arXiv subjects

C. Zhang

Publications and source records attributed to C. Zhang.

At least 19 recordsLinked to original sources

arXiv2026

BICEP/Keck XXII: Analysis of the South Pole Atmosphere for CMB Observations

The South Pole is among the driest sites on Earth, and the resulting high atmospheric transparency makes it an excellent site for millimeter-wave astronomical observations. Nevertheless, for bolometric surveys of the cosmic microwave background (CMB), fluctuations in line-of-sight water vapor drive variable atmospheric emission, which has a significant impact on survey sensitivity. We have deployed a water vapor radiometer (WVR) to characterize these fluctuations in space and time and to compare atmospheric and CMB signals with other sites. This WVR has been monitoring conditions from the South Pole over the past nine years, co-located and co-observing with the BICEP series of telescopes. In this work, we analyze the performance of the WVR and the characteristics of the South Pole atmosphere and demonstrate coherent wind-driven transport of water vapor structures across the site. We show that fluctuations in CMB detector timestreams are well correlated with fluctuations in atmospheric water vapor, providing independent justification for baseline filtering procedures used to remove these fluctuations and providing a new pathway to detect and mitigate the temperature-to-polarization leakage that currently limits the precision of ground-based CMB polarization measurements.

astro-ph.IM↗

arXiv2026

Aliased noise characterization and mitigation in BICEP Array 150, 220 and 270 GHz time-division multiplexed detectors

Early observations with the BICEP Array 150 GHz (BA2-150) and 220/270 GHz (BA3-220/270) receivers revealed detector noise equivalent temperatures (NETs) higher than expected, together with substantial detector-to-detector and module-to-module scatter. Noise measurements acquired with multiplexing off and high frequency sampling demonstrate that this excess originates from elevated high-frequency detector noise that aliases into the science band during time-division multiplexing. We show that the excess high-frequency noise is correlated with anomalously large logarithmic TES transition slopes, {\alpha}, resulting in elevated electrothermal loop gain and operation near the detector stability boundary. Measurements of {\alpha} indicate values substantially larger than expected, consistent with the sharper superconducting transitions introduced by the inverted TES fabrication process adopted for BA2-150 and BA3-220/270 detectors. Operational mitigation strategies were investigated through both increased multiplexing rates and elevated focal-plane operating temperatures. Faster multiplexing reduces aliasing by shifting the multiplexing Nyquist frequency beyond the excess noise roll-off, while elevated bath temperatures reduce TES electrical power and loop gain, improving detector stability and reducing NET by approximately 10%. These results demonstrate the importance of balancing TES responsivity, electrothermal stability, and multiplexed readout performance in next-generation CMB polarimeters.

physics.ins-det↗

arXiv2026

Optical characterization of the BICEP array 150 and 220/270GHz CMB polarimeters in the 2026 season

BICEP Array (BA) is the current-generation instrument in the BICEP series of small-aperture, on-axis refracting telescopes at the South Pole, designed to constrain the tensor-to-scalar ratio $r$ through degree-scale measurements of B-mode polarization in the cosmic microwave background (CMB). As BA pushes to deeper sensitivity, control of instrumental systematics, and beam shape mismatch between the co-located orthogonally polarized detectors in particular, has become an increasingly important factor in translating raw sensitivity into a robust constraint on $r$. In these proceedings we report on the 2026 far field beam mapping (FFBM) campaign, which used a thermal chopped source to characterize the BA2 (150~GHz) and BA3 (220/270~GHz) beams. We fit two-dimensional elliptical Gaussians to each detector's beam, derive per-pair differential parameters (differential pointing, beamwidth, and ellipticity). The resulting high-signal-to-noise array-averaged beam maps are used to compute the beam window function $B_l$ for the power spectrum analysis, while the individual per-detector beams feed dedicated beam convolution simulations used to validate the temperature-to-polarization (T$\rightarrow$ P) deprojection procedure. After correcting for the chopper aperture, the recovered beamwidths follow the expected $\lambda/D$ ordering. We also describe two pipeline improvements carried out during the 2026 campaign: an out-and-back jackknife for noise quantification and an elnod-based gain calibration.

astro-ph.IM↗

arXiv2026

Advanced Time-Division Multiplexed Readout Chain for the BICEP Array 90/150 GHz Receiver

This work presents the current performance of the advanced time-division multiplexed (TDM) readout chain for the BICEP Array 90/150 GHz receiver. BA4-90/150, scheduled for deployment to the South Pole in 2026--27, will use photon-noise-limited, feedhorn-coupled transition edge sensor detectors and an upgraded DC SQUID-based TDM system to map the cosmic microwave background. This new TDM system mitigates readout-induced systematics that are beginning to emerge above the noise floor of the most sensitive maps produced by the BICEP collaboration. Improvements include faster, fully differential SQUID designs, higher TES signal amplification, reduced crosstalk, and hierarchical row-addressing that reduces wiring required for row switching. Measurements made through legacy single-ended warm readout electronics show the upgraded cryogenic readout chain performs as well as or better than the TDM system currently fielded on the BICEP experiment. New warm electronics currently in development at SLAC National Accelerator Laboratory will provide matched fully differential circuits and higher bandwidth, reducing RF susceptibility and aliased noise contributions. On-sky demonstration of this technology will establish a new low-noise, high-bandwidth TDM architecture for future CMB observatories.

astro-ph.IM↗

arXiv2026

Optics and broadband anti-reflection coatings for the BA4-90/150 receiver

The BICEP Array telescopes search for primordial B-mode polarization from inflationary gravitational waves. This signal is exceedingly faint, demanding excellent map depth and systematics control. The new BA4-90/150 receiver introduces a wide 80-169GHz dichroic band, requiring upgrades throughout the optics chain to reduce loss, reflections, and thermal loading. We developed improved anti-reflection (AR) coatings for our HMPE window, HDPE lenses, and nylon infrared filter, extending our AR technology to span more than an octave of bandwidth. The thermal filtering scheme and several mechanical elements were also updated to further suppress optical loss, reflections, and beam truncation. We aim to build on the proven success of deployed BICEP Array (BA) telescopes to produce a new small aperture instrument with the lowest optical systematics to date.

astro-ph.IM↗

arXiv2026

Status of BICEP Array and Integration of the 220/270 GHz Receiver

Measurements of the polarization of the cosmic microwave background are critical for modern cosmology as they constrain the physics of the early universe and cosmic inflation. BICEP Array is using a series of small-aperture polarimeters located at the South Pole to measure this signal with a projected uncertainty on the tensor-to-scalar ratio r of approximately 0.001 by 2034. The 30/40 GHz receiver and the 150 GHz receiver have been observing the cosmic microwave background for multiple years, and data from these receivers will be included in the next published BICEP analysis result which includes all data taken through 2024. The 220/270 GHz receiver is partially completed with seven out of twelve detector modules installed, and is planned to be filled with five additional modules prior to the 2027 observation season. Finishing this receiver is a top priority for BA as the science goals of the experiment require the dust foreground cleaning this receiver will enable. On-site characterization efforts for this receiver from the 2025-26 austral summer show satisfactory spectroscopy, beam pointing, and 220 GHz efficiency, while the 270 GHz efficiency is an active area of work.

astro-ph.IM↗

arXiv2026

Quantifying the systematic impact of differential beam response on the BICEP CMB polarization data from 2016 through 2024

As cosmic microwave background (CMB) polarization experiments, including BICEP3, BICEP Array, and future BICEP experiments, achieve ever-deeper polarization maps in search of primordial B-modes sourced from inflation, constraining instrumental systematics below statistical uncertainties becomes progressively more challenging. Since polarimetry in the BICEP telescopes is performed by pair-differencing co-located, orthogonally polarized detectors, differential beam response leads to temperature-to-polarization ($T \rightarrow P$) leakage, introducing a potential systematic bias on the inferred tensor-to-scalar ratio $r$. To mitigate this leakage, the lowest-order beam mismatch modes are filtered out of the CMB polarization maps through deprojection; however, residual undeprojected modes remain. To quantify this residual contamination, we perform dedicated in situ far-field beam measurements of the BICEP receivers during austral-summer calibration campaigns. We quantify the systematic impact of the undeprojected residuals with a specialized set of timestream simulations based on the measured per-detector beams. These "beam measurement-informed simulations" yield an estimate of the false polarized signal sourced by the undeprojected residuals. We summarize the beam measurements relevant to the BK24 data release and present preliminary residual-leakage results for BICEP3 at 95 GHz. For BICEP3 over 2016-2024, deprojecting all six standard templates together with readout-crosstalk templates and their radially smoothed counterparts reduces the equivalent-$r$ leakage amplitude from $\rho=(4.5\pm0.7)\times10^{-3}$ to $(1.12\pm0.06)\times10^{-3}$. We further describe an ongoing program to extend the deprojection basis beyond its historical six modes, guided by a forward optical model that relates candidate leakage modes to perturbations of physical instrument parameters.

astro-ph.CO↗

arXiv2026

Engineering of titanium transition edge sensor wafers for the BA4-90/150 receiver of BICEP Array

BA4-90/150, the fourth receiver to be deployed in the BICEP Array (BA) series, is a dichroic 90/150 GHz instrument targeting the frequency space where sensitivity to the CMB polarization is maximized. The receiver will be deployed in the 2026-2027 austral summer, and is set to position BA to achieve exceptionally precise measurements of cosmic microwave background (CMB) polarization and strengthen constraints on inflationary models. Recent measurements in existing BA receivers suggest that unexpectedly high loop gain in the titanium (Ti) transition edge sensors (TESs) produces excess high-frequency noise that is consequently aliased down into the science band through the time-division multiplexed readout. To reduce the loop gain, we fabricated and tested prototype Ti TES wafers containing 16 modified detector architectures designed to broaden the superconducting transition and reduce the transition steepness (alpha). We present detector performance results, which will directly inform the final integrated wafer now being designed for full receiver commissioning.

astro-ph.IM↗

arXiv2026

Areal-time disruption prediction and mitigation system for the EXL-50U spherical torus

This work presents a real-time disruption prediction and mitigation system developed for high-current operations in the EXL-50U Spherical Torus. By leveraging Reflective Memory (RFM) technology, the system establishes a low-latency real-time data path, creating a fully integrated pipeline that synchronizes multi-channel diagnostic acquisition, online preprocessing, real-time inference, and Massive Gas Injection (MGI) triggering. At its core, a lightweight prediction model based on a Temporal Convolutional Network (TCN) with a channel attention mechanism extracts disruption precursor features while adaptively weighting the importance of different diagnostic channels. {Tested across discharges \#14036--\#14790, the system achieves a true positive rate of 82.4\% and a false positive rate of 16.5\%, with end-to-end latency below $1~\mathrm{ms}$ in online operation.} Mitigation experiments further show that the MGI system can supply the required gas inventory and trigger a rapid post-injection plasma response, supporting the operational requirements of EXL-50U and providing engineering guidance for future devices such as EHL-2. These results confirm the engineering feasibility of integrated real-time disruption control on EXL-50U, offering a robust basis for future research in higher-parameter fusion devices.

physics.plasm-ph↗

arXiv2026

Apodex 1.1: Scaling Agentic Intelligence for Complex Work

General-purpose language models can reason and synthesize knowledge, but complex work also requires sustained interaction with files, information sources, and executable code, together with state maintenance, failure recovery, and verifiable delivery. We call this \emph{working capability}: sustained, verifiable progress toward a real-world objective. Apodex 1.1 develops this capability along two complementary dimensions. \emph{Environment Scaling} expands the diversity and verifiability of executable file, search, and code environments, while \emph{Agentic Coordination Scaling} trains agents to decompose long-horizon tasks, delegate parallel work, integrate asynchronous results, and replan. A shared execution harness and AgentOS maintain task state and provenance across tools and agents, and training turns environment trajectories and coordination traces into reliable behavior. Across complex professional work, finance, scientific research, mathematics, coding, and search, Apodex 1.1 reaches the leading performance band despite using a substantially smaller model than many frontier systems. The 35B-parameter Apodex 1.1 Mini further retains strong working capability in a locally deployable form. These results ground agentic intelligence in useful, verifiable work completed over time and advance our goal of building a \emph{Heavy-Duty Solver} for ambitious, long-running tasks.

cs.AI↗

arXiv2026

Overview and status of BICEP Array's BA4-90/150 CMB polarimeter

The inflation paradigm postulates a period of rapid expansion in the early Universe, which would generate gravitational waves. These tensor perturbations would produce a faint B-mode signature in the polarization of the cosmic microwave background (CMB), but this signal is orders of magnitude weaker than that from the CMB's other anisotropy and that from astrophysical foregrounds. Placing more-stringent upper limits on this signal or making a definitive detection thus requires exceptional control over instrument and measurement systematics, in addition to extremely-deep maps. The fourth BICEP Array receiver, BA4-90/150, aims to build and improve upon the heritage of the field-leading BICEP series of small-aperture CMB experiments with a dichroic instrument observing in 90 and 150 GHz bands, to advance the search for the inflationary B-mode signal. The instrument will utilize transition-edge-sensor bolometers, which will be read out using a new two-level time-division-multiplexed system and be fed via feedhorn-coupled orthomode transducers and refined cold refractive optics, with the goal of both improving systematics control and sensitivity over existing receivers. With a planned deployment to the South Pole in the 2026-27 austral summer, the instrument will occupy the fourth and final remaining slot in the BICEP Array mount, completing the phaseout of Keck Array receivers. An overview of the BA4-90/150 receiver will be presented, along with a discussion of its current status and future plans for the instrument.

astro-ph.IM↗

arXiv2026

Design and Performance of 220 and 270 GHz Bandpass Filters for BICEP Array

The BICEP Array (BA) is the latest in the BICEP/ Keck series of experiments that aim to measure the polarization of the cosmic microwave background (CMB) with small aperture polarimeters located at the South Pole. To constrain the frequency response of these receivers, each detector is serially coupled to a band-pass filter (BPF). The electric circuits of these BPFs utilize series and shunt capacitors as well as series inductors, but critically do not include shunt inductors which simplifies fabrication. The filters are designed and simulated with Sonnet, and optimized for noise by considering loading from the atmosphere and the CMB. Multiple 220 GHz detector modules have had their frequency response measured at the South Pole. The 270 GHz detector modules have recently begun testing in a lab setting, and their performance in a BA receiver will be measured this winter.

astro-ph.IM↗

arXiv2026

Gamma Backgrounds for Experiments at the High Flux Isotope Reactor

This article describes the deployment of a germanium detector at Oak Ridge National Lab's High Flux Isotope Reactor (HFIR) for the purpose of understanding the energy and spatial distribution of the gamma field in the experiment hall where the Precision Reactor Oscillation and Spectrum Experiment (PROSPECT) took data and future neutrino experiments could be located. The sources from both the reactor and the neutron beamlines are described in detail, along with their temporal variations due to reactor power and their spatial variations due to the geometry of the beamlines and building materials in the vicinity. Additionally, a shielding study was performed to assess the amount that backgrounds in tens of keV range can be mitigated. This work helps inform backgrounds for future experiments at reactors such as IBD-based neutrino measurements and CEvNS measurements.

hep-ex↗

arXiv2026

Origins of Cosmic Rays in the Galactic-extragalactic Transition Energy Range

Cosmic rays arrive at Earth with energies ranging from $10^9$ to over $10^{20}$ eV. One of the open questions in high-energy cosmic ray science concerns the origin of the highest-energy cosmic rays that can be accelerated by Galactic sources, and the transition energy beyond which only extragalactic sources can provide. Measuring the mass composition gives essential information for comparing measurements to source and propagation models, both from the abundances at the source and from the maximum attainable energy which is proportional to the particle charge (and hence its mass). The highest-energy cosmic rays from the Galaxy are found in a range of $10^{16}$ to $10^{18}$ eV which is well suited for radio detection. Building on a decade of experience in measuring cosmic rays at LOFAR, we show that SKA-Low, augmented with an array of small particle detectors, is well suited to advance the field by measuring the mass composition of cosmic rays across this energy range.

astro-ph.HE↗

arXiv2026

Highly filamentary H{\,\small I} gas in the circumgalactic medium and intragalactic medium around NGC 4631

Neutral hydrogen (H{\,\small I}) in the circumgalactic medium (CGM) and intergalactic medium (IGM) traces baryon cycling and galaxy evolution, yet fine filamentary substructures in diffuse CGM/IGM H{\,\small I} remain poorly constrained observationally. We combine FAST FEASTS single-dish and VLA THINGS interferometric H{\,\small I} data of the NGC 4631 group to report the first robust detection of kpc-scale filamentary structures in the CGM/IGM, with widths of $0.5$-$3.3\ \mathrm{kpc}$ and lengths of $6.1$-$49.8\ \mathrm{kpc}$. These features confirm that low-density CGM/IGM gas hosts velocity coherent substructure. From position-velocity kinematic analysis, we identify three filament classes (U-shaped, linear, and wavy), implying diverse formation mechanisms. Our results establish a structural bridge connecting pc-scale interstellar filaments, kpc-scale CGM/IGM filaments, and Mpc-scale cosmic-web filaments, providing key observational support for multiscale gaseous coupling in cosmic ecosystem.

astro-ph.GA↗

arXiv2026

Controlling instrumental systematics for the BICEP inflation survey

The BICEP series of experiments has been observing CMB polarisation from the South Pole for over 20 years, with the goal of constraining inflationary gravitational waves. The upcoming data release, using data taken through 2024, is forecasted to constrain the tensor-to-scalar ratio $r$ at the level of $\sigma(r) \sim 0.005$ (including delensing), with the longer-term goal of reaching $\sigma(r) \sim 0.001$ by 2034. As the survey sensitivity increases, it is crucial to control instrumental systematics to unprecedented levels, and our goal is to limit dominant sources of systematics to 20% of $\sigma(r)$ or lower. Achieving this requires careful instrumental characterisation and dedicated end-to-end studies to evaluate the impact of each systematic on cosmological parameters. We first present the BICEP calibration program, which characterises the optical, spectral, and polarisation response of the receivers. We then describe the analysis strategies implemented to identify and mitigate systematic contamination, and we detail simulations used to evaluate the impact of residual effects. We focus in particular on beam systematics, the dominant source of systematics for BICEP receivers. Finally, we report preliminary estimates of the expected level of systematic contamination for upcoming BICEP results, and we discuss approaches to evaluate and mitigate instrumental systematics for future surveys.

astro-ph.CO↗

arXiv2026

Emergent superconductivity at 16.3 K in an altermagnetic candidate Na$_{2-x}$V$_2$Se$_2$O with broken inversion symmetry

Altermagnets (AMs), characterized by zero net magnetization and momentum-dependent spin splitting, are anticipated to hold significant potential for generating multiple exotic and uncommon superconducting states. However, superconductivity has not yet been realized in AMs to date. Recently, two-dimensional (2D) V$_2$Ch$_2$O (Ch = Se, Te) monolayers, as well as AV$_2$Ch$_2$O (A = K, Rb, Cs) crystals containing [V$_2$Ch$_2$O]$^{\delta -}$ building layers, have been predicted and/or demonstrated to be promising altermagnetic materials. Our preliminary attempts to explore superconductivity in these materials by applying pressure or chemical doping were unsuccessful. Here we report the discovery of superconductivity at a relatively high transition temperature of ~ 16.3 K in a newly synthesized layered compound, Na$_{2-x}$V$_2$Se$_2$O, a variant of AV$_2$Ch$_2$O. In this structure, the [V$_2$Ch$_2$O]$^{\delta -}$ layers are interspersed with double layers of Na$^+$ instead of a single layer of A$^+$, with sodium sites being only half-filled. This new family of layered vanadium oxychalcogenides, lacking inversion symmetry, represents an intriguing platform for exploring altermagnetic superconductors, and holds the potential to reveal novel phenomena, such as topological states, van Hove singularities, and finite-momentum superconductivity. Furthermore, this material acts as a "bridge" between the cuprate/nickelate and iron-pnictide high temperature superconductors, providing new hope and opportunity to expand the category of layered superconductors with higher critical temperatures (T$_c$) and enhancing our understanding of the underlying mechanisms in these systems.

cond-mat.supr-con↗

arXiv2026

Quantum simulation of thermalization dynamics of a nonuniform Dicke model

Previous experimental realizations of Dicke model in atomic or ionic systems are based on global observables assuming uniform spin-boson coupling, while inevitable experimental nonuniformity on the one hand requires site-resolved measurement of spin states, and on the other hand provides potential quantum advantage on the simulation of multi-spin distributions. Here we report the quantum simulation of a nonuniform Dicke-like model in a two-dimensional (2D) crystal of up to 200 ions. We explicitly demonstrate the sensitivity of few-spin observables and multi-spin distributions to the spatial inhomogeneity of the model, and examine the thermalization dynamics of the nonuniform model by measuring the subsystem entropies of selected ion groups. Our work enables the study of Dicke-like models beyond the symmetric subspace, paving the way toward understanding the role of disorder in its thermalization and quantum chaos behavior.

quant-ph↗