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arXiv:2610.11001

Structured leakage in OAM-encoded qubits revealed by distributed quantum feature extraction

Yayu Mo, Nazifa Rumman, Sanjeev Shapkota, Thomas A. Searles, Sanjaya Lohani

quant-phphysics.opticsSubmitted Oct 7, 2026• Announced Oct 9, 2026• 0 citations
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Abstract

Orbital angular momentum (OAM) modes have emerged as an important platform for high-dimensional quantum communication but remain vulnerable to environmental noise and channel imperfections. Here we introduce a logical-qubit representation of OAM propagation in which modes evolve as qubits through noisy quantum channels. In this representation, turbulence manifests as leakage from the logical OAM subspace into turbulence-induced leakage components, providing a unified description of degradation in both single-photon and coherent-beam regimes. We show that turbulence-induced leakage exhibits systematic dependence on turbulence strength and OAM order, revealing structured signatures of information redistribution beyond the logical subspace. Leveraging this insight, we develop a distributed quantum feature-extraction scheme that encodes high-dimensional OAM states into quantum representations and employs quantum convolutional operations to extract turbulence-robust features. Simulations and experimental measurements demonstrate robust extraction of these features across a broad range of turbulence strengths, OAM orders, and propagation conditions. Our results establish a direct link between OAM-channel dynamics, structured leakage, and quantum learning, providing a physics-informed framework for characterizing turbulence-degraded high-dimensional optical states and extracting robust signatures of turbulence-induced information redistribution.

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Orbital angular momentum (OAM) modes have emerged as an important platform for high-dimensional quantum communication but remain vulnerable to environmental noise and channel imperfections. Here we introduce a logical-qubit representation of OAM propagation in which modes evolve as qubits through noisy quantum channels. In this representation, turbulence manifests as leakage from the logical OAM subspace into turbulence-induced leakage components, providing a unified description of degradation in both single-photon and coherent-beam regimes. We show that turbulence-induced leakage exhibits systematic dependence on turbulence strength and OAM order, revealing structured signatures of information redistribution beyond the logical subspace. Leveraging this insight, we develop a distributed quantum feature-extraction scheme that encodes high-dimensional OAM states into quantum representations and employs quantum convolutional operations to extract turbulence-robust features. Simulations and experimental measurements demonstrate robust extraction of these features across a broad range of turbulence strengths, OAM orders, and propagation conditions. Our results establish a direct link between OAM-channel dynamics, structured leakage, and quantum learning, providing a physics-informed framework for characterizing turbulence-degraded high-dimensional optical states and extracting robust signatures of turbulence-induced information redistribution.
Quantum communication with structured photons is topical, owing to the multidimensional state space of spatial modes. However, spatial modes are fragile and their quality degrades when perturbed in traditional communication media such as free-space and optical fibers. Here, we illustrate the effects of atmospheric turbulence on a six-state quantum-key-distribution protocol with orbital angular momentum (OAM) modes. We experimentally characterize the fidelity decay as a function of turbulence strength, showing a concomitant decrease with increasing perturbation, and consider the influence of both mode order and mode size, showing that OAM modes with higher helicity are more resilient to turbulence for this protocol. We outline two approaches to mitigate the photon information loss. In the first, we show that by postselecting on a high-dimensional subspace at the detection side, we can recover information contained in the scattered modes. In the second, we measure the channel operator by means of classically entangled light and mitigate errors on the quantum state through entanglement concentration by means of one-party Procrustean filtering. The tools we provide here will be beneficial for realizing more robust quantum communication with OAM modes of light.
ABSTRACT We present a hybrid quantum–classical convolutional neural network (QC‐CNN) for classifying orbital angular momentum (OAM) modes in free‐space optical (FSO) channels affected by atmospheric turbulence. The approach embeds a convolutional quantum (quanvolutional) preprocessing layer that encodes 2 × 2 image patches into parameterized quantum circuits with rotation and entangling operations, yielding quantum‐extracted features that feed into conventional convolutional layers. We employ a two‐stage training protocol: large‐scale pretraining on 940 000 intensity images spanning a range of turbulence strengths, followed by task‐specific fine‐tuning with selective layer freezing. Turbulence was modeled using a modified Hill–Andrews phase screen and evaluated at multiple strengths over 1000 m propagation distance. The hybrid model achieves high classification accuracies (>97% across strong turbulence scenarios) and reduces bit‐error rates (BER) from up to 0.11 without pretraining to 0.001–0.08 after quantum‐layer pretraining. Results demonstrate that quantum‐inspired preprocessing combined with transfer learning significantly improves resilience to severe atmospheric distortions and offers a scalable route toward real‐time, high‐fidelity OAM demodulation in FSO systems.
The orbital angular momentum (OAM) state of light can potentially be used to implement higher dimensional entangled systems for quantum communication. Unfortunately, optical fibers in use today support only modes with zero OAM values. Free-space quantum communication is an alternative to traditional way of communicating through optical fibers. However the refractive index fluctuation of the atmosphere gives rise to random phase aberrations on a propagating optical beam. To transmit quantum information successfully through a free-space optical channel, one needs to understand how atmospheric turbulence influences quantum entanglement. Here, we present a numerical study of the evolution of quantum entanglement between a pair of qubits. The qubits consist of photons entangled in the OAM basis. The photons propagate in a turbulent atmosphere modeled by a series of consecutive phase screens based on the Kolmogorov theory of turbulence. Maximally entangled initial states are considered, and the concurrence is used as a measure of entanglement. We show how the evolution of entanglement is influenced by various parameters such as the beam waist, the strength of the turbulence and the wavelength of the beam. We restricted our analysis to the OAM values l = ±1 and we compared our results to previous work.
A high-dimensional quantum key distribution (QKD), which adopts degrees of freedom of the orbital angular momentum (OAM) states, is beneficial to realize secure and high-speed QKD. However, the helical phase of a vortex beam that carries OAM is sensitive to the atmospheric turbulence and easily distorted. In this paper, an adaptive compensation method using deep learning technology is developed to improve the performance of OAM-encoded QKD schemes. A convolutional neural network model is first trained to learn the mapping relationship of intensity profiles of inputs and the turbulent phase, and such mapping is used as feedback to control a spatial light modulator to generate a phase screen to correct the distorted vortex beam. Then an OAM-encoded QKD scheme with the capability of real-time phase correction is designed, in which the compensation module only needs to extract the intensity distributions of the Gaussian probe beam and thus ensures that the information encoded on OAM states would not be eavesdropped. The results show that our method can efficiently improve the mode purity of the encoded OAM states and extend the secure distance for the involved QKD protocols in the free-space channel, which is not limited to any specific QKD protocol.
Orbital angular momentum (OAM) entanglement gives access to multiple qubit and high dimensional Hilbert spaces, but is unfortunately susceptible to disturbance, decaying in real-world noisy channels. Here, we show there is an underlying topology arising from OAM entanglement that is robust to such channels, which we demonstrate using atmospheric turbulence -- exemplary of stochastic or chaotic media. Using a quantum channel with various turbulence strengths, we find the OAM topological observable preserved even though the OAM itself is shown to be highly sensitive to the turbulence. We show this is true for mixed states too, with the OAM topology intact even as the purity of the state decreases due to decoherence. Our work offers a new perspective on OAM entanglement preservation, and may easily be extended to other spatial bases, degrees of freedom, as well as complex channels, whether static or dynamic.
Abstract In the last two decades, structured light patterns with orbital angular momentum (OAM) have been extensively utilized in quantum cryptography or quantum key distribution, which is generally considered an unconditionally secure method of secret key transmission between two distant communicating entities. High-dimensional encoding of quantum bits in quantum key distribution improves the security and information-carrying capability of a quantum signal. Significant degradation in the efficiency of such communication is observed due to the aberration of OAM states under atmospheric turbulence. In this paper, we focus on analyzing the impact of atmospheric turbulence on the error-rates of quantum key distribution, which is considered the benchmark in analyzing the security of such communication over the quantum channel in the presence of an intruder (man-in-the-middle). Our results show that turbulence under normal conditions does not significantly affect the successful transmission of secret quantum bits between sender and receiver. Furthermore, we identify an interesting variation in the signature of error-rates to clearly detect the presence of an intruder as a result of medium-level turbulence.