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Vivek Kumar

Publications and source records attributed to Vivek Kumar.

At least 19 recordsLinked to original sources
arXiv2026

X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage

We present an X-ray study of the high-synchrotron-peaked blazar Mrk 421 using ten days of XPoSat/XSPECT observations (2025 January 20-29; 202 ks) and contemporaneous NICER data. The 0.8-15 keV light curve has a fractional variability amplitude of 19.6 +/- 0.3 per cent, and Bayesian blocks resolve 26 flux states spanning a factor of 2.3. The best-constrained flare has a fast rise and a slow decay, with e-folding times of about 3 h and 30 h. For typical jet parameters, the X-ray-emitting electrons cool faster than this, suggesting that the decay traces a gradual weakening of their acceleration. The count-rate distribution is best described by two lognormal components, consistent with multiplicative variability. The power spectrum is a red-noise power law with a slope of about 1.7 over timescales of 25 min to 10 d, typical of blazars. XSPECT and NICER count rates are well correlated over eight matched epochs (r = 0.941). We fitted log-parabola models to day-resolved spectra from XSPECT, NICER and both jointly, and to flux-resolved XSPECT spectra. Both show spectral hardening with brightness. In the flux-resolved spectra, the photon index at 1 keV decreases from 2.24 in the faintest state to 1.96 in the brightest, and the synchrotron peak moves from about 0.66 to 1.06 keV, while the curvature ($β\approx 0.7$) shows no significant change. Power-law fits to Bayesian blocks show that bright blocks at the same rate can differ in photon index by up to $6.4σ$, so count rate alone does not fix the spectral state.

astro-ph.HE↗
arXiv2026

Understanding Work/Life Balance Through Mathematical Model

Work/life balance (WLB) remains a central yet elusive concept in social and organizational sciences, often described qualitatively without a unified quantitative framework. To address this gap, a novel compartmental mathematical model is proposed that formalizes WLB as a dynamic population-level phenomenon. The individuals are categorized into 4 compartments, namely, regularly employed, temporarily employed, imbalanced (infected), and balanced (recovered), representing balanced and imbalanced states. Role-based complaints govern the transitions between states, the conflict conversion rates of these complaints, and the recovery rate. The model, structured as a system of nonlinear ordinary differential equations, is analyzed for the existence of the equilibrium and its stability. In the analysis, the stable disease-free equilibrium is found to exist, representing complete societal balance. The other endemic equilibrium is shown to be infeasible through simulations and thus requires certain parameter constraints, which represent a specific scenario that may occur in the population. However, it is found that the endemic equilibrium always remains unstable. The model's ability to quantify how micro-level stressors spread into macro-level imbalance is demonstrated by numerical simulations, which also validate the analytical results. This work establishes a rigorous mathematical foundation for WLB, enabling future empirical and mathematical calibrations.

math.DS↗
arXiv2026

Monocular 3D bubble reconstruction: DNS-informed ray tracing and deep learning

Bubble size, depth, and spatial distribution govern transport and uniformity in multiphase flows. A single shadowgraphic view cannot separate bubble diameter from focal-plane displacement, as image signatures depend on both, while multicamera systems add optical access, calibration, and cost. We develop a single-camera physics-guided framework combining direct numerical simulation (DNS), validated Monte Carlo ray tracing, preprocessing, and focus-conditioned multitask deep learning to recover three-dimensional bubble position and physical-equivalent diameter from two-dimensional shadowgraphs. DNS-resolved bubble label images rendered at multiple focal-plane settings. The ray tracer reproduces shadowgraphic signatures with errors below $3\%$ (intensity), $1\%$ (apparent size), $15\%$ (blur), $6\%$ (numerical aperture), and $4\%$ (edge-spread-function shape), running $\sim30\times$ faster than the CPU implementation. The dataset has $900$ frames and $87{,}728$ labeled instances, partitioned by DNS timestamp to prevent leakage and expanded to over $10^{5}$ crops by broad-depth controlled-density rendering. A multitask network predicts signed depth and diameter from crops and focal-plane coordinates, achieving $\sim97.6\%$ normalized depth accuracy on held-out data; a closed-loop DNS-to-image-to-reconstruction test of $180$ bubbles gives diameter error below $9.71\%$ ($\sim90.29\%$ accuracy). On experimental images at four radial planes ($142$--$971$ bubbles each), PDF overlap is $68\%$--$92\%$ and log-PDF correlation is $0.72$--$0.87$. At the focal reference plane ($11{,}679$ bubbles), overlap is $\sim86\%$, correlation is $0.93$, and full-distribution, $D_{99.8}$, and composite errors are below $7.7\%$, increasing with focal-plane distance and defining a recoverability limit. Without stereoscopic hardware, the framework offers a scalable diagnostic for dense bubbly flows.

physics.flu-dyn↗
arXiv2026

CHRONOS Science Program

Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS) is a proposed cryogenic torsion-bar interferometer for precision measurements in the 0.1-10 Hz band. A Sagnac speed-meter readout, power and signal recycling, and balanced homodyne detection provide control of the optical response and quantum measurement correlations. This White Paper describes the science program, detector design, calibration, and staged experimental development. The reference sensitivity uses a multi-input quantum model, including rear-mirror vacuum fields and measurement-back-action correlations, combined with classical noise. Two operating points are optimized for complementary power-law-integrated sensitivity objectives. Gravitational-wave forecasts use this common noise budget and state their source and observing assumptions. The program includes intermediate-mass black-hole binaries, stochastic backgrounds, quantum measurement, and terrestrial gravity perturbations. Dark-matter applications are discussed as response and feasibility studies. A differential gravitational calibrator provides a proposed Yukawa-gravity test whose systematic floor is improved by subtracting auxiliary harmonics. These are conditional projections: experimental reach requires validation of the noise, transfer functions, environmental backgrounds, and systematic uncertainties described here.

astro-ph.IM↗
arXiv2026

Quantum noise and torque correlations in a torsion-bar Sagnac speed meter

Quantum fluctuations entering through weakly transmitting mirrors can limit a torsion-bar Sagnac speed meter for low-frequency gravitational-wave detection, even when its optical layout suppresses radiation-pressure back action. We show that, in the Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS), vacuum fields entering from behind the end and middle test masses can dominate the low-frequency quantum noise despite cancellation of the torques on opposite ends of each bar. For a reference configuration with mirror power transmissions of only $10^{-7}$, these fields contribute $89.7\%$ of the total quantum-noise power spectral density at 0.1 Hz. Omitting them would underestimate the amplitude spectral density by a factor of 3.11. Our input-output model connects the correlations produced by successive optical interactions to the forces on each bar and the measured signal. This connection shows why strong torque cancellation does not, by itself, ensure a corresponding improvement in detector sensitivity. Replacing mirror transmission with absorption at fixed reflectivity leaves the excess quantum noise essentially unchanged for the two operating points studied. The predicted signal response and quantum noise agree with independent FINESSE 3 calculations as recycling detuning and homodyne angle are varied. These results establish rear vacuum fields as a limiting noise source in this torsion-bar speed meter and provide quantitative requirements for its mirror and readout design.

gr-qc↗
arXiv2026

Cryogenic Voltage Control of Magnetism in Silicon-Integrated \newline SrTiO$_3$/Fe Heterostructures

Cryogenic electronics forms a rapidly emerging research domain for high-performance and power-efficient computing applications. Incorporating nanomagnetic components in cryogenic circuitry adds highly valuable functionality, facilitating downscaling, reducing energy consumption and introducing time-reversal symmetry breaking. Furthermore, low-temperature environments enhance magnetic stability and switching efficiency at nanoscale dimensions, reinforcing the potential of cryogenic nanomagnets. To fully leverage these opportunities, magnetic control schemes require alternative options to current-based writing, which is the main bottleneck regarding power consumption and downscaling. In this regard, voltage-based gating of the magnetic state could drastically enhance operational efficiency and integration density. In this work, we investigate cryogenic Voltage Control of Magnetism (VCM) in epitaxial SrTiO$_3$/Fe thin film heterostructures on a CMOS compatible Si substrate. We demonstrate and quantify voltage-controlled modifications of the magnetic domain structure, consistent with electric field-controlled magnetic anisotropy at the Fe/SrTiO$_3$ interface. These findings provide a viable material system for the development of next-generation magnetic domain-based devices for classical and quantum computing.

cond-mat.mes-hall↗
arXiv2026

Cascade-driven fragmentation-aggregation transitions in decaying turbulence

In decaying turbulence, the turbulent cascade is not merely a passive background for breakup but an evolving stability boundary that dynamically selects the fragmentation-aggregation pathway. As turbulence decays, the Hinze scale sweeps through the bubble-size distribution, progressively eliminating the breakable population and driving a transition from a mixed breakup-coalescence regime to a self-sustaining pure-coalescence state. Our model predicts a critical turbulence decay exponent $m=5/3$, a transient mixed breakup-coalescence regime, and a late-stage pure-coalescence state with a universal coalescence hazard. Interestingly, the transient breakup acts as a catalyst that amplifies mixed-regime bubble growth relative to pure coalescence. Results from our direct numerical simulations of decaying homogeneous isotropic turbulence as well as high-Reynolds-number duct-flow experiments collapse onto the model predictions when expressed in terms of the measured decay rate. These results replace the conventional instantaneous Hinze criterion with a non-equilibrium, cascade-controlled framework for turbulent fragmentation and aggregation.

physics.flu-dyn↗
arXiv2026

PhaseCoder: Microphone Geometry-Agnostic Spatial Audio Understanding for Multimodal LLMs

Current multimodal LLMs process audio as a mono stream, ignoring the rich spatial information essential for embodied AI. Existing spatial audio models, conversely, are constrained to fixed microphone geometries, preventing deployment across diverse devices. We present PhaseCoder, a transformer-only spatial audio encoder that is agnostic to microphone geometry. PhaseCoder takes raw multichannel audio and microphone coordinates as inputs to perform localization and produces robust spatial embeddings. We demonstrate that Gemma 3n LLM can be fine-tuned to reason over "Spatial Audio Tokens" produced by PhaseCoder. We show our encoder achieves state-of-the-art results on microphone-invariant localization benchmarks and, for the first time, enables an LLM to perform complex spatial reasoning and targeted transcription tasks from an arbitrary microphone array.

cs.SD↗
arXiv2026

On the scaling of bubble interactions in dynamic turbulence: theoretical, numerical, and experimental study

This study investigates dilute bubbly decaying homogeneous isotropic turbulence at high Reynolds number using theory, direct numerical simulation, and experiments. The turbulent kinetic energy and dissipation rate follow power-law decay, while the bubble population reorganizes relative to the evolving Hinze scale. When the dissipation decays sufficiently rapidly, the Hinze scale grows faster than the characteristic bubble diameter, driving the population from super-Hinze toward sub-Hinze sizes. The system passes through a mixed regime in which coalescence dominates but breakup remains active, followed by a pure-coalescence regime. Residual breakup in the mixed regime increases the number of small bubbles and enhances coalescence, leading to faster growth of the characteristic bubble size. DNS of dilute bubble-laden turbulence shows decay exponents close to single-phase turbulence and a bubble-size distribution that shifts toward smaller diameter relative to the Hinze scale. Before the transition, the distribution exhibits two power-law ranges associated with capillary effects and inertial breakup; after the transition, it approaches a single capillary-dominated scaling. Theory and DNS predict distinct growth laws for bubble diameter in the mixed and pure-coalescence regimes, together with corresponding scalings for number density, interfacial area, and coalescence rate. These predictions are further assessed in a spatially developing pump-driven bubbly duct flow at higher Reynolds number. Despite confinement, inhomogeneity, and wall production, the measured trends agree with the theoretical and DNS-based scalings. The results identify Hinze-scale drift as the organizing mechanism for bubble interactions in both idealized and practical decaying turbulent flows, and provide guidance for population-balance and interfacial-area-transport models.

physics.flu-dyn↗
arXiv2026

Proximity-induced Rashba spin-orbit interaction in BaMnO$_\text{3}|$KTaO$_\text{3}$ heterostructure for antiferromagnetic spintronics

Antiferromagnetic spintronics, a promising technology for ultra-fast electronic devices, faces several challenges, including the lack of materials simultaneously hosting robust antiferromagnetism and adequate Rashba-like interaction. We design a heterostructure of BaMnO$_3|$KTaO$_3$ with the idea of proximity-inducing strong Rashba spin-orbit interaction from KTaO$_3$ part to BaMnO$_3$ part, where the latter is already a robust antiferromagnet. Within our DFT calculations, the heterostructure reveals BaMnO$_3$ bands near the Fermi level with a significant magnetic moment per Mn atom and a decent ordering temperature. Further, the BaMnO$_3$ bands in the heterostructure exhibit linear Rashba interaction with a sizable Rashba coefficient, owing to its proximity to KTaO$_3$. Our work can motivate future research by demonstrating the road map to proximity-induced Rashba interaction for antiferromagnetic spintronics.

cond-mat.mtrl-sci↗
arXiv2026

Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS) for gravitational wave detection

We present the optical design and sensitivity modeling of the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS), a triangular Sagnac speed-meter interferometer incorporating power and signal recycling. Using ABCD-matrix analysis and \textsc{Finesse3} simulations, we obtain stable optical eigenmodes with mode-matching efficiencies above 99.5%. The optimized configuration achieves a quantum-noise-limited strain sensitivity of $h \simeq 3\times10^{-18},\mathrm{Hz^{-1/2}}$ at 1 Hz, with a ring-cavity finesse of $\mathcal{F}\simeq3.1\times10^{4}$ and a round-trip Gouy phase of $ψ\approx153^{\circ}$. The low-frequency quantum noise is primarily governed by the power-recycling cavity detuning, while the signal-recycling cavity produces an approximately uniform quadrature rotation. An optimal homodyne angle of $ζ\simeq46^{\circ}$ provides the best sensitivity near 1 Hz. Assuming an end-mirror reflectivity of $R_{\mathrm{ETM}}=99.9999%$ at 10 K, CHRONOS can achieve quantum-noise-limited performance on a laboratory scale. Its projected science reach includes intermediate-mass black-hole binaries out to approximately $271,\mathrm{Mpc}$, a 10-year stochastic-background sensitivity of $Ω_{\mathrm{GW}}\simeq4.7\times10^{-4}$ at $2.15,\mathrm{Hz}$, constraints on Yukawa-type deviations from Newtonian gravity, and prompt gravity-gradient signals from nearby earthquakes.

physics.ins-det↗
arXiv2026

Improving calibration accuracy with torque coupled gravity field calibrator for sub-Hz gravitational wave observation in CHRONOS

A fundamental challenge in low-frequency gravitational-wave detectors is the limited signal-to-noise ratio (SNR) of calibration lines, particularly in torsion-bar systems where the response is governed by rotational dynamics. In this work, we resolve this issue by optimizing the geometrical configuration of a torque-coupled gravity field calibrator (GCal), achieving an improvement in calibration-line SNR by more than an order of magnitude compared to conventional layouts. For the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed-meter (CHRONOS), the calibration signal appears as a monochromatic line within the $0.1$--$10~\mathrm{Hz}$ band. At $1~\mathrm{Hz}$, the strain-equivalent calibration amplitude reaches $|h_{\rm GCal}| = 1.16 \times 10^{-14}$, corresponding to an SNR density of $|h_{\rm GCal}|/S_h = 4.16 \times 10^{3}$. This demonstrates for the first time that a high-SNR calibration line can be directly injected into the sub-Hz band of a torsion-bar detector. A first-order perturbative error propagation analysis yields a total fractional systematic uncertainty of $δh_{\rm GCal}/h_{\rm GCal} = 0.24\%$, dominated by geometric alignment uncertainties, while contributions from mass uncertainties and the gravitational constant remain subdominant. The corresponding absolute systematic uncertainty is $δh_{\rm GCal} \sim 10^{-17}$ at $1~\mathrm{Hz}$. These results establish torque-coupled gravitational calibration as a practical solution to the longstanding low-SNR problem in sub-Hz torsion-bar detectors and provide a robust pathway toward precision absolute calibration in the low-frequency regime.

gr-qc↗
arXiv2026

Raman Spectroscopic Investigation of Kitaev Quantum Spin Liquids

Quantum spin liquids, a highly topologically entangled, dynamically correlated state where quantum fluctuations preclude any long-range ordering down to absolute zero. In the search for a topologically robust qubit, the scientific community has been in continuous hunt for real quantum spin liquid systems. Alexei Kitaev in his exactly solvable model for a spin-1/2 two-dimensional honeycomb lattice, presented a system that hosts a topologically protected state (Majorana zero-modes). Under an applied external field, the Kitaev spin liquids turn into a topologically non-trivial chiral spin-liquid state with non-abelian anionic excitations, which is crucial for quantum computing. Earlier theoretical predictions advocated that Kitaev physics can be realized in spin-orbit-coupled Mott insulators such as honeycomb irradiates and ruthenates. However, the experimental findings continuously challenge the theoretical aspects, indicating the presence of non-Kitaev interactions in real materials, where the dimensionality, disorder (vacancy), chemical composition, generalized spin-S, and external perturbations (pressure, magnetic field, temperature) can actively tune the Kitaev interactions and the ground state excitations. In this review article, a comprehensive discussion is included with an updated literature survey in the context of the potential of Raman spectroscopy as a probe for Kitaev quantum spin liquids.

cond-mat.str-el↗
arXiv2026

Noise budget of Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS)

CHRONOS is a proposed gravitational-wave detector designed to operate in the sub-Hz frequency range (0.1 to 10 Hz), a largely unexplored band due to strong noise sources that hamper ground-based detectors. It employs cryogenic operation, a cross torsion-bar configuration, a triangular Sagnac interferometer, and a speed meter readout scheme to overcome key noise limitations, targeting a strain sensitivity of $h \sim 10^{-18} Hz^{-1/2}$ around 2 Hz and a stochastic gravitational wave background of $Ω_{GW}$ approximately $2 \times 10^{-3}$ at 2 Hz. Using analytical and interferometric simulations with FINESSE3, we evaluate the noise budget of CHRONOS and characterize the relative contributions of quantum, thermal, and environmental noise sources. Our results demonstrate that CHRONOS achieves competitive sensitivity at low frequencies. The feasibility of using CHRONOS in an earthquake early-warning system by detecting prompt gravity-gradient signals is also investigated, and is predicted to be faster by approximately 2.92 to 6.90 seconds within 40 km. These findings highlight the scientific potential of CHRONOS, bridging gravitational-wave astronomy and geophysical monitoring, and motivating further development of low-frequency detector technologies.

physics.ins-det↗
arXiv2026

Apple Intelligence Foundation Language Models

We present foundation language models developed to power Apple Intelligence features, including a ~3 billion parameter model designed to run efficiently on devices and a large server-based language model designed for Private Cloud Compute. These models are designed to perform a wide range of tasks efficiently, accurately, and responsibly. This report describes the model architecture, the data used to train the model, the training process, how the models are optimized for inference, and the evaluation results. We highlight our focus on Responsible AI and how the principles are applied throughout the model development.

cs.AI↗
arXiv2026

Quantum interpretations, causality and quantum computation

The interpretation of quantum mechanics continues to be debated, and quantum nonlocality accentuates the puzzle. Quantum interpretations can be classified broadly into two types: realist interpretations, which assert that quantum states describe objective reality (even if hidden or branching), and subjective interpretations, which treat quantum states as observer-dependent information or beliefs about the system. Here we study the implication of quantum interpretations for causal explanations of Bell nonlocal correlations, and show that a given interpretation type carries an inherent commitment to a preferred causal structure. Specifically, we find that realist interpretations entail a classical causal model, and thus require Fine-Tuning to prevent superluminal signaling, while subjective interpretations are found to entail a framework of nonclassical causal models. The implications of our results for one-way quantum computation and computation-based Bell nonlocality are studied.

quant-ph↗
arXiv2026

Variational Probe and Measurement Optimization for Structured Phase Estimation

We present a proof-of-principle study of variational quantum sensing for estimating a structured linear function of local phase parameters, in which each qubit in a spin-1/2 array accumulates a phase phi_i = alpha_i theta with known weights alpha and a global parameter theta. In a hardware-motivated regime of shallow circuits and shallow decoding measurements, we optimize the probe state with respect to the classical Fisher information (CFI) using the Covariance Matrix Adaptation Evolution Strategy. The variational ansatz is built from dipolar-interacting gates and global rotations on a polygon-centered qubit layout. To assess whether the standard Ramsey readout extracts all available information, we introduce a shallow global decoder and optimize it independently with the encoder frozen. For uniform (alpha_i = 1/N) and weighted-central (alpha_c = 1, alpha_i = 0.5) encodings with N = 2-8 qubits and depths L = 1-3, the optimized probes approach the respective entanglement-enhanced precision bounds, which reduce to the Heisenberg limit only for uniform encoding. The decoder provides systematic but modest CFI gains. For uniform encoding, these gains are smallest at the deepest circuits, confirming that fixed Ramsey readout is near-optimal for well-converged probes. For weighted encoding, a persistent component remains, reflecting the broken permutation symmetry of the generator under unequal weights. At large N, the weighted-encoding CFI also exhibits non-monotonic growth with system size, revealing an expressivity limit of the polygon-symmetric ansatz under asymmetric encoding.

quant-ph↗
arXiv2026

Mass-Transfer Control With Microbubbles in Highly Turbulent Decaying Flows

We hypothesize that combining extreme turbulence with a minute reduction in surface tension $σ$ (surface tension of the liquid) using surfactant provides a simple and scalable route for controlling micron scale bubble size in gas--liquid systems. To test this, we generate high-intensity turbulence using a multiphase pump [turbulent intensity $\ge 40\%$; Taylor Reynolds number $Re_λ=\mathcal{O}(10^3)$; bulk Reynolds number $Re=\mathcal{O}(10^5)$] feeding a straight duct, which produces a decaying turbulent flow where, without additives, bubble coalescence dominates and causes monotonic downstream growth in the mean diameter $d_\mathrm{avg}$ of the bubbles. This growth is governed by the turbulent dissipation rate $\varepsilon$. High-speed imaging, back-lit shadowgraph and particle shadow velocimetry (PSV) quantify bubble statistics ($d_\mathrm{avg}$, and the bubble-size distribution) and turbulence metrics (turbulent kinetic energy $k$, turbulence intensity $\mathcal{I}$, and dissipation rate $\varepsilon$). We then introduce a minute amount ($\sim 0.01\%$ critical micelle concentration) of additive that produces a slight reduction in $σ$, used here only as an interfacial tuning knob because the same change in surface tension can be achieved with non surface active agents. This small decrease in $σ$ enhances breakup, slightly suppresses coalescence, and makes smaller bubbles more breakup prone, resulting in reduced $d_\mathrm{avg}$ and a narrower bubble-size distribution. Turbulence statistics remain unchanged within experimental uncertainty, indicating that the effect arises entirely from interface rather than hydrodynamic changes. Overall, combining extreme turbulence with a minute reduction in surface tension offers a low complexity and tunable lever for setting bubble-size distributions and intensifying mass transfer in industrial multiphase flows.

physics.flu-dyn↗