arXiv subjects

Andrew Forbes

Publications and source records attributed to Andrew Forbes.

At least 37 records · Page 2Linked to original sources

arXiv2026

Particle-like topologies of light in turbulent complex media

The basic building blocks of many forms of optical topologies are particle-like singularities in phase and polarisation, giving rise to lines of darkness that weave complex threads in 3D space. Although known for half a century since seminal work on dislocations in wave trains, their behaviour in complex media remains under debate, especially with respect to their relative stability. Here we show that polarisation and phase vortices behave identically in one-sided turbulent complex channels. We perform complementary numerical and experimental studies using atmospheric turbulence as a test case, demonstrating agreement and equivalent dynamics. Our work addresses open questions on optical topologies and will be relevant to their harnessing for applications such as sensing, communication, imaging, and information transfer in noisy or complex environments.

physics.optics↗

arXiv2026

Fractional optical skyrmions

Optical topologies in the form of Skyrmions have attracted significant interest of late, where their integer Skyrmion number has been shown to be robust to complex media. Here we create the first fractional Skyrmions by structuring light as a vectorial superposition of non-integer orbital angular momentum. We unravel the map structure to reveal a new phenomenon, the abrupt transition jumps in skyrmion number, which serves to reinforce the integer nature of skyrmion topologies. Our experimental demonstration agrees well with simulation, opening a new spectrum of optical topologies to explore, with exciting possibilities in optical communication and sensing.

physics.optics↗

arXiv2026

Topological robustness of optical skyrmions through a real-world free-space link

Structured light offers a promising solution for the increasing data demands of modern optical networks, opening up new degrees of freedom that can be leveraged for greater channel capacity and more bits per photon. However, its implementation is hindered by real-world distortions, for example, atmospheric turbulence in free-space, with severe and rapidly evolving phase perturbations that alter the amplitude, phase and vectorial polarization structure of the beam. Here, we demonstrate that optical topologies in the form of skyrmions are highly resilient to the effects of real-world atmospheric turbulence. We create and transmit these particle-like topologies of light through a 270~m free-space optical link, revealing their robustness across a wide variety of conditions and turbulence strengths. While we observe severe distortion in the states' underlying degrees of freedom, we show that the topological numbers are preserved in all cases. We account for fast changes to the medium, where the channel produces statistically averaged outcomes, by probing the state's decoherence, showing that while the degree of polarisation consequently decays, the topology remains intact. Using topology, we show information can be transmitted through the channel with almost perfect fidelity (>98%) in most cases, only decreasing to 86% in the most severe conditions tested. Our work is the first to demonstrate the potential for optical topologies as reliable and robust information carriers in a real-world environment and points to the potential for other complex channels too, offering attractive features for classical and quantum communication alike.

physics.optics↗

arXiv2025

Deep diffractive optical neural networks for detecting Skyrmionic topologies of light

Optical Skyrmions are topological forms of structured light with the potential of an infinite encoding alphabet that is immune to disturbance. This attractive prospect is hindered by the lack of any topological detector, a challenging problem due to the non-orthogonal nature of the topological invariant (N). Here we demonstrate the first deterministic detector for Skyrmionic topologies of light using a deep diffractive optical neural network. Our network uses two independent processing channels of 5 diffractive layers each to map incoming topologies to spatially separated Gaussian channels from which N can be detected. We overcome the complexity of the training by using a spatial mode basis rather than pixels, reducing the training variables by x1000 compared to current methods. We use the detector on an input set of 81 input topologies, showing high accuracy even in the presence of significant levels of noise. Finally, to show the practical utility of the device, we transmit and receive an image encoded in a 14-level topological alphabet with no discernible cross-talk. Our work offers a new paradigm for the emergent field of diffractive optical networks and can easily be extended to other forms of optical topologies, setting a clear pathway for their deployment in real-world applications.

physics.optics↗

arXiv2025

Structured Light at the Extreme: Harnessing Spatiotemporal Control for High-Field Laser-Matter Interactions

This review charts the emerging paradigm of intelligent structured light for high-field laser-matter interactions, where the precise spatiotemporal and vectorial control of light is a critical degree of freedom. We outline a transformative framework built upon three synergistic pillars. First, we survey the advanced electromagnetic toolkit, moving beyond conventional spatial light modulators to include robust static optics and the promising frontier of plasma light modulators. Second, we detail the optimization engine for this high-dimensional design space, focusing on physics-informed digital twins and AI-driven inverse design to automate the discovery of optimal light structures. Finally, we explore the groundbreaking applications enabled by this integrated approach, including programmable electron beams, orbital-angular-momentum-carrying γ-rays, compact THz accelerators, and robust communications. The path forward necessitates overcoming grand challenges in material science, real-time adaptive control at MHz rates, and the extension of these principles to the quantum realm. This review serves as a call to action for a coordinated, interdisciplinary effort to command, rather than merely observe, light-matter interactions at the extreme.

physics.optics↗

arXiv2025

Dual-wavelength quantum skyrmions from liquid crystal topological defect

We propose a spin-orbit strategy for generating dual-wavelength quantum skyrmions, realized either as entangled photon pairs at dual wavelengths or as heralded single-photon states at a given wavelength -- regimes neither previously conceptualized nor demonstrated. By coupling a two-photon entangled state to an electrically tunable liquid crystal topological defect, we engineer both nonlocal and local skyrmionic topologies in a reconfigurable platform. We highlight with examples how this approach may open new directions for engineered topological quantum states that exploit the topological richness of liquid crystals.

quant-ph↗

arXiv2025

Encoding orbital angular momentum of light in space with optical catastrophes

Light beams carrying orbital angular momentum (OAM) possess an unbounded set of orthogonal modes, offering significant potential for optical communication and security. However, exploiting OAM beams in space has been hindered by the lack of a versatile design toolkit. Here, we demonstrate a strategy to tailor OAM across multiple transverse planes by shaping optical caustics leveraging on catastrophe theory. With complex-amplitude metasurfaces fabricated using two-photon polymerization lithography, we construct these caustics to steer Poynting vectors and achieve arbitrary shapes of OAM beams. Interestingly, we use such an approach to realize hidden OAM along the propagation trajectory, where the intensity of the beam is spread out thus avoiding detection. The OAM of these beams can be intrinsic, which avoids OAM distortions arising from the mixing of intrinsic and extrinsic components. By exploiting this intrinsic nature of OAM, we demonstrate the detection of encoded information in optical encryption. Our approach provides a unique framework for dynamic control of OAM in space, with promising applications in optical trapping and sensing, high-capacity data storage, and optical information security.

physics.optics↗

arXiv2025

Quantifying inconsistency in one-stage individual participant data meta-analyses of treatment-covariate interactions: a simulation study

It is recommended that measures of between-study effect heterogeneity be reported when conducting individual-participant data meta-analyses (IPD-MA). Methods exist to quantify inconsistency between trials via I^2 (the percentage of variation in the treatment effect due to between-study heterogeneity) when conducting two-stage IPD-MA, and when conducting one-stage IPD-MA with approximately equal numbers of treatment and control group participants. We extend formulae to estimate I^2 when investigating treatment-covariate interactions with unequal numbers of participants across subgroups and/or continuous covariates. A simulation study was conducted to assess the agreement in values of I^2 between those derived from two-stage models using traditional methods and those derived from equivalent one-stage models. Fourteen scenarios differed by the magnitude of between-trial heterogeneity, the number of trials, and the average number of participants in each trial. Bias and precision of I^2 were similar between the one- and two-stage models. The mean difference in I^2 between equivalent models ranged between -1.0 and 0.0 percentage points across scenarios. However, disparities were larger in simulated datasets with smaller samples sizes with up to 19.4 percentage points difference between models. Thus, the estimates of I^2 derived from these extended methods can be interpreted similarly to those from existing formulae for two-stage models.

stat.ME↗

arXiv2025

Orbital angular momentum of entangled photons as a probe for relativistic effects

Orbital angular momentum (OAM) as both classical and quantum states of light has proven essential in numerous applications, from high-capacity information transfer to enhanced precision and accuracy in metrology. Here, we extend OAM metrology to relativistic scenarios to determine the Lorentz factor of a moving reference frame, exploiting the fact that OAM is not Lorentz invariant. We show that the joint OAM spectrum from entangled states is modified by length contraction when measured by two observers moving relative to the entanglement source. This relative motion rescales the spatial dimensions, thus breaking the orthogonality of the OAM measurement process and resulting in a broadening of the joint OAM spectrum that can precisely determine the Lorentz factor. We experimentally simulate velocities up to $0.99c$, confirm the predicted broadening, and use the measurement outcomes to extract the Lorentz factor. Our work provides a pathway for novel measurement techniques suitable for relativistic conditions that leverage OAM structured light as a resource.

quant-ph↗

arXiv2025

Evaluating Informative Cluster Size in Cluster Randomized Trials

In cluster randomized trials, the average treatment effect among individuals (i-ATE) can be different from the cluster average treatment effect (c-ATE) when informative cluster size is present, i.e., when treatment effects or participant outcomes depend on cluster size. In such scenarios, mixed-effects models and generalized estimating equations (GEEs) with exchangeable correlation structure are biased for both the i-ATE and c-ATE estimands, whereas GEEs with an independence correlation structure or analyses of cluster-level summaries are recommended in practice. However, when cluster size is non-informative, mixed-effects models and GEEs with exchangeable correlation structure can provide unbiased estimation and notable efficiency gains over other methods. Thus, hypothesis tests for informative cluster size would be useful to assess this key phenomenon under cluster randomization. In this work, we develop model-based, model-assisted, and randomization-based tests for informative cluster size in cluster randomized trials. We construct simulation studies to examine the operating characteristics of these tests, show they have appropriate Type I error control and meaningful power, and contrast them to existing model-based tests used in the observational study setting. The proposed tests are then applied to data from a recent cluster randomized trial, and practical recommendations for using these tests are discussed.

stat.ME↗

arXiv2025

Wavelength-adaptive spin-orbit orbital angular momentum management in three-wave mixing

Here we propose the use of an adjustable liquid crystal spin-orbit device to shape bi-colour structured light to create bimodal states. We demonstrate the proof-of-principle for two individual wavelengths in a nonlinear optics framework. The spin-orbit device has an inhomogeneous optical axis orientation and birefringence, allowing it to modulate two wavelengths of light with pre-selected transmission functions by simply tuning a voltage. We combine this bi-colour functionality in a nonlinear optical experiment by employing three-wave mixing in a periodically poled crystal to show how the combined effect of linear spin-orbit transformation rules and nonlinear selection rules gives rise to novel approaches for light to modulate light, and light to unravel light. We show that the roles of the nonlinear crystal and spin-orbit device can be switched to either characterise the device with known light, or unravel unknown light with the device. This synergy between spin-orbit and nonlinear optics offers a novel paradigm where light manipulates and reveals its own structure across spectral domains.

physics.optics↗

arXiv2025

Spatially-Resolved Atmospheric Turbulence Sensing with Two-Dimensional Orbital Angular Momentum Spectroscopy

Atmospheric turbulence characterization is crucial for technologies like free-space optical communications. Existing methods using a spatially-integrated one-dimensional (1D) orbital angular momentum (OAM) spectrum, P(m), obscure the heterogeneous nature of atmospheric distortions. This study introduces a two-dimensional (2D) OAM spectroscopy, P(m, n), which resolves the OAM spectrum (topological charge m) across discrete radial annuli (index n). Integrating this high-dimensional spectral analysis with a Support Vector Machine (SVM) classifier significantly improves the accuracy of atmospheric turbulence parameter inversion. The full potential of complex probe beams, such as multi-ringed Bessel-Gaussian beams, is realized with this radially-resolved 2D analysis. Through a co-design of the probe beam's spatial structure and the OAM spectral analysis dimensionality, a median classification accuracy of 85.47% was achieved across 20 turbulence conditions, a 23% absolute improvement over 1D techniques. The radial index also mitigates insufficient OAM spectral range, and a targeted feature-selection protocol addresses noise from low signal-to-noise ratio outer radial regions. This framework emphasizes co-design of the optical probe field and its OAM spectral analysis for enhanced fidelity in turbulence characterization.

physics.optics↗

arXiv2025

Topological robustness of classical and quantum optical skyrmions in atmospheric turbulence

The degradation of classical and quantum structured light induced by complex media constitutes a critical barrier to its practical implementation in a range of applications, from communication and energy transport to imaging and sensing. Atmospheric turbulence is an exemplary case due to its complex phase structure and dynamic variations, driving the need to find invariances in light. Here we construct classical and quantum optical skyrmions and pass them through experimentally simulated atmospheric turbulence, revealing the embedded topological resilience of their structure. In the quantum realm, we show that while skyrmions undergo diminished entanglement, their topological characteristics maintain stable. This is paralleled classically, where the vectorial structure is scrambled by the medium yet the skyrmion remains stable by virtue of its intrinsic topological protection mechanism. Our experimental results are supported by rigorous analytical and numerical modelling, validating that the quantum-classical equivalence of the topological behaviour is due to the non-separability of the states and one-sided nature of the channel. Our work blurs the classical-quantum divide in the context of topology and opens a new path to information resilience in noisy channels, such as terrestrial and satellite-to-ground communication networks.

physics.optics↗

arXiv2025

Machine learning assisted speckle and OAM spectrum analysis for enhanced turbulence characterisation

Atmospheric turbulence degrades the performance of free-space optical (FSO) communication and remote sensing systems by introducing phase and intensity distortions. While a majority of research focuses on mitigating these effects to ensure robust signal transmission, an underexplored alternative is to leverage the transformation of structured light to characterize the turbulent medium itself. Here, we introduce a deep learning framework that fuses post-propagation intensity speckle patterns and orbital angular momentum (OAM) spectral data for atmospheric turbulence parameter inference. Our architecture, based on a modified InceptionNet backbone, is optimized to extract and integrate multi-scale features from these distinct optical modalities. This multimodal approach achieves validation accuracies exceeding 80%, substantially outperforming conventional single-modality baselines. The framework demonstrates high inference accuracy and enhanced training stability across a broad range of simulated turbulent conditions, quantified by varying Fried parameters (r0) and Reynolds numbers (Re). This work presents a scalable and data-efficient method for turbulence characterization, offering a pathway toward robust environmental sensing and the optimization of dynamic FSO systems.

physics.optics↗

arXiv2025

Seeing through randomness with topological light

Transmitting structured light robustly through complex random media is crucial in many applications, from sensing to communication. Unfortunately, the spatial structure of light is distorted in such media due to refractive index inhomogeneities that cause multiple scattering, requiring mitigating strategies such as iterative optimisation and adaptive optics. Here, we use topological light to see through random media without the need for any corrective measures. Using skyrmions as our optical topology, we first demonstrate their robustness to randomness using controlled digital random phase masks before showing the universality of the approach with physical samples, from biological tissue to highly scattering materials. We benchmark the invariance of the topology against orbital angular momentum (OAM) and show no modal crosstalk using topology in channels where orbital angular momentum exhibits crosstalk greater than 70%. With the control in hand, we transmit images encoded into an alphabet of 10 topological numbers and show information transfer with high fidelity in regimes where traditional degrees of freedom, such as OAM, fail. Our work represents an important step towards noise-free transmission through noisy channels with the spatial structure of light without the need for active compensation strategies, opening potential applications in imaging, sensing and communicating with topology.

physics.optics↗

arXiv2025

Unveiling the link between quantum ghost imaging and Grover's quantum searching algorithm

Photonic quantum technologies have become pivotal in the implementation of communication, imaging and computing modalities. Among these applications, quantum ghost imaging (GI) exploits photon correlations to surpass classical limits, with recent advances in spatial-mode encoding and phase imaging. In parallel, all-optical computing offers powerful, passive-light processing capabilities. Here, we explore the intersection of these domains, revealing a conceptual and operational link between GI and Grover's quantum search algorithm (GSA) which is designed to search for elements in an unstructured database. Here, the elements are encoded as phases in the position basis states of photons. To show this, we use entangled photon pairs, with one photon encoding the oracle features while the other photon is used to find the marked element.

quant-ph↗

arXiv2025

A reconfigurable arbitrary retarder array as complex structured matter

Tuneable retarder arrays, such as spatially patterned liquid crystal devices, have given rise to impressive photonic functionality, fuelling diverse applications ranging from microscopy and holography to encryption and communications. Presently these solutions are limited by the controllable degrees of freedom of structured matter, hindering applications that demand photonic systems with high flexibility and reconfigurable topologies. Here we demonstrate a compound modulator that implements a synthetic tuneable arbitrary retarder array as virtual pixels derived by cascading low functionality tuneable devices, realising full dynamic control of its arbitrary elliptical axis geometry, retardance value, and induced phase. Our approach offers unprecedented functionality that is user-defined and possesses high flexibility, allowing our modulator to act as a new beam generator, analyser, and corrector, opening an exciting path to tuneable topologies of light and matter.

physics.optics↗

arXiv2025

Disorder-enabled Synthetic Metasurfaces

Optical metasurfaces have catalyzed transformative advances across imaging, optoelectronics, quantum information processing, sensing, energy conversion, and optical computing. Yet, despite this rapid progress, most research remains focused on optimizing single functionalities, constrained by the persistent challenge of integrating multiple functions within a single device. Here, we demonstrate that engineered structural disorder of metapixels, used to implement a photonic function, can significantly reduce the area required across the entire aperture without compromising optical performance. The unallocated space can then be repurposed to encode functionally distinct metapixels without increasing the design complexity, each independently addressable via various optical degrees of freedom. As a proof of concept, we present a synthetic achromatic metalens featuring 11 spectrally distinct lens profiles encoded through nonlocal metapixels engineered to support sharp resonances via quasi bound states in the continuum. This large-scale metalens with 8.1 mm aperture achieves diffraction-limited achromatic focusing across the 1200 to 1400 nm spectral window. We further incorporate polarization-selective metapixels to implement momentum-space distinct gratings, enabling single-shot, high spatial resolution polarimetric imaging of arbitrarily structured light fields, including radial and azimuthal vector beams and optical skyrmions. Altogether, this disorder-enabled synthetic metasurface platform establishes a versatile foundation for unifying diverse photonic functionalities within a single optical element, marking a substantial step toward compact, high-density, multifunctional optical devices.

physics.optics↗