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arXiv:2508.13638 · Physical Review A

Engineering of maximally entangled orbital angular momentum states via path identity

Richard Bernecker, Baghdasar Baghdasaryan, S. Fritzsche

Published Nov 14, 2025• 2 citations
arXivHTMLPDF

Abstract

Cutting-edge quantum technologies lean on sources of high-dimensional entangled states (HDESs) that reliably prepare high-fidelity target states. The idea to overlap photon paths from distinct but indistinguishable sources was recently introduced for the creation of HDESs, known as entanglement by path identity. In this regard, the use of orbital angular momentum (OAM) modes is promising, as they offer a high-dimensional and discrete Hilbert space to encode information. While entanglement by path identity with OAM has been verified experimentally, a detailed investigation of how the OAM distribution of photon pairs can be engineered to maximize the entanglement is lacking. We address this gap and identify an optimal dimensionality for maximally entangled states (MESs) when the spatial engineering of the pump beam and the path-identity approach are combined. Our theoretical study reveals notable limitations for the fidelity of high-dimensional target states. We also establish the equivalence of entangled biphoton states pumped by a spatially engineered beam and generated via path identity. These findings constitute a valuable step toward the optimized preparation of MESs in high dimensions.

DOI
10.1103/9qrm-chgg

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ObjectiveQuantum communication is an emerging mode of communication with broad application prospects, combining with classical information science and technology. It offers unconditional security and efficient transmission of information, making it a hot research topic in todayƳs communication field. High-dimensional quantum entanglement often exhibits strong nonlocality, enhancing the security in quantum communication processes and playing a crucial role in quantum communication and computing. To meet the demand for high-dimensional entangled states in quantum information science, this paper proposes a method for generating high-dimensional Bell states based on the Orbital Angular Momentum (OAM) of photons, capable of producing the complete set of Bell state basis vectors with high fidelity.MethodsInitially, the Spontaneous Parametric Down-Conversion (SPDC) process involving photon OAM is studied. Furthermore, a Genetic Algorithm (GA) is employed to optimize the quantum state of the pump light, aiming to obtain the required high-dimensional maximally entangled states. We analyze the characteristics of Laguerre-Gaussian (LG) beams in detail and explore their application in the SPDC process. By optimizing the parameters of the pump light, maximally entangled states are generated in three-dimensional, four-dimensional, and five-dimensional spaces.ResultsA comparison of the spatial mode spectral distribution shows that the optimized entangled states possess higher fidelity to the target states. Additionally, by simulating the generation of 16 Bell states, it is verified that the produced quantum states form a complete basis for a four-dimensional Hilbert space.ConclusionThe proposed scheme not only effectively generates high-dimensional entangled states but also enables longer-distance quantum communication and potentially global quantum communication systems. Moreover, it provides new avenues for the application of high-dimensional entangled states in the fields of quantum communication and computing, significantly contributing to the advancement of quantum information science.

•Feb 1, 2026•0 citations
Cutting-edge quantum technologies lean on sources of high-dimensional entangled states (HDESs) that reliably prepare high-fidelity target states. The idea to overlap photon paths from distinct but indistinguishable sources was recently introduced for the creation of HDESs, known as entanglement by path identity. In this regard, the use of orbital angular momentum (OAM) modes is promising, as they offer a high-dimensional and discrete Hilbert space to encode information. While entanglement by path identity with OAM has been verified experimentally, a detailed investigation of how the OAM distribution of photon pairs can be engineered to maximize the entanglement is lacking. We address this gap and identify an optimal dimensionality for maximally entangled states (MESs) when the spatial engineering of the pump beam and the path-identity approach are combined. Our theoretical study reveals notable limitations for the fidelity of high-dimensional target states. We also establish the equivalence of entangled biphoton states pumped by a spatially engineered beam and generated via path identity. These findings constitute a valuable step toward the optimized preparation of MESs in high dimensions.
Photon pairs generated from spontaneous parametric down-conversion are a well-established method to realize entangled bipartite photonic systems. Laguerre-Gaussian modes, which carry orbital angular momentum (OAM), are commonly exploited to engineer high-dimensional entangled quantum states. For Hilbert spaces with dimension d>2d>2d>2, maximally entangled states (MESs) help to improve the capacity and security of quantum communication protocols, among several other promising features. However, the direct generation of MESs in well-defined high-dimensional subspaces of the infinite OAM basis has remained a challenge. Here, we formalize how the spatial distribution of the pump beam and the nonlinear profile of the crystal can be simultaneously utilized to generate MESs without additional spatial filtering of OAM modes within a subspace. We illustrate our approach with maximally entangled qutrits (d=3)(d=3)(d=3) and ququints (d=5)(d=5)(d=5).
High-dimensional entanglement is a valuable resource for quantum communication, and photon pairs entangled in orbital angular momentum (OAM) are commonly used for encoding high-dimensional quantum states. However, methods for the preparation of maximally entangled states of arbitrary dimensionality are still lacking, and currently used approaches essentially rely on filtering and entanglement concentration. Here, we experimentally realize a method for the generation of high-dimensional maximally entangled OAM states of photon pairs which does not require any of these procedures. Moreover, the prepared state is restricted to the subspace of the specified dimensionality, thus requiring minimal spatial postselection.
High-dimensional maximally entangled orbital angular momentum (OAM) states are a promising resource for enhancing information capacity and robustness in quantum communication. However, it still lacks an effective method to increase the state dimensionality. Here, we theoretically propose an efficient scheme to generate maximally entangled OAM states of ultrahigh dimensionality by manipulating the radial components of a Laguerre-Gaussian (LG) pump beam. By optimizing the complex amplitudes of multiple radial modes of the LG pump light, one can feasibly achieve 101-dimensional OAM-based maximally entangled states. Our scheme has potential applications in high capacity quantum communication networks.
High-dimensional quantum entanglement is an important resource for fundamental research and quantum technological application. Recently, the orbital angular momentum (OAM) of light has received much attention to realize high-dimensional quantum systems, and generating and manipulating the OAM entanglement has become an important topic. Here, we propose an inverse design technique to manipulate the OAM entanglement of the photon pair generated through spontaneous parametric down-conversion. The proposed technique directly tailors the OAM states of entangled photon pairs by shaping the incident pump beam. In particular, we construct a physics-informed model by leveraging the backpropagation algorithm and adhering to the overlap integral relation. Our model automatically optimizes diffractive masks, allowing the structured pump to quantitatively modulate the OAM distribution. The generation of maximally entangled states in different dimensions is demonstrated numerically and experimentally.
We present an experimental demonstration of a general entanglement-generation framework, where the form of the entangled state is independent of the physical process used to produce the particles. It is the indistinguishability of multiple generation processes and the geometry of the setup that give rise to the entanglement. Such a framework, termed entanglement by path identity, exhibits a high degree of customizability. We employ one class of such geometries to build a modular source of photon pairs that are high-dimensionally entangled in their orbital angular momentum. We demonstrate the creation of three-dimensionally entangled states and show how to incrementally increase the dimensionality of entanglement. The generated states retain their quality even in higher dimensions. In addition, the design of our source allows for its generalization to various degrees of freedom and even for the implementation in integrated compact devices. The concept of entanglement by path identity itself is a general scheme and allows for construction of sources producing also customized states of multiple photons. We therefore expect that future quantum technologies and fundamental tests of nature in higher dimensions will benefit from this approach.
ObjectiveQuantum communication is an emerging mode of communication with broad application prospects, combining with classical information science and technology. It offers unconditional security and efficient transmission of information, making it a hot research topic in todayƳs communication field. High-dimensional quantum entanglement often exhibits strong nonlocality, enhancing the security in quantum communication processes and playing a crucial role in quantum communication and computing. To meet the demand for high-dimensional entangled states in quantum information science, this paper proposes a method for generating high-dimensional Bell states based on the Orbital Angular Momentum (OAM) of photons, capable of producing the complete set of Bell state basis vectors with high fidelity.MethodsInitially, the Spontaneous Parametric Down-Conversion (SPDC) process involving photon OAM is studied. Furthermore, a Genetic Algorithm (GA) is employed to optimize the quantum state of the pump light, aiming to obtain the required high-dimensional maximally entangled states. We analyze the characteristics of Laguerre-Gaussian (LG) beams in detail and explore their application in the SPDC process. By optimizing the parameters of the pump light, maximally entangled states are generated in three-dimensional, four-dimensional, and five-dimensional spaces.ResultsA comparison of the spatial mode spectral distribution shows that the optimized entangled states possess higher fidelity to the target states. Additionally, by simulating the generation of 16 Bell states, it is verified that the produced quantum states form a complete basis for a four-dimensional Hilbert space.ConclusionThe proposed scheme not only effectively generates high-dimensional entangled states but also enables longer-distance quantum communication and potentially global quantum communication systems. Moreover, it provides new avenues for the application of high-dimensional entangled states in the fields of quantum communication and computing, significantly contributing to the advancement of quantum information science.