Ian DSouza | Searcharxiv

arXiv2026

Towards a Digital Twin for the Ground to QEYSSat Quantum Link

Simulations of physical systems require high-fidelity models to accurately represent reality. Simple models may be analytically tractable, but may not be sufficiently representative of reality for the given application. The cost of this simplicity is accuracy, or in the case of quantum key distribution, provable security. Sources of this accuracy gap include the difficulty of modelling physical effects which do not lend themselves well to analytical descriptions, such as afterpulsing. Here, we introduce a novel Monte Carlo based photon emission, transmission, and detection simulator, designed in the context of the Quantum Encryption and Science Satellite (QEYSSat) mission. Within this simulator, every major physical effect a photon may experience during an experiment, from emission to detection, can be accounted for in a probabilistic manner. This methodology allows for the inclusion of experimental parameters which are relevant for a satellite mission, and their impacts on secure key lengths. This simulator serves as a comprehensive baseline to predict and validate experimental data for the upcoming QEYSSat mission.

quant-ph↗

arXiv2026

Stellar Disruption of Axion Minihalos and Consequences for Direct Axion Detection

Scenarios such as the QCD axion with the Peccei-Quinn symmetry broken after inflation predict an enhanced matter power spectrum on sub-parsec scales. These theories lead to the formation of dense dark matter structures known as minihalos, which provide insights into early Universe dynamics and have implications for direct detection experiments. We examine the mass loss of minihalos during stellar encounters, building on previous studies that derived formulas for mass loss and performed N-body simulations. We propose a new formula for the mass loss that accounts for changes in the minihalo profile after disruption by a passing star. We also investigate the mass loss for multiple stellar encounters. We demonstrate that accurately assessing the mass loss in minihalos due to multiple stellar encounters necessitates considering the alterations in the minihalo's binding energy after each encounter, as overlooking this aspect results in a substantial underestimation of the mass loss. We further extend our analysis to the Galactic environment by more accurately incorporating multiple stellar encounters and dynamical relaxation timescales, simulating minihalo orbits in the Galactic potential. Our results show stellar interactions are more destructive than previously estimated, reducing minihalo mass retention at the solar system to ~30%, compared to earlier estimates of ~60%. This enhanced loss arises from cumulative energy injections when relaxation periods between stellar encounters are accounted for. The altered minihalo mass function implies a larger fraction of axion dark matter occupies inter-minihalo space, potentially increasing the local axion density and improving haloscope detection prospects. This thesis highlights the significance of detailed modeling of stellar disruptions in shaping the axion dark matter distribution.

astro-ph.CO↗

arXiv2025

Semi-analytic Orbits: A Practical Implementation of Lynden-Bell's Planar Orbits and Extension to Vertical Oscillations

We present a practical implementation of the perturbation theory derived by Lynden-Bell (2015) for describing, to arbitrary precision, the orbit of a particle in an arbitrary spherically-symmetric potential. Our implementation corrects minor but important errors in the initial derivation, and extends the formalism in two ways. First, a numerical method is developed to efficiently and precisely solve the analogue to the Kepler problem, and second, a method is introduced to track the particle's vertical oscillations about an axisymmetric disk, even when the vertical oscillation frequency varies with radius. While not as flexible as numerical integration, this method guarantees conservation of energy, angular momentum, and related quantities, and may be used to evaluate a particle's position and velocity in constant time. Our implementation is written in Python and is pip installable as the package lbparticles.

astro-ph.GA↗

arXiv2024

Enhanced Disruption of Axion Minihalos by Multiple Stellar Encounters in the Milky Way

If QCD axion dark matter formed post-inflation, axion miniclusters emerged from isocurvature fluctuations and later merged hierarchically into minihalos. These minihalos, potentially disrupted by stellar encounters in the Milky Way, affect axion detectability. We extend prior analyses by more accurately incorporating multiple stellar encounters and dynamical relaxation timescales, simulating minihalo orbits in the Galactic potential. Our results show stellar interactions are more destructive than previously estimated, reducing minihalo mass retention at the solar system to ~30%, compared to earlier estimates of ~60%. This enhanced loss arises from cumulative energy injections when relaxation periods between stellar encounters are accounted for. The altered minihalo mass function implies a larger fraction of axion dark matter occupies inter-minihalo space, potentially increasing the local axion density and improving haloscope detection prospects. This work highlights the significance of detailed modeling of stellar disruptions in shaping the axion dark matter distribution.

astro-ph.CO↗

arXiv2024

The Disruption of Dark Matter Minihalos by Successive Stellar Encounters

Scenarios such as the QCD axion with the Peccei-Quinn symmetry broken after inflation predict an enhanced matter power spectrum on sub-parsec scales. These theories lead to the formation of dense dark matter structures known as minihalos, which provide insights into early Universe dynamics and have implications for direct detection experiments. We examine the mass loss of minihalos during stellar encounters, building on previous studies that derived formulas for mass loss and performed N-body simulations. We propose a new formula for the mass loss that accounts for changes in the minihalo profile after disruption by a passing star. We also investigate the mass loss for multiple stellar encounters. We demonstrate that accurately assessing the mass loss in minihalos due to multiple stellar encounters necessitates considering the alterations in the minihalo's binding energy after each encounter, as overlooking this aspect results in a substantial underestimation of the mass loss.

astro-ph.GA↗

arXiv2021

Repeated radiation damage and thermal annealing of avalanche photodiodes

Avalanche photodiodes (APDs) are well-suited for single-photon detection on quantum communication satellites as they are a mature technology with high detection efficiency without requiring cryogenic cooling. They are, however, prone to significantly increased thermal noise caused by in-orbit radiation damage. Previous work demonstrated that a one-time application of thermal annealing reduces radiation-damage-induced APD thermal noise. Here we examine the effect of cyclical proton irradiation and thermal annealing. We use an accelerated testing environment which emulates a realistic two-year operating profile of a satellite in low-Earth-orbit. We show that repeated thermal annealing is effective at maintaining thermal noise of silicon APDs within a range suitable for quantum key distribution throughout the nominal mission life, and beyond. We examine two strategies -- annealing at a fixed period of time, and annealing only when the thermal noise exceeds a pre-defined limit. We find both strategies exhibit similar thermal noise at end-of-life, with a slight overall advantage to annealing conditionally. We also observe that afterpulsing probability of the detector increases with cumulative proton irradiation. This knowledge helps guide design and tasking decisions for future space-borne quantum communication applications.

physics.ins-det↗