arXiv subjects

John O'Hara

Publications and source records attributed to John O'Hara.

9 recordsLinked to original sources

arXiv2025

Site-controlled quantum dot arrays edge-coupled to integrated silicon nitride waveguides and devices

The scalability of quantum photonic integrated circuits opens the path towards large-scale quantum computing and communication. To date, this scalability has been limited by the stochastic nature of the quantum light sources. Moreover, hybrid integration of different platforms will likely be necessary to combine state-of-the-art devices into a functioning architecture. Here, we demonstrate the active alignment and edge-coupling of arrays of ten site-controlled gallium arsenide quantum dots to an array of ten silicon nitride single-mode waveguides, at cryogenic temperatures. The coupling is facilitated by the fabrication of nanopillars, deterministically self-aligned around each quantum dot, leading to a high-yield and regular array of single-photon sources. An on-chip beamsplitter verifies the triggered emission of single photons into the silicon nitride chip. The low inhomogeneous broadening of the ensemble enables us to observe the spectral overlap of adjacent site-controlled emitters. Across the array of waveguides, the signal collected from each coupled quantum dot is consistently and reproducibly 0.17 relative to the free-space collection from the very same single-photon source. Comparing measurement with waveguide simulations, we infer that absolute coupling efficiencies of $\approx 5 \%$ are currently obtained between our quantum dots and the waveguides.

physics.optics↗

arXiv2025

Self-aligned pillar arrays embedding site-controlled single quantum dots for enhanced non-classical light emission

This work presents a foundational approach for fabricating arrays of self-aligned micro- and nanopillar structures incorporating individual site-controlled quantum dots (QDs) for enhanced light extraction. This method leverages the non-planar surface morphology of pyramidal QD samples to define dielectric masks self - aligned to the QD positions. The mask size, and consequently the lateral dimensions of the pillars, is precisely controlled through a chemical mechanical polishing step, obviating the need for any additional lithography step for creating the pillar. This fabrication technique offers several key advantages, including precise control over the pillar sites, and fully deterministic embedding of QD structures. The functionality of the structures was validated by integrating single In0.25Ga0.75As QDs - upon two-photon excitation of the biexciton state, the emission of single and polarization-entangled photon pairs was observed. Additionally, an extra fabrication step to deposit dome-like structures atop the pillars was demonstrated, effectively enhancing light extraction efficiency up to 12%.

physics.optics↗

arXiv2023

A new encoding of implied volatility surfaces for their synthetic generation

In financial terms, an implied volatility surface can be described by its term structure, its skewness and its overall volatility level. We use a PCA variational auto-encoder model to perfectly represent these descriptors into a latent space of three dimensions. Our new encoding brings significant benefits for synthetic surface generation, in that (i) scenario generation is more interpretable; (ii) volatility extrapolation achieve better accuracy; and, (iii) we propose a solution to infer implied volatility surfaces of a stock from an index to which it belongs directly by modelling their relationship on the latent space of the encoding. All these applications, and the latter in particular, have the potential to improve risk management of financial derivatives whenever data is scarce.

q-fin.PR↗

arXiv2021

The Efficient Hedging Frontier with Deep Neural Networks

The trade off between risks and returns gives rise to multi-criteria optimisation problems that are well understood in finance, efficient frontiers being the tool to navigate their set of optimal solutions. Motivated by the recent advances in the use of deep neural networks in the context of hedging vanilla options when markets have frictions, we introduce the Efficient Hedging Frontier (EHF) by enriching the pipeline with a filtering step that allows to trade off costs and risks. This way, a trader's risk preference is matched with an expected hedging cost on the frontier, and the corresponding hedging strategy can be computed with a deep neural network. We further develop our framework to improve the EHF and find better hedging strategies. By adding a random forest classifier to the pipeline to forecast market movements, we show how the frontier shifts towards lower costs and reduced risks, which indicates that the overall hedging performances have improved. In addition, by designing a new recurrent neural network, we also find strategies on the frontier where hedging costs are even lower.

q-fin.CP↗

arXiv2021

Dispersion from Diffuse Reflectors and its Effect of Terahertz Wireless Communication Performance

This work investigates the temporal dispersion of a wireless terahertz communication signal caused by reflection from a rough (diffuse) surface, and its subsequent impact on symbol error rate versus data rate. Broadband measurements of diffuse reflectors using terahertz time-domain spectroscopy were used to establish and validate a scattering model that uses stochastic methods to describe the effects of surface roughness on the phase and amplitude of a reflected terahertz signal, expressed as a communication channel transfer function. The modeled channel was used to simulate a quadrature phase shift keying (QPSK)- modulated wireless communication link to determine the relationships between symbol error rate and data rate as a function of surface roughness. The simulations reveal that surface roughness from wall texturing results in group delay dispersion that limits achievable data rate with low errors. A distinct dispersion limit in surface roughness is discovered beyond which unacceptable numbers of symbol errors begin to accrue for a given data rate.

eess.SP↗

arXiv2021

Emulating UAV Motion by Utilizing Robotic Arm for mmWave Wireless Channel Characterization

In this paper, millimeter wave (mmWave) wireless channel characteristics (Doppler spread and path loss modeling) for Unmanned Aerial Vehicles (UAVs) assisted communication is analyzed and studied by emulating the real UAV motion using a robotic arm. The motion considers the actual turbulence caused by the wind gusts to the UAV in the atmosphere, which is statistically modeled by the widely used Dryden wind model. The frequency under consideration is 28 GHz in an anechoic chamber setting. A total of 11 distance points from 3.5 feet to 23.5 feet in increments of 2 feet were considered in this experiment. At each distance point, 3 samples of data were collected for better inference purposes. In this emulated environment, it was found out that the average Doppler spread at these different distances was around -20 Hz and +20 Hz at the noise floor of -60 dB. On the other hand, the path loss exponent was found to be 1.843. This study presents and lays out a novel framework of emulating UAV motion for mmWave communication systems, which will pave the way out for future design and implementation of next generation UAV-assisted wireless communication systems.

eess.SP↗

arXiv2020

Towards Non-Contact Glucose Sensing in Aqueous Turbid Medium at ~1.1 Meters Distance

This work demonstrates a non-contact diffuse reflectance approach with a working distance of ~1.1 meters for the potential of glucose sensing. Non-contact diffuse reflectance over 1.1-1.3 micrometers was developed according to a center-illumination-area-detection (CIAD) geometry. The modeled response of diffuse reflectance in the CIAD geometry was examined with phantoms by altering independently the size of the collection geometry and the reduced scattering and absorption properties of the medium. When applied to aqueous turbid medium containing glucose control solutions with the cumulative volume varying over three orders of magnitude, a linear relationship expected for the diffuse reflectance as a function of the medium absorption/reduced-scattering property was observed for four conditions of the glucose-medium composition that differed either in the effective glucose concentration or the host medium scattering property. The cumulation of glucose up to 17.8mg/dL and 8.9mg/dL in the host medium having the same optical properties resulted in linear regression slopes of 0.0032 and 0.0030, respectively. The cumulation of the glucose up to 17.8mg/dL in an aqueous host medium that differed two folds in the reduced scattering property caused the linear regression slope to differ between 0.0032 and 0.0019. The R^2 values of all cases were all greater than 0.987. A provisional patent (US#63/053,004) has been obtained for this work.

physics.med-ph↗

arXiv2019

Light Scattering from Solid-State Quantum Emitters: Beyond the Atomic Picture

Coherent scattering of light by a single quantum emitter is a fundamental process at the heart of many proposed quantum technologies. Unlike atomic systems, solid-state emitters couple to their host lattice by phonons. Using a quantum dot in an optical nanocavity, we resolve these interactions in both time and frequency domains, going beyond the atomic picture to develop a comprehensive model of light scattering from solid-state emitters. We find that even in the presence of a cavity, phonon coupling leads to a sideband that is completely insensitive to excitation conditions, and to a non-monotonic relationship between laser detuning and coherent fraction, both major deviations from atom-like behaviour.

quant-ph↗

arXiv2016

On-chip interference of single photons from an embedded quantum dot and an external laser

In this work, we demonstrate the on-chip two-photon interference between single photons emitted by a single self-assembled InGaAs quantum dot and an external laser. The quantum dot is embedded within one arm of an air-clad directional coupler which acts as a beam-splitter for incoming light. Photons originating from an attenuated external laser are coupled to the second arm of the beam-splitter and then combined with the quantum dot photons, giving rise to two-photon quantum interference between dissimilar sources. We verify the occurrence of on-chip Hong-Ou-Mandel interference by cross-correlating the optical signal from the separate output ports of the directional coupler. This experimental approach allows us to use classical light source (laser) to assess in a single step the overall device performance in the quantum regime and probe quantum dot photon indistinguishability on application realistic time scales.

cond-mat.mes-hall↗