arXiv subjects

Rajeev Singh

Publications and source records attributed to Rajeev Singh.

At least 37 records · Page 2Linked to original sources

arXiv2025

Entanglement and particle production from cosmological perturbations: a quantum optical simulation approach

In this work, we develop a computational framework based on the Gaussian formalism and symplectic circuit representation to explore cosmological perturbations during inflation. These tools offer an efficient means to study entanglement generation and particle production, particularly when analytical methods become insufficient and numerical simulations are essential. By evolving an initial Bunch-Davies vacuum through a two-mode squeezer, we simulate the behavior of the von Neumann entropy and logarithmic negativity across a wide range of cosmological backgrounds, each characterized by a distinct equation of state. The von Neumann entropy obtained via QuGIT simulations is compared with analytic Rényi entropy bounds, thereby validating the accuracy of our circuit implementation of the cosmological squeezing Hamiltonian in both accelerating and decelerating scenarios. We further investigate the role of thermal noise and demonstrate how the von Neumann entropy and logarithmic negativity are affected by its presence.

gr-qc↗

arXiv2025

Hybrid Photon-magnon Systems: Exploring the Purcell Effect

We present a novel approach to observing the Purcell effect in a photon-magnon coupled (PMC) hybrid system consisting of a yttrium iron garnet (YIG) thin film and a hexagonal ring resonator (HRR) arranged in a planar geometry. This hybrid system has been designed and simulated using the commercial electromagnetic full-wave simulator CST Microwave Studio for various values of damping constant (alpha) of the YIG film while keeping the HRR properties constant. Our results reveal that as the magnon damping increases, the anti-crossing behavior between photon and magnon modes in the transmission spectra diminishes, transitioning the coupled modes into the Purcell regime. This transition is attributed to an enhanced spontaneous emission rate of microwave photons when coupled to lossy magnons, driving the PMC system into the Purcell regime. To elucidate this behavior, we developed a comprehensive theoretical framework based on a quantum model, which accurately describes the observed Purcell phenomena and provides estimations of the PMC strength (g/2pi). Notably, by tuning alpha from 1.4 x 10^-5 to 2.8 x 10^-2, we achieved precise control over (g/2pi) ranging from 63 MHz to 127 MHz. This study highlights the Purcell effect's role in enhancing photon decay rates and establishes a clear relationship with PMC strength. Our work offers a comprehensive method for controlling photon resonance dissipation, opening new avenues for exploring the Purcell effect and its applications in on-chip functional devices leveraging magnon-photon interactions for quantum technologies.

quant-ph↗

arXiv2025

Multimode Photon-Photon Coupling

This study investigates a planar hybrid system consisting of three complementary splitring resonators (CSRRs), designed to examine interactions among multiple photon modes at room temperature. The system was modeled and simulated using the full-wave electromagnetic solver CST Microwave Studio. Analysis of the transmission spectra (|S21|) as a function of frequency for different CSRR dimensions revealed distinct anti-crossing behavior, indicative of strong photon-photon coupling (PPC). To explain this phenomenon, we present theoretical framework that quantitatively captures the observed mode hybridization and provides estimates of the coupling strength, which are further validated experimentally. This work not only elucidates the fundamental dynamics of PPC in planar systems but also offers practical guidance for designing hybrid platforms with tunable photon interactions, paving the way for future advancements in planar magnonic and hybrid photonic technologies.

quant-ph↗

arXiv2025

Dynamic control of photon-magnon interactions via secondary magnon excitation

Photon-mediated magnon-magnon coupling between spatially separated Yttrium Iron Garnet (YIG) and permalloy (NiFe) thin films on a planar hexagonal ring resonator shows clear signatures of magnon-magnon interaction are observed without direct dipolar interaction between the magnetic films. The coupling strength between the hexagonal ring resonator and the permalloy film increases with the thickness of the YIG film, despite a fixed permalloy film thickness. This suggests the presence of an indirect interaction channel mediated by resonator photons. A theoretical model is presented that accurately reproduces the observed transmission spectra and reveals a nontrivial interdependence between the individual coupling strengths of YIG and permalloy to the resonator. These results highlight the importance of indirect interactions and potential crosstalk pathways in designing hybrid magnonic systems and scalable quantum architectures, while demonstrating the feasibility of cost-effective, planar configurations for experimental implementation. These insights are valuable for advancing low-loss, coherent information transfer in hybrid quantum devices.

quant-ph↗

arXiv2025

Unlocking Photon Magnon Interplay via Saturation Magnetization

Photon magnon hybrid systems present a promising platform for the development of next generation devices in quantum information processing and quantum sensing technologies. In this study, we investigate the control of photon magnon coupling (PMC) strength through systematic variation of the saturation magnetization in a planar hexagonal ring resonator (HRR) integrated with a yttrium iron garnet (YIG) thin film configuration. Using full wave numerical simulations in CST Microwave Studio, we demonstrate that tuning the Ms of the YIG film from 1750 Oe to 900 Oe enables systematic control over the coupling strength across the 127 to 51 MHz range at room temperature. To explain the observed PMC dynamics, we develop a semiclassical analytical model based on electromagnetic theory that accurately reproduces the observed coupling behavior, revealing the key role of spin density in mediating the light matter interaction. The model is further extended to include the effects of variable magnon damping across different Ms values, enabling broader frequency control. These findings establish Ms as a key tuning parameter for tailoring PMC, with direct implications for the design of tunable hybrid systems for reconfigurable quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Some insights on the partonic collectivity in heavy-ion collisions

Elliptic flow and Number of Constituent Quarks scaling provide crucial insights into the underlying dynamics and degrees of freedom in heavy-ion collisions. This article provides some insights on the transition from hadronic to partonic collectivity, revealing possible key signatures of medium evolution. At low energies around ($\sqrt{s_{\rm NN}} \leq 4.0$ GeV), $v_2$ may be negative and NCQ scaling might break down. We expect that as the collision energy rises beyond 4.0 GeV, $v_2$ may become positive and NCQ scaling gradually restores. The transition in $v_2$ behavior, coupled with the breakdown and restoration of NCQ scaling, highlights the increasing significance of partonic interactions at higher energies, marking the onset of partonic collectivity and the emergence of quark-gluon plasma (QGP)-like properties.

nucl-th↗

arXiv2025

HQNN-FSP: A Hybrid Classical-Quantum Neural Network for Regression-Based Financial Stock Market Prediction

Financial time-series forecasting remains a challenging task due to complex temporal dependencies and market fluctuations. This study explores the potential of hybrid quantum-classical approaches to assist in financial trend prediction by leveraging quantum resources for improved feature representation and learning. A custom Quantum Neural Network (QNN) regressor is introduced, designed with a novel ansatz tailored for financial applications. Two hybrid optimization strategies are proposed: (1) a sequential approach where classical recurrent models (RNN/LSTM) extract temporal dependencies before quantum processing, and (2) a joint learning framework that optimizes classical and quantum parameters simultaneously. Systematic evaluation using TimeSeriesSplit, k-fold cross-validation, and predictive error analysis highlights the ability of these hybrid models to integrate quantum computing into financial forecasting workflows. The findings demonstrate how quantum-assisted learning can contribute to financial modeling, offering insights into the practical role of quantum resources in time-series analysis.

q-fin.ST↗

arXiv2025

Relativistic Viscous Hydrodynamics in the Density Frame: Numerical Tests and Comparisons

We conduct a numerical study of relativistic viscous fluid dynamics in the Density Frame for one-dimensional fluid flows. The Density Frame is a formulation of relativistic viscous hydrodynamics that is first-order in time, requires no auxiliary fields, and has no non-hydrodynamic modes. We compare our results to QCD kinetic theory simulations and find excellent agreement within the regime of applicability of hydrodynamics. Additionally, the Density Frame results remain well-behaved and robust near the boundary of applicability. We also compare our findings to the second-order-in-time hydrodynamic theory developed by Bemfica, Disconzi, Noronha, and Kovtun (BDNK) and a well-known Müller-Israel-Stewart-type hydrodynamics code, MUSIC, which is commonly used to simulate heavy-ion collisions.

nucl-th↗

arXiv2025

Stochastic relativistic viscous hydrodynamics from the Metropolis algorithm

We propose an algorithm for simulating stochastic relativistic fluid dynamics based on Metropolis updates. Each step of the algorithm begins with an update based on ideal hydrodynamics. This is followed by proposing random (spatial) momentum transfers between fluid cells, keeping the total energy fixed. These proposals are then accepted or rejected using the change in entropy as a statistical weight. The algorithm reproduces relativistic viscous hydrodynamics in the ``Density Frame", which is a formulation of viscous hydrodynamics we review and clarify here. This formulation is first order in time and requires no auxiliary dynamical fields such as $Π^{μν}$. The only parameters are the shear and bulk viscosities and the equation of state. By adopting the 3+1 split of general relativity, we extend the Metropolis algorithm to general space-time coordinates, such as Bjorken coordinates, which are commonly used to simulate heavy-ion collisions.

nucl-th↗

arXiv2025

Field redefinition and its impact in relativistic hydrodynamics

In this paper, we explore the impact of field redefinition on the spectrum of linearized perturbations in relativistic hydrodynamics. We observe that the spectrum of hydrodynamics modes is never affected by the local field redefinition, however, the spectrum of the non-hydrodynamic modes is affected. Through an appropriate all-order redefinition, non-hydrodynamic modes can be eliminated, leading to a new frame where the spectrum contains only hydrodynamic modes. We also observe that the resulting stress-energy tensor may have an infinite series in momentum space, with a convergence radius linked to the eliminated non-hydrodynamic mode. In certain special cases, higher-order terms in the stress-energy tensor under field redefinition may cancel, indicating that non-hydrodynamic modes are mere artefacts of the fluid variable choice and hold no physical significance, even if they appear to violate physical constraints. Using a special toy example, we find a criterion to distinguish between physical and unphysical non-hydrodynamic modes.

nucl-th↗

arXiv2025

A note on the canonical approach to hydrodynamics and linear response theory

This note provides a comprehensive examination of the various approaches to formulating relativistic hydrodynamics, with a particular emphasis on the canonical approach. Relativistic hydrodynamics plays a crucial role in understanding the behavior of fluids in high-energy astrophysical phenomena and heavy-ion collisions. The canonical approach is explored in detail, highlighting its foundational principles, mathematical formulations, and practical implications in modeling relativistic fluid dynamics. Following this, we delve into the linear response theory, elucidating its relevance in the context of hydrodynamics. We analyze the response of relativistic fluids to external perturbations, discussing the theoretical framework and key results. This dual focus aims to bridge the gap between theoretical foundations and practical applications, offering a robust perspective on the dynamic interplay between relativistic hydrodynamics and linear response theory.

hep-th↗

arXiv2025

Photon-photon coupling induced bound state in the continuum and transparency

This study presents the coherent and dissipative coupling realized in the hybrid photonic resonators that have been achieved via the constructive and destructive interference of the photonic resonator fields with the radiation of a common transmission line fed with microwave photons. In the dissipative coupling regime we have found the coexistence of a peculiar phenomenon bound state in the continuum (BIC) near the crossing of frequency of the uncoupled resonators by satisfying the Friedrich-Wintgen BICs condition. Again just by rotating one of the samples and with the dynamic adjustment of a parameter we have achieved coupling induced transparency between the photonic resonators. This transition from BIC in the absorption regime to transparency opens avenues for different sorts of plain or programmable oscillators, filters, quantum information processors, sensors etc.

quant-ph↗

arXiv2024

State-dependent mobility edge in kinetically constrained models

In this work, we show that the kinetically constrained quantum East model lies between a quantum scarred and a many-body localized system featuring an unconventional type of mobility edge in the spectrum. We name this scenario $\textit{state-dependent}$ mobility edge: while the system does not exhibit a sharp separation in energy between thermal and non-thermal eigenstates, the abundance of non-thermal eigenstates results in slow entanglement growth for $\textit{many}$ initial states, such as product states, below a finite energy density. We characterize the state-dependent mobility edge by looking at the complexity of classically simulating dynamics using tensor network for system sizes well beyond those accessible via exact diagonalization. Focusing on initial product states, we observe a qualitative change in the dynamics of the bond dimension needed as a function of their energy density. Specifically, the bond dimension typically grows $\textit{polynomially}$ in time up to a certain energy density, where we locate the state-dependent mobility edge, enabling simulations for long times. Above this energy density, the bond dimension typically grows $\textit{exponentially}$ making the simulation practically unfeasible beyond short times, as generally expected in interacting theories. We correlate the polynomial growth of the bond dimension to the presence of many non-thermal eigenstates around that energy density, a subset of which we compute via tensor network. The outreach of our findings encompasses quantum sampling problems and the efficient simulation of quantum circuits beyond Clifford families.

cond-mat.stat-mech↗

arXiv2024

Unveiling Magnon-Magnon Coupling and Its Dynamic Control in Nanomagnets

Hybrid magnonics, exploring the coupling between magnons and quantum systems, is an exciting field for developing next-generation information technologies. Achieving a strong and tunable magnon-magnon coupling (MMC) in confined nanomagnets is crucial for the on-chip integration of these hybrid systems and advancing the field. In this work, we numerically investigate the interactions between different magnon modes excited within an elliptical magnonic nano-disc (EMND), demonstrating an anti-crossing effect in the dispersion spectra. A comprehensive theoretical framework was presented that explains this anti-crossing phenomenon as a result of MMC and provide estimates for the strength of the coupling (g). Furthermore, we show that this intermodal coupling can be tuned from a strong coupling regime (g = 300 MHz) to a weak coupling regime by varying the direction of the external magnetic field and the intrinsic properties of the EMND. Our combined numerical and theoretical findings offer new insights into MMC, significantly advancing the field of quantum magnonics and magnon-based quantum information technology.

cond-mat.mes-hall↗

arXiv2024

Relativistic fluid dynamics in a 'hydro' frame

In this letter, we investigate how field redefinition influences the spectrum of linearized perturbations in relativistic fluid dynamics. We show that the hydrodynamic modes do not get affected under local field redefinition, whereas the non-hydrodynamic modes do. These non-hydrodynamic modes can be removed through a suitable all-order field redefinition. This process leads to a new frame containing only hydrodynamic modes, which we refer to as the hydro frame. Additionally, we demonstrate that the resulting stress-energy tensor may constitute an infinite series in momentum space, with the radius of convergence associated with the removed non-hydrodynamic mode, highlighting its role in the hydrodynamic expansion's validity.

nucl-th↗

arXiv2024

Emergence of coupling induced transparency by tuning purely dissipative couplings

Controlled transitions between coupling induced transparency (CIT) and coupling induced absorption (CIA) are effects of both fundamental importance as well as potential applications in various devices. We have explored these peculiar phenomena in multi-mode coupled hybrid quantum systems by considering a tunable mode (TM) and several static modes (SMs). The individual SMs and TM are designed such that they show CIA, but upon coupling different SMs we observe a transition from CIA to CIT. Quite remarkably we are able to achieve CIT using only purely dissipative couplings, whereas it is well known that CIT appears with coherent coupling. We have developed a robust quantum theory based formalism which is able to capture the transition between CIA to CIT and have the capability to explain the inter-transition (CIT to CIA) as well as intra-transitions (CIA to CIA, CIT to CIT etc.) in a multimode hybrid quantum system all with just linear approach. A general model is developed for hybrid quantum systems having N modes. We have then explicitly described two sets of hybrid systems, the first set is of three modes, 1TM coupled with 2SMs, and the second set is of four modes, 1TM coupled with 3SMs. Later we have generalised it for hybrid quantum systems having N number of modes. The results provide a pathway for designing hybrid systems that can control the group velocity of light, offering potential applications in the fields of optical switching and quantum information technology. Our finding and formulation that in a single hybrid quantum system we can achieve controllable inter-transitions and intra-transitions of CIT/ CIA may open a tool and guidance for its application in quantum technology and quantum materials as the TMs/SMs may be well extended to other real/ quasi-particles also.

quant-ph↗

arXiv2024

Study of angular observables in exclusive semileptonic $B_c$ decays

In this work, we investigate angular observables such as the longitudinal polarization of charged leptons, $τ$-polarization, and forward-backward asymmetry in semileptonic $B_c$ decays. Additionally, we provide predictions for lepton flavor violating observables, the $\mathcal{R}$ ratios in the decay channels $B_c \rightarrow η_c (J/ψ) l ν_l$ and $B_c \rightarrow D (D^*) l ν_l$ across the entire $q^2$ region. Our analysis is conducted within the Relativistic Independent Quark Model, focusing on the potential model-dependent aspects of these observables. We compare our model predictions with existing lattice predictions, highlighting the strong applicability of our framework in describing $B_c$ decays. Considering the forthcoming experimental upgrades and the Run 3 data results on $B_c$ meson decays, rapid confirmation of these quantities could indicate significant discoveries of physics beyond the Standard Model. This will open up new avenues for understanding the complex flavor dynamics in heavy meson decays.

hep-ph↗

arXiv2024

The stochastic relativistic advection diffusion equation from the Metropolis algorithm

We study an approach to simulating the stochastic relativistic advection-diffusion equation based on the Metropolis algorithm. We show that the dissipative dynamics of the boosted fluctuating fluid can be simulated by making random transfers of charge between fluid cells, interspersed with ideal hydrodynamic time steps. The random charge transfers are accepted or rejected in a Metropolis step using the entropy as a statistical weight. This procedure reproduces the expected strains of dissipative relativistic hydrodynamics in a specific (and non-covariant) hydrodynamic frame known as the density frame. Numerical results, both with and without noise, are presented and compared to relativistic kinetics and analytical expectations. An all order resummation of the density frame gradient expansion reproduces the covariant dynamics in a specific model. In contrast to all other numerical approaches to relativistic dissipative fluids, the dissipative fluid formalism presented here is strictly first order in gradients and has no non-hydrodynamic modes. The physical naturalness and simplicity of the Metropolis algorithm, together with its convergence properties, make it a promising tool for simulating stochastic relativistic fluids in heavy ion collisions and for critical phenomena in the relativistic domain.

nucl-th↗