arXiv subjects

Andrew Yu

Publications and source records attributed to Andrew Yu.

7 recordsLinked to original sources

arXiv2026

Explicit Matrices over $\mathbb Z_2$ with CNOT and Row Complexity $4n-\mathrm{o}(n)$ and Local Logic Gates

In this article, we present an explicit family of invertible $n\times n$ matrices over $\mathbb Z_2$ whose CNOT and row complexity is at least $4n-\text{o}(n)$; equivalently, reducing these matrices to the identity requires at least $4n-\text{o}(n)$ elementary row operations. Moreover, the same complexity lower bound holds in the stronger computational model where the CNOT gates are replaced by arbitrary local linear logic gates, namely arbitrary invertible linear transformations acting on pairs of coordinates. Let $G_n$ denote the permutation group generated by local logic gates acting on the set of binary strings of length $n$. We prove that $G_n$ is naturally isomorphic to the group of all invertible affine transformations of the vector space $\mathbb Z_2^n$, thus reducing the problem of estimating the quantum complexity of permutations in $G_n$ to the row reduction complexity of invertible matrices over $\mathbb Z_2$. As an application, we show that the permutations associated with our explicit matrices have quantum complexity at least $4n-\text{o}(n)$.

quant-ph↗

arXiv2026

Structure and Dynamics of the Inner Corona Measured from the DEB Initiative 2024 Eclipse Image Sequence

The Dynamic Eclipse Broadcast (DEB) Initiative citizen science program observed coronal visible continuum brightness during the 2024 April 8 total eclipse from locations across North America. We present results from 11 DEB sites spanning 2700 km of distance and showing 49 minutes of evolution. The coronal brightness radial profiles from these telescopes are tightly correlated from 1.2 to 4.0 solar radii and comparable to published photometric coronal intensities. The coronal flattening parameter is measured from 1.4 to 2.8 solar radii. A Ludendorff index of 0.0761 +/- 0.0007 is computed but the extrapolation techniques used by some to calculate this index are shown to disagree with this direct measurement, and alternate structure parameters are suggested. Measured radial velocities are compared with an MHD model of the corona during the eclipse from Y. Li et al. (2026). A polar downflow is measured with an average radial velocity of -37 +/- 3 km s-1 and a deceleration of 14 +/- 3 m s-2 at a speed and position which agrees with the model. The predicted mixture of outflows and downflows at low heights is seen, as well as outflows in two western regions of the corona. The fastest observed outflow has a radial speed of 105 km s-1 and is likely associated with a transient event not predicted by the model. Future DEB Initiative eclipse experiments can more tightly constrain models of the inner corona by using both coronal intensity and radial velocity measurements.

astro-ph.SR↗

arXiv2026

Magnetospheric flows in X-ray pulsars II: Heating, cooling and ionization degree at sub-critical accretion

Magnetospheric accretion flows in X-ray pulsars shape their spectra, polarization, and variability. We model the thermal balance of the flow enveloping the neutron star magnetosphere in the sub-critical regime ($L \lesssim 10^{37}\,\mathrm{erg\,s^{-1}}$), where radiation forces do not control the dynamics and single Compton scatterings dominate. The energy budget includes Compton heating by surface X-rays, compressional (adiabatic) heating in the converging flow, and radiative cooling dominated by free-free emission and contributed also by cyclotron emission. We show that the interplay of these processes leads to efficient cooling of the flow in the inner magnetosphere. We compute the flow temperature profile as a function of luminosity and find that near the stellar surface the temperature can fall to a few tens of eV at $L < 10^{35}\,\mathrm{erg\,s^{-1}}$. Under such conditions, the accreting plasma, modelled here as pure hydrogen, is no longer fully ionized. In the strong magnetic fields typical for X-ray pulsars, such temperatures permit partial recombination of electrons and protons into neutral hydrogen. As a result, a significant fraction of the flow becomes weakly ionized, while external illumination ionizes this gas only partially within a geometrically thin layer immediately above the neutron star surface. This implies that magnetospheric accretion at low luminosities proceeds through a partially ionized medium, in contrast to the commonly assumed fully ionized flow.

astro-ph.HE↗

arXiv2024

Forte : Finding Outliers with Representation Typicality Estimation

Generative models can now produce photorealistic synthetic data which is virtually indistinguishable from the real data used to train it. This is a significant evolution over previous models which could produce reasonable facsimiles of the training data, but ones which could be visually distinguished from the training data by human evaluation. Recent work on OOD detection has raised doubts that generative model likelihoods are optimal OOD detectors due to issues involving likelihood misestimation, entropy in the generative process, and typicality. We speculate that generative OOD detectors also failed because their models focused on the pixels rather than the semantic content of the data, leading to failures in near-OOD cases where the pixels may be similar but the information content is significantly different. We hypothesize that estimating typical sets using self-supervised learners leads to better OOD detectors. We introduce a novel approach that leverages representation learning, and informative summary statistics based on manifold estimation, to address all of the aforementioned issues. Our method outperforms other unsupervised approaches and achieves state-of-the art performance on well-established challenging benchmarks, and new synthetic data detection tasks.

cs.LG↗

arXiv2024

Overcoming Ambient Drift and Negative-Bias Temperature Instability in Foundry Carbon Nanotube Transistors

Back-end-of-line (BEOL) logic integration is emerging as a complementary scaling path to supplement front-end-of-line (FEOL) Silicon. Among various options for BEOL logic, Carbon Nanotube Field-Effect Transistors (CNFETs) have been integrated within commercial silicon foundries, and complex CNFET circuits (e.g., RISC-V core, SRAM arrays) have been demonstrated. However, there lacks comprehensive studies that analyze the ambient drift (i.e., air-stability) and reliability of CNFETs. Here, for the first time, we thoroughly characterize and demonstrate how to overcome ambient drift and negative bias temperature instability (NBTI) in CNFETs using the following techniques: (1) Silicon Nitride encapsulation to limit ambient atmosphere induced threshold voltage shift (~8x reduction of median VT shift over 90 days) and (2) AC/pulsed operation to significantly improve CNFET NBTI vs. DC operation across a wide frequency range (e.g., 20% duty cycle AC operation at 10 MHz could extend CNFET NBTI time-to-failure by >10000x vs. DC for a target VT shift tolerance < 100 mV with gate stress bias VGS,stress = -1.2 V at 125 C).

cs.ET↗

arXiv2023

Sharp bound for embedded eigenvalues of Dirac operators with decaying potentials

We study eigenvalues of the Dirac operator with canonical form \begin{equation} L_{p,q} \begin{pmatrix} u \\ v \end{pmatrix}= \begin{pmatrix} 0 & -1 \\ 1 & 0 \end{pmatrix}\frac{d}{dt} \begin{pmatrix} u \\ v \end{pmatrix}+\begin{pmatrix} -p & q \\ q & p \end{pmatrix}\begin{pmatrix} u \\ v \end{pmatrix},\nonumber \end{equation} where $ p$ and $q$ are real functions. Under the assumption that \begin{equation} \limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}<\infty,\nonumber \end{equation} the essential spectrum of $L_{p,q}$ is $(-\infty,\infty)$. We prove that $L_{p,q}$ has no eigenvalues if $$\limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}<\frac{1}{2}.$$ Given any $A\geq \frac{1}{2}$ and any $\lambda\in\R$, we construct functions $p$ and $q$ such that $\limsup_{x\to \infty}x\sqrt{p^2(x)+q^2(x)}=A$ and $\lambda$ is an eigenvalue of the corresponding Dirac operator $L_{p,q}$. We also construct functions $p$ and $q$ so that the corresponding Dirac operator $L_{p,q}$ has any prescribed set {(finitely or countably many)} of eigenvalues.

math.SP↗

arXiv2022

Quantum Complexity of Permutations

Let $S_n$ be the symmetric group of all permutations of $\{1, \cdots, n\}$ with two generators: the transposition switching $1$ with $2$ and the cyclic permutation sending $k$ to $k+1$ for $1\leq k\leq n-1$ and $n$ to $1$ (denoted by $\sigma$ and $\tau$). In this article, we study quantum complexity of permutations in $S_n$ using $\{\sigma, \tau, \tau^{-1}\}$ as logic gates. We give an explicit construction of permutations in $S_n$ with quadratic quantum complexity lower bound $\frac{n^2-2n-7}{4}$. We also prove that all permutations in $S_n$ have quadratic quantum complexity upper bound $3(n-1)^2$. Finally, we show that almost all permutations in $S_n$ have quadratic quantum complexity lower bound when $n\rightarrow \infty$.

quant-ph↗