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Coalescence of limit cycles in the presence of noise
Get PDFFunding: S.S. was partially supported by NSF-MPS-PHY Award No. 2207383.Complex dynamical systems may exhibit multiple steady states, including time-periodic limit cycles, where the final trajectory depends on initial conditions. With tuning of parameters, limit cycles can proliferate or merge at an exceptional point. Here we ask how dynamics in the vicinity of such a bifurcation are influenced by noise. A pitchfork bifurcation can be used to induce bifurcation behavior. We model a limit cycle with the normal form of the Hopf oscillator, couple it to the pitchfork, and investigate the resulting dynamical system in the presence of noise. We show that the generating functional for the averages of the dynamical variables factorizes between the pitchfork and the oscillator. The statistical properties of the pitchfork in the presence of noise in its various regimes are investigated and a scaling theory is developed for the correlation and response functions, including a possible symmetry-breaking field. The analysis is done by perturbative calculations as well as numerical means. Finally, observables illustrating the coupling of a system with a limit cycle to a pitchfork are discussed and the phase-phase correlations are shown to exhibit nondiffusive behavior with universal scaling.Peer reviewe
Concurrent linear deracemization of secondary benzylic alcohols via simultaneous photocatalysis and whole-cell biocatalysis
Get PDFFunding: This work was supported by the UKRI Biotechnology and Biological Sciences Research Council (BBSRC) Grant number: BB/M010996/1 via an EASTBIO Doctoral Training Partnership studentship to W.Y.W.W. S.W. acknowledges a Future Leaders Fellowship from UKRI (MR/S033882/1). C.P.J. acknowledges funding from the Royal Society (University Research Fellowship URF\R1\180017, URF\R\231016, and associated Enhancement Award RGF\EA\181022).Photobiocatalysis enables remarkable synthetic transformations by combining the exquisite stereoselectivity of enzymes with the mild generation of high-energy intermediates by photocatalysis, but practical applications remain limited due to enzyme photodamage. The deracemization of secondary alcohols is a key model reaction for photobiocatalytic protocols due to the importance of the enantioenriched products. However, current strategies rely on the temporal separation of catalytic cycles to circumvent incompatibilities, precluding photobiocatalytic transformations that require the in situ generation of reactive intermediates. We report a single-step concurrent linear deracemization protocol by combining a water-soluble photocatalyst (sodium anthraquinone-2-sulfonate) with a promiscuous alcohol dehydrogenase (Geotrichum candidum acetophenone reductase) encapsulated in lyophilized microbial whole cells. Insights into enzyme selectivity and system dynamics from molecular docking and kinetic modeling guided the optimization of the multicomponent system. Our approach represents a modular and generalizable strategy for developing photobiocatalytic cascades operating under mutually compatible conditions, wherein spatial separation mitigates photodamage and enables simultaneous dual catalytic turnover.Peer reviewe
TRIDENT : a rapid 3D radiative-transfer model for exoplanet transmission spectra
Get PDFFunding: R.J.M. acknowledges conference travel support from the Royal Astronomical Society (RAS). R.J.M. and N.K.L. acknowledge support from NASA grant No. 80NSSC20K0586 issued through the James Webb Space Telescope Guaranteed Time Observer Program.Transmission spectroscopy is one of the premier methods used to probe the temperature, composition, and cloud properties of exoplanet atmospheres. Recent studies have demonstrated that the multidimensional nature of exoplanet atmospheres—due to nonuniformities across the day–night transition and between the morning and evening terminators—can strongly influence transmission spectra. However, the computational demands of 3D radiative-transfer techniques have precluded their usage within atmospheric retrievals. Here we introduce TRIDENT, a new 3D radiative-transfer model which rapidly computes transmission spectra of exoplanet atmospheres with day–night, morning–evening, and vertical variations in temperature, chemical abundances, and cloud properties. We also derive a general equation for transmission spectra, accounting for 3D atmospheres, refraction, multiple scattering, ingress/egress, grazing transits, stellar heterogeneities, and nightside thermal emission. After introducing TRIDENT's linear-algebra-based approach to 3D radiative transfer, we propose new parametric prescriptions for 3D temperature and abundance profiles and 3D clouds. We show that multidimensional transmission spectra exhibit two significant observational signatures: (i) day–night composition gradients alter the relative amplitudes of absorption features; and (ii) morning–evening composition gradients distort the peak-to-wing contrast of absorption features. Finally, we demonstrate that these signatures of multidimensional atmospheres incur residuals >100 ppm compared to 1D models, rendering them potentially detectable with the James Webb Space Telescope. TRIDENT's rapid radiative transfer, coupled with parametric multidimensional atmospheres, unlocks the final barrier to 3D atmospheric retrievals.Peer reviewe
Gemini/GMOS transmission spectroscopy of the grazing planet candidate WD 1856+534 b
Get PDFFunding: H.D.-L. acknowledges support from the Villum Foundation. S.B. was supported by the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20190624PRD2.WD 1856+534 b is a Jupiter-sized, cool giant planet candidate transiting the white dwarf WD 1856+534. Here, we report an optical transmission spectrum of WD 1856+534 b obtained from ten transits using the Gemini Multi-Object Spectrograph. This system is challenging to observe due to the faintness of the host star and the short transit duration. Nevertheless, our phase-folded white light curve reached a precision of 0.12%. WD 1856+534 b provides a unique transit configuration compared to other known exoplanets: the planet is 8× larger than its star and occults over half of the stellar disk during mid-transit. Consequently, many standard modeling assumptions do not hold. We introduce the concept of a "limb darkening corrected, time-averaged transmission spectrum" and propose that this is more suitable than (Rp,λ/Rs)2 for comparisons to atmospheric models for planets with grazing transits. We also present a modified radiative transfer prescription. Though the transmission spectrum shows no prominent absorption features, it is sufficiently precise to constrain the mass of WD 1856+534 b to be >0.84 MJ (to 2σ confidence), assuming a clear atmosphere and a Jovian composition. High-altitude cloud decks can allow lower masses. WD 1856+534 b could have formed either as a result of common envelope evolution or migration under the Kozai-Lidov mechanism. Further studies of WD 1856+534 b, alongside new dedicated searches for substellar objects around white dwarfs, will shed further light on the mysteries of post-main-sequence planetary systems.Peer reviewe
POSEIDON : a multidimensional atmospheric retrieval code for exoplanet spectra
Get PDFFunding: RJM acknowledges financial support from the UK’s Science and Technology Facilities Council (STFC) during the early development of POSEIDON and support from NASA Grant 80NSSC20K0586 issued through the James Webb Space Telescope Guaranteed Time Observer Program. Most recently, RJM acknowledges support from NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.Exoplanet atmospheres are a dynamic and fast-changing field at the frontier of modern astronomy. Telescope observations can reveal the chemical composition, temperature, cloud properties, and (potentially) the habitability of these remote worlds. Astronomers can measure these atmospheric properties by observing how the fraction of starlight blocked by a planet passing in front of its host star changes with wavelength — a technique called transmission spectroscopy. Since the wavelengths where different atoms and molecules absorb are already known (from laboratory measurements or quantum mechanics), astronomers can compare models of exoplanet spectra to observations to infer the chemical composition of exoplanets. POSEIDON is a Python package for the modelling and analysis of exoplanet spectra. POSEIDON has two main functions: (i) computation of model spectra for 1D, 2D, or 3D exoplanet atmospheres; and (ii) a Bayesian fitting routine (‘atmospheric retrieval’) that can infer the range of atmospheric properties consistent with an observed exoplanet spectrum.Peer reviewe
A catalogue of exoplanet atmospheric retrieval codes
Get PDFExoplanet atmospheric retrieval is a computational technique widely used to infer properties of planetary atmospheres from remote spectroscopic observations. Retrieval codes typically employ Bayesian sampling algorithms or machine learning approaches to explore the range of atmospheric properties (e.g., chemical composition, temperature structure, aerosols) compatible with an observed spectrum. However, despite the wide adoption of exoplanet retrieval techniques, there is currently no systematic summary of exoplanet retrieval codes in the literature. Here, we provide a catalogue of the atmospheric retrieval codes published to date, alongside links to their respective code repositories where available. Our catalogue will be continuously updated via a Zenodo archive.Non peer reviewe
Composite neural network pruning for edge computing
Get PDFDeep neural networks (DNNs) are the foundation of modern machine learning applications, supporting various technologies from data analytics and chatbots to generative AI systems. However, DNNs require significantly more compute, memory, and energy resources than traditional non-AI workloads. As model architectures scale in size and complexity, most notably with the development of large language models (LLMs), the gap between computational demands and the capabilities of available deployment hardware continues to widen.
Simultaneously, there is a growing demand to move inference closer to data sources through edge computing, where models must run on devices with limited compute power, memory, and energy capacity. This creates a fundamental challenge: How can increasingly complex DNNs be deployed efficiently on resource-constrained hardware?
Model compression aims to reduce the size and computational cost of DNNs by removing redundant components while retaining those critical to model accuracy. Compressed models are faster, smaller, and more energy-efficient. However, existing compression methods are often slow, incur significant overheads, degrade accuracy, or fail to scale to modern models like LLMs. Furthermore, many techniques are optimised for cloud environments and are less suited for edge deployment.
This thesis identifies three key challenges in compressing DNNs for the edge: (1) maintaining high model accuracy after compression, (2) generating compressed models before training to save compute and memory resources, and (3) scaling compression techniques to large and modern architectures such as LLMs.
To address these, this thesis introduces a new method called composite pruning, which combines the advantages of unstructured pruning and structured pruning into a unified pruning framework. Composite pruning is used to build three pruning systems—each targeting a specific challenge and demonstrating practical improvements across a range of DNN architectures, including convolutional and transformer-based models. The result is a new pruning paradigm that enables the deployment of high-quality, compressed models in edge environments
Measurement-induced dynamical quantum thermalization
Get PDFFunding: This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—Cluster of Excellence “Matter and Light for Quantum Computing”, ML4Q (390534769), and through the DFG Collaborative Research Center CRC 185 OSCAR (277625399). S.B. acknowledges financial support by the Deutsche Akademische Austauschdienst (DAAD, German Academic Exchange Service) through the Working Internships in Science and Engineering (WISE) Program, 2019 (57460839).One of the fundamental problems of quantum statistical physics is how an ideally isolated quantum system can ever reach thermal equilibrium behavior despite the unitary time evolution of quantum-mechanical systems. Here, we study, via explicit time evolution for the generic model system of an interacting, trapped Bose gas with discrete single-particle levels, how the measurement of one or more observables subdivides the system into observed and non-observed Hilbert subspaces and the tracing over the non-measured quantum numbers defines an effective, thermodynamic bath, induces the entanglement of the observed Hilbert subspace with the bath, and leads to a bi-exponential approach of the entanglement entropy and of the measured observables to thermal equilibrium behavior as a function of time. We find this to be more generally fulfilled than in the scenario of the eigenstate thermalization hypothesis (ETH), namely for both local particle occupation numbers and non-local density correlation functions, and independent of the specific initial quantum state of the time evolution.Peer reviewe
Layering mechanisms, compaction, and liquid migration in the layered nepheline syenites of the Ilímaussaq complex, South Greenland
No full textUnderstanding the mechanics of mineral layering and subsequent modification within igneous intrusions is of fundamental importance for magmatic differentiation, the accumulation of crystal-poor melts, and the formation of ore deposits. Layered igneous intrusions offer a unique opportunity to study these broad processes as they commonly preserve multiple stages of their crystallisation history. The Ilímaussaq complex, South Greenland, is of particular interest as it displays clear modal layers that host the world’s second most valuable rare earth element (REE) deposit.
This thesis investigates three key processes at Ilímaussaq: (1) the initial development of mineral layers; (2) if and how compaction occurred in the layered mush; and (3) late-stage liquid migration through the mush and how this enhanced or diminished REE concentration. This is achieved by quantifying and recording rock fabrics and textures through combined stratigraphic logging, rock magnetic, petrographic, and crystallographic preferred orientation methods.
Detailed stratigraphic logs, magnetic fabrics, and CPO reveal sub-horizontal silicate mineral foliations across all igneous layers, consistent with layer formation through gravitational settling of crystals. These foliations are often oblique to the modal layer contacts, suggesting post-cumulus modification, likely due to compaction. Rock magnetic fabrics also show sub-vertical patterns orthogonal to igneous layering, indicating vertical liquid migration through the mush.
The data presented in this thesis support a new model for REE enrichment in layered igneous intrusions, where igneous layering forms through gravitational settling of crystal mats, followed by mechanical compaction. During compaction, a saline, hydrous liquid migrates vertically through the crystal pile. The trapping and movement of this late-stage liquid through impermeable and permeable layers plays a key role in REE concentration. This study highlights the value of structural and textural analysis in understanding layered intrusions, especially when paired with robust field context. The methods and insights from Ilímaussaq are broadly applicable to layered intrusions worldwide."This work was supported by IAPETUS2 [grant number NE/S007431/1]. The field expedition to Ilímaussaq, South Greenland in 2022 was funded by the Geological Society of London, the Gino Watkins Memorial Fund, the Arctic Club, the Society of Economic Geologists, the Cambridge Arctic Shelf Programme, the Henry Emeleus award of the Volcano and Magmatic Studies Group, the Hazel-Prichard Student Bursary, the Edinburgh Geological Society, and the School of Earth and Environmental Sciences at the University of St Andrews."--Fundin
Testing the metabolic feasibility of Noachian hot springs on Mars
No full textMars exploration missions aim to search for signs of past microbial life during the planet’s most habitable era, the Noachian period. Volcanism during this period led to surface hydrothermal environments, which are hypothesised to be a conducive environment for prebiotic chemistry and are often inhabited by microorganisms employing primitive metabolisms. Modern terrestrial hot springs and their microbial inhabitants serve as a living representation of possible geobiological cycling on Noachian Mars. Biological fixation of nitrogen is a key part in unravelling the habitability of early martian environments. The nitrogen cycle on Mars remains enigmatic, and early martian communities would need to convert N₂ into bioavailable forms. Likewise, fixation of inorganic carbon forms the basis of the biological carbon cycle and is predicted to be a keystone metabolism for a Noachian biosphere.
This thesis will establish biological carbon and nitrogen cycling within Mars analogue hot springs and identify how these metabolic pathways manifest in the sedimentary record. Thermodynamic habitability estimates of analogue environments provide geochemical context to detailed genomic characterisation of key chemolithoautotrophic metabolisms. Through novel coupling of metagenomic analysis with stable isotope geochemistry, I test the hypothesis that δ¹³C and δ¹⁵N fractionations record the carbon and nitrogen fixation pathways described above.
Results reveal thermodynamically feasible carbon, iron and sulfur metabolisms and a ubiquitous reliance on biological fixation of inorganic N₂ and carbon within the hot spring communities. Deep geothermal processes indirectly influence biogeochemistry by controlling solubility of bio-essential metals used in metalloenzyme structures. This is reflected in biomass and sedimentary δ¹⁵N signatures, which are lighter than those seen for conventional N₂ fixation in acidic, sulfidic springs due to scarcity of molybdenum which is required for the MoFe nitrogenase enzyme. δ¹⁵N values instead agree with those produced by rare alternative nitrogenases which utilise vanadium and iron detected in these springs.
Overall, this thesis broadens our knowledge of keystone metabolisms within Mars analogue hot spring microbial communities and highlights the potential of nitrogen stable isotopes to record dominant biological nitrogen processes within Mars hot spring deposits."This work was supported by the UK Space Agency, Science and Technology Faculties Council
[ST/W002337/1, 2021-2025]."--Fundin
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