$ such sources firing simultaneously decreases exponentially with $N$, imposing severe limitations on the practically achievable number of coincident photons. We tackle this challenge with a quantum interference buffer (QIB), which combines three functionalities: firstly, it stores polarization qubits, enabling the use of polarization-entangled states as resource; secondly, it implements entangled-source multiplexing, greatly enhancing the resource-state generation rates; thirdly, it implements time-multiplexed, on-demand linear optical networks for interfering subsequent states. Using the QIB, we multiplex 21 Bell-state sources and demonstrate a nine-fold enhancement in the generation rate of four-photon GHZ states. The enhancement scales exponentially with the photon number; larger states benefit more strongly. Multiplexed photon entanglement and interference will find diverse applications in quantum photonics, allowing for practical realisations of multi-photon protocols.
Recent advances have lead towards first prototypes of a quantum internet in which entanglement is distributed by sources producing bipartite entangled states with high fidelities. This raises the question which states can be generated in quantum networks based on bipartite sources using local operations and classical communication. In this work we study state transformations under finite rounds of local operations and classical communication in networks based on maximally entangled two-qubit states. We first derive the symmetries for arbitrary network structures as these determine which transformations are possible. Then we show that contrary to tree graphs for which it has already been shown that any state within the same entanglement class can be reached there exist states which can be reached probabilistically but not deterministically if the network contains a cycle. Furthermore, we provide a systematic way to determine states which are not reachable in networks consisting of a cycle. Moreover, we provide a complete characterization of the states which can be reached in a cycle network with a protocol where each party measures only once and each step of the protocol results in a deterministic transformation. Finally, we present an example which cannot be reached with such a simple protocol.
Entanglement generation in discrete time quantum walks is deemed to be another key property beyond the transport behaviors. The latter has been widely used in investigating the localization or topology in quantum walks. However, there are few experiments involving the former for the challenges in full reconstruction of the final wave function. Here, we report an experiment demonstrating the enhancement of the entanglement in quantum walks using dynamic disorder. Through reconstructing the local spinor state for each site, von Neumann entropy can be obtained and used to quantify the coin-position entanglement. We find that the enhanced entanglement in the dynamically disordered quantum walks is independent of the initial state, which is different from the entanglement generation in the Hadamard quantum walks. Our results are inspirational for achieving quantum computing based on quantum walks.
We study the ground-state entanglement in systems of spins forming the boundary of a quantum spin network in arbitrary geometries and dimensionality. We show that as long as they are weakly coupled to the bulk of the network, the surface spins are strongly entangled, even when distant and non directly interacting, thereby generalizing the phenomenon of long-distance entanglement occurring in quantum spin chains. Depending on the structure of the couplings between surface and bulk spins, we discuss in detail how the patterns of surface entanglement can range from multi-pair bipartite to fully multipartite. In the context of quantum information and communication, these results find immediate application to the implementation of quantum routers, that is devices able to distribute quantum correlations on demand among multiple network nodes.
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