arXiv subjects

William Liu

Publications and source records attributed to William Liu.

9 recordsLinked to original sources

arXiv2026

A metastable state in $\mathrm{Yb}^+$ with lifetime greater than 10 minutes

We find through direct, time-domain measurement of trapped $\mathrm{Yb}^+$ ions that the excited ${}^3[11/2]_{9/2}^o$ state has a collision-free lifetime of 17.8(6) min, long enough for use in atomic clockwork or quantum information processing. We locate and benchmark a broad ``repump'' transition at $701\, \mathrm{nm}$ that readily transfers population back into the Doppler cooling cycle with the ground state, which may allow for quantum nondemolition measurement of sub-structure in the metastable manifold. Using a bi-stable Coulomb-crystal manometer, we find that the collisional quenching of this metastable ${}^3[11/2]_{9/2}^o$ manifold by background gas is more efficient than for ${}^2\mathrm{F}_{7/2}^o$, but nonetheless we observe an average lifetime exceeding 10 minutes at the base pressure of our apparatus.

physics.atom-ph↗

arXiv2026

Sub-hertz optical transitions in excited Yb$^+$

We present the observation of three semi-forbidden transitions in singly-ionized ytterbium from the metastable $^{2}\mathrm{F}^o_{7/2}$ state. Owing to the long lifetimes of both the upper and lower states involved, these transitions are narrow and complement those already frequently used in this atom for quantum information and searches for new physics. We report the absolute frequencies of these electric quadrupole transitions, their isotope shifts, hyperfine structure, and measurements of the quadrupole transition moments. We find that the spontaneous lifetimes of these excited states are limited by slow magnetic dipole emission to lower-lying, odd-parity states, the lowest of which is accessible by laser excitation from ${}^2\mathrm{F}_{7/2}^o$ and is an attractive candidate for hosting a long-lived qubit.

physics.atom-ph↗

arXiv2025

Finite Element and Machine Learning Modeling of Autogenous Self-Healing Concrete

A time-dependent modeling framework for autogenous self-healing concrete that couples moisture diffusion with damage evolution was developed. Water transport follows Fick's second law with a damage-dependent diffusivity obtained by power-law interpolation between intact concrete and crack space. Healing reduces damage in proportion to the product of local moisture and a smoothed cement availability field computed via a novel Helmholtz filtering approach that models the spatial extent over which cement clinker can travel and form crystals. Two finite element variants were implemented in FEniCSx: a Crack Diffusion Model (CDM) with standard diffusion and a Crack Membrane Model (CMM) that introduces a novel threshold-based gating mechanism to control cross-crack water transport until a critical moisture threshold is reached. Key control parameters are the initial crack orientation and size, the diffusion coefficients of intact and cracked concrete, the healing rate constant, and the cement availability smoothing parameter. Simulations show that healing time varies non-monotonically with crack orientation, peaking near $45^\circ$ and $135^\circ$ and minimizing near $90^\circ$. The dependence on crack width reverses with material parameters. The CMM reproduces staged moisture penetration with delayed gate activation but lengthens total healing time, whereas the CDM is efficient for parametric sweeps. Machine learning regression models were trained on finite element simulation data to predict healing times $H(\sigma,\gamma,\beta)$ with high accuracy ($R^2 > 0.999$), dramatically reducing computational time. SHAP analysis demonstrated that crack orientation influenced healing time the most, followed by crack width and cement availability smoothing. The modeling framework provides a versatile tool for guiding future laboratory studies and implementations of self-healing concrete.

cs.CE↗

arXiv2023

A Theory Guide to Using Control Functions to Instrument Hazard Models

I develop the theory around using control functions to instrument hazard models, allowing the inclusion of endogenous (e.g., mismeasured) regressors. Simple discrete-data hazard models can be expressed as binary choice panel data models, and the widespread Prentice and Gloeckler (1978) discrete-data proportional hazards model can specifically be expressed as a complementary log-log model with time fixed effects. This allows me to recast it as GMM estimation and its instrumented version as sequential GMM estimation in a Z-estimation (non-classical GMM) framework; this framework can then be leveraged to establish asymptotic properties and sufficient conditions. Whilst this paper focuses on the Prentice and Gloeckler (1978) model, the methods and discussion developed here can be applied more generally to other hazard models and binary choice models. I also introduce my Stata command for estimating a complementary log-log model instrumented via control functions (available as ivcloglog on SSC), which allows practitioners to easily instrument the Prentice and Gloeckler (1978) model.

econ.EM↗

arXiv2023

Eigenvalue-less matrices from SL$_3(\mathbb{F}_7)$ to PSL$_3(\mathbb{F}_7)$

We analyse the set of matrices in SL$_3(\mathbb{F}_7)$ without eigenvalues explicitly, extracting nice bijections between the 18 equally sized conjugacy classes contained within. In doing so, we discover a set of $18$ commuting matrices for which every conjugacy class is represented and tells us how to decide when collections of commuting matrices are simultaneously conjugate. The main innovation is the proofs are accessible to undergraduates and do not rely on computer calculations nor very general theory. It is also rare that the details of special cases are written down.

math.GM↗

arXiv2022

Jumping kinetic Monte Carlo: Fast and accurate simulations of partially delocalised charge transport in organic semiconductors

Developing devices using disordered organic semiconductors requires accurate and practical models of charge transport. In these materials, charge transport occurs through partially delocalised states in an intermediate regime between localised hopping and delocalised band conduction. Partial delocalisation can increase mobilities by orders of magnitude over conventional hopping, making it important for materials and device design. Although delocalisation, disorder, and polaron formation can be described using delocalised kinetic Monte Carlo (dKMC), it is a computationally expensive method. Here, we develop jumping kinetic Monte Carlo (jKMC), a model that approaches the accuracy of dKMC with a computational cost comparable to conventional hopping. jKMC achieves its computational performance by modelling conduction using identical spherical polarons, yielding a simple delocalisation correction to the Marcus hopping rate that allows polarons to jump over their nearest neighbours. jKMC can be used in regimes of partial delocalisation inaccessible to dKMC to show that modest delocalisation can increase mobilities by as much as two orders of magnitude.

physics.chem-ph↗

arXiv2018

Information and Communications Technologies for Sustainable Development Goals: State-of-the-Art, Needs and Perspectives

In September 2015, the United Nations General Assembly accepted the 2030 Development Agenda, which has included 92 paragraphs, and the Paragraph 91 defined 17 sustainable development goals (SDGs) and 169 associated targets. The goal of this paper is to discover the correlations among SDGs and information and communications technologies (ICTs). This paper discusses the roles and opportunities that ICTs play in pursuing the SDGs. We identify a number of research gaps to those three pillars, social, economic, and environmental perspectives, of sustainable development. After extensive literature reviews on the SDG-related research initiatives and activities, we find that the majority of contributions to SDGs recognized by the IEEE and ACM research communities have mainly focused on the technical aspects, while there are lack of the holistic social good perspectives. Therefore, there are essential and urgent needs to raise the awareness and call for attentions on how to innovate and energize ICTs in order to best assist all nations to achieve the SDGs by 2030.

cs.CY↗

arXiv2016

Efficient Integer Coefficient Search for Compute-and-Forward

Integer coefficient selection is an important decoding step in the implementation of compute-and-forward (C-F) relaying scheme. Choosing the optimal integer coefficients in C-F has been shown to be a shortest vector problem (SVP) which is known to be NP hard in its general form. Exhaustive search of the integer coefficients is only feasible in complexity for small number of users while approximation algorithms such as Lenstra-Lenstra-Lovasz (LLL) lattice reduction algorithm only find a vector within an exponential factor of the shortest vector. An optimal deterministic algorithm was proposed for C-F by Sahraei and Gastpar specifically for the real valued channel case. In this paper, we adapt their idea to the complex valued channel and propose an efficient search algorithm to find the optimal integer coefficient vectors over the ring of Gaussian integers and the ring of Eisenstein integers. A second algorithm is then proposed that generalises our search algorithm to the Integer-Forcing MIMO C-F receiver. Performance and efficiency of the proposed algorithms are evaluated through simulations and theoretical analysis.

cs.IT↗

arXiv2009

Cross-Scale: Multi-Scale Coupling in Space Plasma, Assessment Study Report

Driven by the support and interest of the international space plasma community to examine simultaneous physical plasma scales and their interactions, the Cross-Scale Mission concept was submitted and accepted as an ESA Cosmic Vision M-class candidate mission. This report presents an overview of the assessment study phase of the 7 ESA spacecraft Cross-Scale mission. Where appropriate, discussion of the benefit of international collaboration with the SCOPE mission, as well as other interested parties, is included.

physics.plasm-ph↗