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Alok Kumar

Publications and source records attributed to Alok Kumar.

At least 19 recordsLinked to original sources
arXiv2026

Deterministic Universal Logical Gates for Finite-Energy GKP Qubits in Trapped Neutral Atoms

We present a universal logical gate set for finite-energy Gottesman-Kitaev-Preskill (GKP) qubits encoded in the harmonic motional states of trapped neutral atoms. Internal electronic states serve as ancilla for implementing state-dependent conditional displacements in phase space, enabling arbitrary single-qubit phase gates. A Transient Rydberg excitation-mediated atomic dipole-dipole interaction enables a controlled-Z gate without invoking the blockade mechanism. The gates operate under magic-trapping conditions, allowing continuous trapping throughout the gate sequence, thereby preserving the motional encoding without intermediate measurement or feedforward. We assess the experimental feasibility of the proposed protocols using neutral $^{88}\mathrm{Sr}$ atoms and obtain average gate fidelities of $0.986$ for single-qubit phase gates and $0.985$ for the controlled-Z gate at a finite squeezing parameter of $\Delta=0.25$ (12 dB of squeezing), increasing to $0.997$ and $0.995$ respectively as $\Delta$ reaches $0.1$. These results provide a route toward universal, measurement-free logical control of motional GKP qubits in neutral-atom architectures.

quant-ph↗
arXiv2026

Nemotron 3 Ultra: Open, Efficient Mixture-of-Experts Hybrid Mamba-Transformer Model for Agentic Reasoning

We introduce Nemotron 3 Ultra, a 550 billion total and 55 billion active parameter Mixture-of-Experts Hybrid Mamba-Attention language model. We pre-trained Nemotron 3 Ultra on 20 trillion text tokens, then extended the context length to 1M tokens, and post-trained using Supervised Fine Tuning (SFT), Reinforcement Learning (RL), and Multi-teacher On-Policy Distillation (MOPD). Nemotron 3 Ultra is our most capable model yet, employing multiple key technologies - LatentMoE, Multi Token Prediction (MTP), NVFP4 pre-training, multi-environment RLVR, MOPD, and reasoning budget control. Nemotron 3 Ultra achieves up to ~6x higher inference throughput as compared to state-of-the-art publicly available LLMs while attaining on-par accuracy. The state-of-the-art accuracy, high inference throughput, and 1M token context length make Nemotron 3 Ultra ideal for long-running autonomous agentic tasks. We open-source the base, post-trained, and quantized checkpoints, along with the training data and recipe on HuggingFace.

cs.CL↗
arXiv2026

Nemotron 3 Super: Open, Efficient Mixture-of-Experts Hybrid Mamba-Transformer Model for Agentic Reasoning

We describe the pre-training, post-training, and quantization of Nemotron 3 Super, a 120 billion (active 12 billion) parameter hybrid Mamba-Attention Mixture-of-Experts model. Nemotron 3 Super is the first model in the Nemotron 3 family to 1) be pre-trained in NVFP4, 2) leverage LatentMoE, a new Mixture-of-Experts architecture that optimizes for both accuracy per FLOP and accuracy per parameter, and 3) include MTP layers for inference acceleration through native speculative decoding. We pre-trained Nemotron 3 Super on 25 trillion tokens followed by post-training using supervised fine tuning (SFT) and reinforcement learning (RL). The final model supports up to 1M context length and achieves comparable accuracy on common benchmarks, while also achieving up to 2.2x and 7.5x higher inference throughput compared to GPT-OSS-120B and Qwen3.5-122B, respectively. Nemotron 3 Super datasets, along with the base, post-trained, and quantized checkpoints, are open-sourced on HuggingFace.

cs.LG↗
arXiv2026

Interfacial Charge Transfer Driven Enhanced Transport and Thermal Stability in Graphene-MoS2 Vertical Heterostructure Field-Effect Transistors

In this work, we demonstrate interfacial charge transfer-driven transport enhancement in few-layer graphene monolayer MoS2 vertical heterostructure field-effect transistor. Raman scattering and Raman intensity mapping results confirm the successful stacking of FL graphene on ML MoS2. Pronounced photoluminescence (PL) quenching of MoS2 and spectral redshift in the heterostructure suggest efficient interlayer charge transfer and strong electronic coupling at the vdW interface. Electrical measurements show enhanced drain current, field-effect mobility, and conductivity in Gr-MoS2 device compared to pristine MoS2 transistor with Ag contacts. The energy band considerations under equilibrium and gate bias conditions suggest improved Fermi-level alignment and reduced effective Schottky barrier effects at the graphene-MoS2 interface, enabling efficient carrier injection. Temperature-dependent transport (300-400 K) reveals phonon-dominated mobility and conductivity degradation in both devices; however, the heterostructure exhibits significantly suppressed performance degradation. The mobility enhancement factor increases from ~1.6 at 300 K to ~4.0 at 400 K, accompanied by a corresponding improvement in conductivity stability, demonstrating superior thermal robustness for the Gr-MoS2 heterostructure. The power-law analysis indicates that transport in pristine MoS2 is influenced by both intrinsic phonon scattering and additional thermally activated extrinsic processes such as contact and interfacial effects, whereas the weaker temperature dependence in the Gr-MoS2 device reflects moderated extrinsic contributions and transport behaviour approaching a predominantly phonon-limited regime. These findings demonstrate graphene contact engineering as a viable pathway toward improved performance and thermally stable two-dimensional semiconductor electronics.

cond-mat.mes-hall↗
arXiv2026

Quantum Resistance in Multilayer Graphene-BiFeO3 Memristor for Brain-Inspired Computing

In the era of big data and the Internet of Things, quantum-level control of conductance states offers a promising route toward high-density data storage and brain-inspired neuromorphic computing. Although quantum conductance (QC) phenomena have been demonstrated in various metal oxide memristors, achieving reliable and precise control over quantized states remains in its infancy. Here, we demonstrate bidirectional quantum conductance states in multifunctional BiFeO3 (BFO) perovskite memristors integrated with multilayer-graphene contacts, enabling higher-order tunability and revealing the potential of perovskite-2D heterostructures for quantum-engineered memory and computing devices. XPS analysis provides detailed insights into oxygen vacancy dynamics in BFO, whereas first-principles density functional theory calculations clearly reveal a strong localized electric field at the graphene-BFO interface. Our devices exhibit current-controlled higher-order QC transitions facilitated by quantum point contact formation, giving rise to quantized conductance states during both SET and RESET processes. Time-lag correlation maps quantify the stochastic evolution of QC states under dynamic voltage-pulse tuning schemes. Notably, the quantized conductance states effectively emulate synaptic potentiation and depression, enabling precise weight modulation for high-accuracy image and digit recognition in convolutional neural networks. These findings establish perovskite-2D heterostructures as promising candidates for QC-driven resistive switching and demonstrate their potential for developing controllable quantum memristors.

cond-mat.mes-hall↗
arXiv2026

Charge Transfer with a Spin. I: A Generalized CASSCF Framework for Investigating Charge Transfer in the Presence of Spin-Orbit Coupling

We present a generalized extension of the recently developed electron/hole-transfer Dynamically-weighted State-Averaged Constrained CASSCF (eDSC/hDSC) method to model charge transfer in the presence of spin-orbit coupling (SOC) for systems containing an odd number of electrons. Our approach incorporates complex-valued spinor orbitals and incorporates four electronic configurations in describing ground-excited state curve crossings between Kramers-restricted doublet states. The method achieves smooth potential energy surfaces and rapid self-consistent field (SCF) convergence across a wide range of spin-orbit coupling strengths, providing an efficient framework for investigating charge transfer processes in the presence of nontrivial spin degrees of freedom.

physics.chem-ph↗
arXiv2026

Charge Transfer with a Spin. II: A Framework for Diabatization which Localizes Charge and Spin

We investigate a diabatization procedure that localizes charges (in real space) and localizes spins (in spin space) for open-shell systems that exhibit charge transfer in the presence of spin-orbit coupling. The procedure is applied to a two-state crossing between pairs of Kramers-restricted doublet states (which can also be considered effectively a four-state crossing). To generate the relevant electronic states, we employ the recently developed electron/hole-transfer Dynamically-weighted State-Averaged Constrained CASSCF (eDSC/hDSC) method that treats systems with an odd number of electrons. To generate the relevant diabatic states, we employ a two-step optimization over complex-unitary rotations that sequentially maximizes dipole and spin moments through iterative Jacobi sweeps; the resulting update rules are effectively equivalent to those of approximate joint diagonalization (AJD) applied to charge and spin. The method converges rapidly and yields smooth diabatic potential energy surfaces that preserve dipole and spin properties (e.g., a slowly varying pseudospin texture) along the reaction coordinate while maintaining time-reversal symmetry.

physics.chem-ph↗
arXiv2025

Coalitional Game Framework for Multicast in Wireless Networks

We consider a wireless network in which there is a transmitter and a set of users, all of whom want to download a popular file from the transmitter. Using the framework of cooperative game theory, we investigate conditions under which users have incentives to cooperate among themselves to form coalitions for the purpose of receiving the file via multicast from the transmitter. First, using the solution concept of core, we investigate conditions under which it is beneficial for all users to cooperate, i.e., the grand coalition is stable. We provide several sets of sufficient conditions under which the core is non-empty as well as those under which the core is empty. Next, we use the concept of $\mathbb{D}_c$-stability to identify a set of sufficient conditions under which the users in the network form a certain fixed number of coalitions such that all the users within each coalition cooperate among themselves. Our analytical results show how the values of different system parameters, e.g., data rates of different users, transmit and receive power, file size, bandwidth cost, etc., influence stability properties of coalitions, and provide a systematic approach to evaluating cooperation of users for multicast. We also study cooperation among different users using numerical computations. The problem of coalition formation in the context of multicast addressed in this paper is fundamental, and our analysis provides new insights into the feasibility of stable cooperative multicast strategies, contributing to a deeper understanding of cooperation in wireless networks.

cs.NI↗
arXiv2025

Heavy Particle Motion in Rotational Vortices

This study examines the motion of spherical inertial particles in a three-dimensional rotating cylindrical vortex - a simplified model of geophysical flow structures such as oceanic eddies. The analytical vortex formulation enables the isolation of the key mechanisms that govern particle transport in rotating flows. Using Lagrangian particle tracking simulations, we investigate the influence of drag, buoyancy, virtual mass, Coriolis, and Magnus lift forces across a range of particle sizes, densities, and vortex rotation rates. Results show that particle aggregation and periodic stability depend on both particle inertia and flow parameters. Rotational lift forces, though often neglected for spherical particles, become dominant at moderate particle Stokes numbers and introduce slow vertical oscillations in both particle position and spin. The balance of forces determines whether particles settle into stable periodic orbits or escape the vortex. Our analysis reveals unique equilibrium positions that exhibit bifurcations, with multiple or vanishing steady states depending on particle and flow characteristics. Our results demonstrate how particle aggregation and orbital stability stem from the complex coupling between rotational lift, Coriolis, drag, buoyancy, and virtual mass forces. This model may promote informed modeling of dispersed phase transport in marine flows and industrial mixing processes.

physics.flu-dyn↗
arXiv2020

Analog quantum simulation of chemical dynamics

Ultrafast chemical reactions are difficult to simulate because they involve entangled, many-body wavefunctions whose computational complexity grows rapidly with molecular size. In photochemistry, the breakdown of the Born-Oppenheimer approximation further complicates the problem by entangling nuclear and electronic degrees of freedom. Here, we show that analog quantum simulators can efficiently simulate molecular dynamics using commonly available bosonic modes to represent molecular vibrations. Our approach can be implemented in any device with a qudit controllably coupled to bosonic oscillators and with quantum hardware resources that scale linearly with molecular size, and offers significant resource savings compared to digital quantum simulation algorithms. Advantages of our approach include a time resolution orders of magnitude better than ultrafast spectroscopy, the ability to simulate large molecules with limited hardware using a Suzuki-Trotter expansion, and the ability to implement realistic system-bath interactions with only one additional interaction per mode. Our approach can be implemented with current technology; e.g., the conical intersection in pyrazine can be simulated using a single trapped ion. Therefore, we expect our method will enable classically intractable chemical dynamics simulations in the near term.

quant-ph↗
arXiv2012

Effective Action and Mean Fermion Number Density of Graphene in Constant Magnetic Field at Finite Temperature and Density

The effective action and the mean fermion number density of graphene in constant external magnetic field at finite temperature and density are calculated. Closed expressions for these are given and their variation with temperature are studied. It is found that the mean fermion number density peaks around a particular temperature, depending on the chemical potential at low temperatures. This feature is interpreted as 'condensation of fermions ${\bar{\psi}}{\psi}$' in graphene. In future, it is interesting to extend and explore this calculation and the work of the reference [20] for the case of Graphyne [23].

hep-th↗
arXiv2010

Matsubara Frequency Sums

We cannot use directly the results of zero-temperature at finite temperature, for at finite temperature the average is to be carried over all highly degenerate excited states unlike zero-temperature average is only on unique ground state. One of the formal way to take into account the finite temperature into quantum field theory is due to Matsubara, to replace temporal component of eigenvalues $k_{4}$ by $\omega_{n}=\frac{2\pi n}{\beta}$ $(\frac{2\pi (n+{1/2})}{\beta})$ with summation over all integer values of $n$. The summation is done with the infinite series expansion of $\coth (\pi y)$. With the chemical potential $\mu$, $\omega_{n}$ will be replaced by $\omega_{n} - \mu$ in the eigenvalues and the summation over $n$ cannot be done easily. Various methods exist to evaluate it. We use the infinite series expansion of $\coth (\pi y)$ to work operationally for such Matsubara frequency sums.

physics.gen-ph↗
arXiv2010

Covariant and Contravariant Vectors

Vector is a physical quantity and it does not depend on any co-ordinate system. It need to be expanded in some basis for practical calculation and its components do depend on the chosen basis. The expansion in orthonormal basis is mathematically simple. But in many physical situations we have to choose an non-orthogonal basis (or oblique co-ordinate system). But the expansion of a vector in non-orthogonal basis is not convenient to work with. With the notion of contravariant and covariant components of a vector, we make non-orthogonal basis to behave like orthonormal basis. The same notion appears in quantum mechanics as Ket and Bra vectors and we compare the two equivalent situation via the completeness relation. This notion appears in the differential geometry of a metric manifold for tangent vectors at a point, where it takes into account the non-orthogonality of basis as well as non-Euclidean geometry.

math-ph↗
arXiv2009

Fermion Wavefunctions in Magnetized branes: Theta identities and Yukawa couplings

Computation of Yukawa couplings, determining superpotentials as well as the K\"{a}hler metric, with oblique (non-commuting) fluxes in magnetized brane constructions is an interesting unresolved issue, in view of the importance of such fluxes for obtaining phenomenologically viable models. In order to perform this task, fermion (scalar) wavefunctions on toroidally compactified spaces are presented for general fluxes, parameterized by Hermitian matrices with eigenvalues of arbitrary signatures. We also give explicit mappings among fermion wavefunctions, of different internal chiralities on the tori, which interchange the role of the flux components with the complex structure of the torus. By evaluating the overlap integral of the wavefunctions, we give the expressions for Yukawa couplings among chiral multiplets arising from an arbitrary set of branes (or their orientifold images). The method is based on constructing certain mathematical identities for general Riemann theta functions with matrix valued modular parameter. We briefly discuss an application of the result, for the mass generation of non-chiral fermions, in the SU(5) GUT model presented by us in arXiv:0709.2799.

hep-th↗
arXiv2008

$w_{\infty}$ 3-algebra

We use `lone-star' product of the $W_{\infty}$ generators as well as their commutation relations to obtain a $w_{\infty}$ 3-algebra by applying appropriate double scaling limits on the generators. We show explicitly that "Fundamental Identity Condition" (FI) of 3-algebra is satisfied.

hep-th↗
arXiv2007

Chiral Symmetry Breaking in Gribov's Approach to QCD at Low Momentum

We consider Gribov's equation for inverse quark Green function with and without pion correction. With polar parametrization of inverse quark Green function, we relate the dynamical mass function without pion correction, $M_{0}(q^2)$ and with pion correction, $M(q^2)$ at low momentum. A graph is plotted for $M(q^2)$ and $M_{0}(q^2)$ with q for low momentum. It is found that at low momenta pion corrections are small.

hep-th↗