arXiv subjects

Ali Vahedi

Publications and source records attributed to Ali Vahedi.

At least 19 recordsLinked to original sources

arXiv2026

Infrared Memory and Scrambling in a Dynamically Opened Coupled-SYK Majorana Junction

We study a dynamically opened Majorana junction formed by two coupled Sachdev--Ye--Kitaev (SYK) systems. A time-dependent bilinear coupling drives the system from a nearly decoupled to a strongly hybridized regime, providing an interacting realization of a Majorana shutter. Using a low-frequency analysis of the large-$N$ Keldysh theory around the conformal $q=4$ SYK saddle, we find that the opening retains a strong infrared memory, with the associated anomalous response becoming increasingly sensitive to the infrared cutoff. Exact finite-dimensional simulations of two $N=8$ SYK clusters show enhanced inter-sector entanglement, parity transfer, and cross-sector operator scrambling after the opening. These results connect the infrared structure of conformal SYK dynamics with finite-size many-body scrambling and show how a dynamically opened interacting Majorana system can retain long-wavelength memory beyond the quadratic limit.

hep-th↗

arXiv2026

A Truncated Majorana

We investigate the quantum field dynamics of truncating a Majorana wave packet via a time-dependent channel shutter. Modeling truncation as a controlled modification of chiral propagation rather than a literal spatial cut, we analyze a $1+1$D massless Majorana field subjected to a time-dependent rotation between its chiral components. We demonstrate that this protocol manifests two complementary physical limits. Globally, a mismatch between early- and late-time channel identifications induces a fermionic Bogoliubov transformation. We prove this asymptotic change universally generates an infrared soft-mode memory, $\beta(\omega, \nu) \propto (\omega+\nu)^{-1}$, leading to a logarithmic divergence in the Hilbert--Schmidt norm. This orthogonality-catastrophe-like obstruction persists even under infinitely smooth switching in the massless limit. Conversely, in the number-conserving regime where pair production vanishes, the shutter acts as a purely causal filter. We establish exact local field identities showing that, restricted to the even local observable algebra, the retained sector is exactly equivalent to a single-particle state while the discarded sector reduces to the vacuum. Ultimately, we show that global infrared memory and local causal truncation are complementary diagnostics of the same dynamical operation. These results provide a rigorous field-theoretic foundation for time-dependent control in topological platforms, cleanly separating effective operational benchmarks from microscopic boundary dynamics.

hep-th↗

arXiv2026

Source-Aware Recovery and Security Bounds in Temporal-Mode Quantum Communication

We analyze a temporal-mode extension of the two-way LM05 quantum communication primitive, in which a message block is encoded into a fixed-weight binary occupation vector (the support) alongside classical ordering information. We demonstrate that applying an independent-slot recovery model to this correlated source is fundamentally inconsistent. For an ideal erasure channel, we derive the exact maximum-a-posteriori (MAP) recovery probability, showing that source-aware decoding outperforms independent guessing by orders of magnitude. We extend this analysis to a realistic threshold-detector model, quantifying the degradation caused by dark counts and inefficiency. Furthermore, we clarify the quantum-classical information split: in the ideal single-photon limit, preparation averaging renders Eve's state independent of the support, yielding zero Holevo information. We establish conditional security bounds using entropic uncertainty relations and identify photon-number-splitting as the primary practical vulnerability. These results provide a rigorous source-aware recovery benchmark and define the physical conditions under which the quantum layer offers a key-consumption advantage over classical constant-weight coding.

quant-ph↗

arXiv2026

Nonlinear response of the chiral magnetic effect in the D3/D7 holographic model

We investigate the nonlinear response of the chiral magnetic current to an external magnetic field in a holographic setup. Using the D3/D7 brane system, where the chiral magnetic effect (CME) can be realized by considering rotating probe D7-branes, corresponding to introducing an axial chemical potential, we analyze the current-magnetic field relation beyond the linear regime. Focusing on the vicinity of the phase boundary between the insulating phase and the CME phase, we find that the chiral magnetic current exhibits a multi-valued dependence on the magnetic field, indicating a highly nonlinear response characteristic of this model. We further study the dynamical stability of the insulating phase near the transition point, and show that the presence of both an axial chemical potential and an external magnetic field cooperatively stabilize the system. Our results clarify the interplay between the axial chemical potential and the magnetic field in determining the phase structure and stability of the system, and reveal new nonlinear aspects of chiral transport in holographic gauge theories.

hep-th↗

arXiv2024

Testing loop quantum gravity by quasi-periodic oscillations: rotating blackholes

We investigate a compelling model of a rotating black hole that is deformed by the effects of loop quantum gravity (LQG). We present a simplified metric and explore two distinct geometries: one in which the masses of the black hole and white hole are equal, and another in which they differ. Our analysis yields the radius of the innermost stable circular orbits (ISCO), as well as the energy and angular momentum of a particle within this framework. Additionally, we find the frequency of the first-order resonance separately. We constrain the model by the quasi-periodic oscillations (QPO) of the X-ray binary GRO J1655-40. We show that $\lambda=0.15^{+0.23}_{-0.14}$ at $1\sigma$ confidence level for equal mass black hole and white hole geometry. For the other geometry we get $\lambda=0.11^{+0.07}_{-0.07}$ at $1\sigma$ confidence level.We encounter a degeneracy in the parameter space that hinders our ability to constrain $\lambda$ with greater precision.

gr-qc↗

arXiv2024

Probing the warped vacuum geometry around a Kerr black hole by quasi-periodic oscillations

We investigate quasi-periodic oscillations (QPOs) in the context of a new rotating black hole solution that incorporates a cosmological constant. Recent work by the authors in \cite{Ovalle:2022eqb} interpreted the cosmological constant, denoted as $\Lambda$, as a form of vacuum energy and employed a gravitational decoupling approach to derive an extended Kerr-de Sitter black hole solution, which is geometrically richer than the classical case. In this study, we derive the expressions for timelike circular geodesics within this solution and, using a relativistic precision model, calculate the corresponding frequencies of the QPOs. To constrain our model, we apply Bayesian formalism, utilizing data from three well-known microquasars: GRO 1655-40, XTE 1550-564, and GRS 1915+105. Our analysis reveals that$\Lambda$ is degenerate and correlated with other parameters. Finally, we perform a Bayesian model comparison with the Kerr metric and find that the Kerr metric is favored among the models considered.

gr-qc↗

arXiv2024

Non-Linear Dynamics and Critical Phenomena in the Holographic Landscape of Weyl Semimetals

This study presents a detailed analysis of critical phenomena in a holographic Weyl semi-metal (WSM) using the $D3/D7$ brane configuration. The research explores the non-linear response of the longitudinal current $ J $ when subjected to an external electric field $ E $ at both zero and finite temperatures. At zero temperature, the study identifies a potential quantum phase transition in the $ J $-$ E $ relationship, driven by background parameters the particle mass, and axial gauge potential. This transition is characterized by a unique reconnection phenomenon resulting from the interplay between WSM-like and conventional nonlinear conducting behaviors, indicating a quantum phase transition. Additionally, at non-zero temperature with dissipation, the system demonstrates first- and second-order phase transitions as the electric field and axial gauge potential are varied. The longitudinal conductivity is used as an order parameter to identify the current-driven phase transition. Numerical analysis reveals critical exponents in this non-equilibrium phase transition that show similarities to mean-field values observed in metallic systems.

hep-th↗

arXiv2023

Cross-correlation Power Spectra and Cosmic Birefringence of the CMB via Photon-neutrino Interaction

In the context of the standard model of particles, the weak interaction of cosmic microwave background (CMB) and cosmic neutrino background (C$ν$B), can generate non-vanishing TB and EB power spectra in the order of one loop forward scattering, in the presence of scalar perturbation, which is in contrast with the standard scenario cosmology. Comparing our results with the current experimental data may provide, significant information about the nature of C$ν$B, including CMB-C$ν$B forward scattering for TB, TE, and EB power spectra. To this end, different cases were studied, including Majorana C$ν$B and Dirac C$ν$B. On the other hand, it was shown that the mean opacity due to cosmic neutrino background could behave as an anisotropic birefringent medium and change the linear polarization rotation angle. Considering the contributions from neutrino and anti-neutrino forward scattering with CMB photons (in the case of Dirac neutrino), we introduce relative neutrino and anti-neutrino density asymmetry ($δ_ν=\frac{Δn_ν}{n_ν}=\frac{n_ν-n_{\barν}}{n_ν}$). Then, using the cosmic birefringence angle reported by the Planck data release $β=0.30^\circ\pm0.11^\circ$ ($68\%C.L.$), some constraints can be put on $δ_ν$. Also, the value of cosmic birefringence due to Majorana C$ν$B medium is estimated at about $β|_ν\simeq0.2$ rad. In this respect, since Majorana neutrino and anti-neutrino are exactly the same, both CB contributions will be added together. However, this value is at least two orders larger than the cosmic birefringence angle reported by the Planck data release, $β=0.30^\circ\pm0.11^\circ$ ($68\%C.L.$).

hep-ph↗

arXiv2021

Gravitational Waves in a Closed Spacetime via Deviation Equation

Within the closed universe, we obtain the amplitude and frequency of gravitational waves in the terms of discrete wave numbers, wave propagation time, and cosmological constant using the deviation equation in the first-order perturbed metric. We demonstrate that the cosmological constant effect on GWs is only seen in the early universe. Also, by considering the time evolution of a gravitational wave in a closed spacetime, we investigate its effect on a circle of nearby massless particles, which will be compared with this case in the flat spacetime. Expanding the universe has effective damping on GWs; thus, we suggest it can be used as a tool to characterize the large-scale curvature of the universe

gr-qc↗

arXiv2021

Are latest detected events of gravitational waves in favor of some models of inflation based on string theory?

The general potential of power-law inflation is as $ V(ϕ)\propto ϕ^{n} $ with scalar field $ϕ$. The behavior of inflation is often known as power-law expansion like $S(η)\propto η^{1+β}$ with $1+β<0$. In this paper, the theoretical spectra of relic gravitational waves are compared with the measured strain sensitivity of Advanced LIGO and VIRGO, corresponding to the latest detected events of gravitational waves. The results show tight constraints on $β$ and $n$. Also, the obtained constraints indicate that special types of the potential of inflation, prototype, and KKLTI models, which are originated from string theory, are more suitable than other models. Our analysis shows that there exist some more chances for the detection of relic gravitational waves.

astro-ph.CO↗

arXiv2021

Aschenbach effect for spinning particles in Kerr-(A)dS spacetime

A non-monotonic behavior of the velocity gradient of a test particle revolving around a rapidly rotating black hole in the locally non-rotating frame of reference is known as the Aschenbach effect. This effect can serve as a distinguishing signature of rapidly rotating black holes, being potentially useful for the measurements of the astrophysical black hole spins. This paper is the generalization of our previous research to the motion of spinning particles around a rotating black hole with non-zero cosmological constant. We show that both the particle's spin $s$ and the cosmological constant $Λ$ modify the critical value of the black hole spin $a_c$, for which the Aschenbach effect can be observed; $a_c$ can increase or decrease depending on the signs of $s$ and $Λ$. We also found that the particle's spin $s$ can mimic the effect of the cosmological constant $Λ$ for a given $a_c$, causing thus a discrepancy in the measurements of $s$, $Λ$ and $a_c$ in the Aschenbach effect.

gr-qc↗

arXiv2020

Aschenbach effect for spinning particles in Kerr spacetime

The orbital velocity profile of circular timelike geodesics in the equatorial plane of a Kerr black hole has a non-monotonic radial behavior, provided that the spin parameter $a$ of the black hole is bigger than a certain critical value $a_c \approx 0.9953 M$. Here the orbital velocity is measured with respect to the Locally Non-Rotating Frame (LNRF), and the non-monotonic behavior, which is known as the Aschenbach effect, occurs only for co-rotating orbits. Using the Mathisson-Papapetrou-Dixon equations for a massive spinning particle, we investigate the Aschenbach effect for test particles with spin. In addition to the black-hole spin, the absolute value of the particle's spin and its orientation (parallel or anti-parallel to the black-hole spin) also play an important role for the Aschenbach effect. We determine the critical value $a_c$ of the spin parameter of the Kerr black hole where the Aschenbach effect sets in as a function of the spin of the probe. We consider not only black holes ($a^2 \le M^2$) but also naked singularities ($a^2>M^2$). Whereas for spinless (geodesic) particles the orbital velocity is always monotonically decreasing if the motion is counter-rotating, we find that for spinning particles in counter-rotating motion with anti-parallel spin around a naked singularity the orbital velocity is increasing on a certain radius interval.

gr-qc↗

arXiv2020

B-mode Power Spectrum of CMB via Polarized Compton Scattering

In this work, according to some evidence from being an asymmetry in the number density of left and right-handed electrons, $δ_L$, in-universe motivate us to calculate the dominated contribution of this asymmetry in the generation of B-mode power spectrum $C_{ B\,l}^{(S)}$. Note, in the standard cosmological scenario, Compton scattering in the presence of scalar matter perturbation can not generate magnetic like pattern in linear polarization while in the case of polarized Compton scattering, we have shown $C_{B\,l}^{(S)}\propto δ_L^2$. We add up the spectrum of the B-mode generated by the polarized Compton scattering to the spectra produced by weak lensing effects and Compton scattering in the presence of tensor perturbations. The results show a significant amplification in $C_{B\,l}$ in large scale $l<500$ for $δ_L>10^{-6}$ which will be observable in future high resolution B-mode polarization detection. Finally, we have shown that $C_{ B\,l}^{(S)}$ generated by polarized Compton scattering can suppress the tensor to scalar ratio, $r$ parameter so that this contamination can be comparable to a primordial tensor-to-scalar ratio spatially for $δ_L>10^{-5}$.

astro-ph.CO↗

arXiv2018

Ground State Instability in Non-relativistic QFT and Euler-Heisenberg Lagrangian via Holography

We study the ground state instability of a strongly coupled QFT with the $z=2$ Schrödinger symmetry in a constant electric field using probe branes holography. The system is $N_f$ $\mathcal{N}=2$ hypermultiplet fermions at zero charge density in the supergravity Schrödinger background. We show that the instability occurs due to Schwinger-like effect and an insulator state will undergo a transition to a conductor state. We calculate the decay rate of instability and pair production probability by using the $gauge/gravity$ duality. At zero temperature for massive fermions, we suggest that the instability occurs if the critical electric field is larger than the confining force between fermions, which is proportional to an effective mass. We demonstrate that, at zero temperature, the Schrödinger background simulates the role of a crystal lattice for massive particles. We also show that at finite 't Hooft coupling for particles with a mass higher than $\frac{\sqrtλ}{π\b}$, in this background, instability does not occur, no matter how large the external electric field is, meaning that we have a \textit{perfect insulator}. Moreover, we derive Euler-Heisenberg effective Lagrangian for the non-relativistic strongly correlated quantum theory from probe branes holography in Schrödinger spacetime.

hep-th↗

arXiv2018

Non-Equilibrium Critical Phenomena From Probe Brane Holography in Schrödinger Spacetime

We study the non-equilibrium steady-state phase transition from probe brane holography in $z=2$ Schrödinger spacetime. Concerning differential conductivity, a phase transition could occur in the conductor state. Considering constant current operator as the external field and the conductivity as an order parameter, we derive scaling behavior of order parameter near the critical point. We explore the critical exponents of the non-equilibrium phase transition in two different Schrödinger spacetimes, which originated $1)$ from supergravity, and $2)$ from AdS blackhole in the light-cone coordinates. Interestingly, we will see that even at the zero charge density, in our first geometry, the dynamical critical exponent of $z=2$ has a major effect on the critical exponents.

hep-th↗

arXiv2018

Generation of Circular Polarization of CMB via Polarized Compton Scattering

The standard scenario of cosmology predicts a measurable amount for linear polarization of the Cosmic Microwave Background radiation (CMB) via Thomson scattering, while through this scenario, the generation of circular polarization is excluded. On the another hand, the circular polarization of CMB has not been excluded in observational evidence. The generation of CMB photons circular polarization via their Compton scattering with polarized cosmic electrons is considered in this paper. Our motivation for considering polarized Compton scattering comes from the effects of the external magnetic field in large scale, the chiral magnetic instability and new physics interactions of the cosmic electrons. It is shown that damping term of polarized Compton scattering in the presence of scalar perturbation can generate circular polarization in CMB radiation, so that the power spectrum of circular polarization of CMB $C_l^{V(S)}$ is proportional to the power spectrum of temperature anisotropy of CMB $C_l^{I(S)}$ and also $δ^2$ which is a fraction of polarized electron number density to the total one with net Left- or Right-handed polarizations. We have discussed that at least we need $δ<10^{-4}$ to find consistency with a reported upper limit of CMB circular polarization.

astro-ph.CO↗

arXiv2016

Equilibrium Instability of Chiral Mesons in External Electromagnetic Field via AdS/CFT

We study the equilibrium instability of chiral quarkonia in a plasma in the presence of constant magnetic and electric field and at finite axial chemical potential using AdS/CFT duality. The model in use is a supersymmetric QCD at large 't$\,$Hooft coupling and number of colors. We show that the presence of the magnetic field and the axial chemical potential even in the absence of the electric field make the system unstable. In a gapped system, a stable/unstable equilibrium state phase transition is observed and the initial transition amplitude of the equilibrium state to the non-equilibrium state is investigated. We demonstrate that at zero temperature and large magnetic field the instability grows linearly by increasing the quarkonium binding energy. In the constant electric and magnetic field, the system is in a equilibrium state if the Ohm's law and the chiral magnetic effect cancel their effects. This happens in a sub-space of $(E,B,T,μ_5)$ space with constraint equation $σ_B B =- σE$, where $σ$ and $σ_B$ are called electric and chiral magnetic conductivity, respectively. We analyze the decay rate of a gapless system when this constraint is slightly violated.

hep-th↗

arXiv2013

Non-equilibrium Phase Transition from AdS/CFT

Using AdS/CFT correspondence we study non-equilibrium phase transition in the presence of a constant external magnetic field. The transition occurs when the sign of differential conductivity reverses. Utilizing numerical method we show that the type of transition depends on the value of magnetic field as well as the temperature of gauge theory. Moreover we show that this transition does not depend on the supersymmetry and the subspace on which the fundamental matter fields live.

hep-th↗