arXiv subjects

Ramis Movassagh

Publications and source records attributed to Ramis Movassagh.

48 records · Page 3Linked to original sources

arXiv2013

Optical "Bernoulli" forces

By Bernoulli's law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure. Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion. Does a rotating body immersed in a stream of photons experience a Bernoulli-like force? We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave. In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive (ε>ε_{0}), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction. Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force.

physics.optics↗

arXiv2013

A Time-Asymmetric Process in Central Force Scatterings

This article puts forth a process applicable to central force scatterings. Under certain assumptions, we show that in attractive force fields a high speed particle with a small mass speeding through space, statistically loses energy by colliding softly with large masses that move slowly and randomly. Furthermore, we show that the opposite holds in repulsive force fields: the small particle statistically gains energy. This effect is small and is mainly due to asymmetric energy exchange of the transverse (i.e., perpendicular) collisions. We derive a formula that quantifies this effect (Eq. 12). We then put this work in a broader statistical context and discuss its consistency with established results.

physics.class-ph↗

arXiv2013

Unfrustration Condition and Degeneracy of Qudits on Trees

We generalize the previous results of [1] by proving unfrustration condition and degeneracy of the ground states of qudits (d-dimensional spins) on a k-child tree with generic local interactions. We find that the dimension of the ground space grows doubly exponentially in the region where rk<=(d^2)/4 for k>1. Further, we extend the results in [1] by proving that there are no zero energy ground states when r>(d^2)/4 for k=1 implying that the effective Hamiltonian is invertible.

quant-ph↗

arXiv2012

Eigenvalues and Low Energy Eigenvectors of Quantum Many-Body Systems

I first give an overview of the thesis and Matrix Product States (MPS) representation of quantum spin chains with an improvement on the conventional notation. The rest of this thesis is divided into two parts. The first part is devoted to eigenvalues of quantum many-body systems (QMBS). I introduce Isotropic Entanglement, which draws from various tools in random matrix theory and free probability theory (FPT) to accurately approximate the eigenvalue distribution of QMBS on a line with generic interactions. Next, I discuss the energy distribution of one particle hopping random Schrödinger operator in 1D from FPT in context of the Anderson model. The second part is devoted to ground states and gap of QMBS. I first give the necessary background on frustration free (FF) Hamiltonians, real and imaginary time evolution within MPS representation and a numerical implementation. I then prove the degeneracy and FF condition for quantum spin chains with generic local interactions, including corrections to our earlier assertions. I then summarize my efforts in proving lower bounds for the entanglement of the ground states, which includes some new results, with the hope that they inspire future work resulting in solving the conjecture given therein. Next I discuss two interesting measure zero examples where FF Hamiltonians are carefully constructed to give unique ground states with high entanglement. One of the examples (i.e., $d=4$) has not appeared elsewhere. In particular, we calculate the Schmidt numbers exactly, entanglement entropies and introduce a novel technique for calculating the gap which may be of independent interest. The last chapter elaborates on one of the measure zero examples (i.e., $d=3$) which is the first example of a FF translation-invariant spin-1 chain that has a unique highly entangled ground state and exhibits signatures of a critical behavior.

quant-ph↗

arXiv2012

Criticality without frustration for quantum spin-1 chains

Frustration-free (FF) spin chains have a property that their ground state minimizes all individual terms in the chain Hamiltonian. We ask how entangled the ground state of a FF quantum spin-s chain with nearest-neighbor interactions can be for small values of s. While FF spin-1/2 chains are known to have unentangled ground states, the case s=1 remains less explored. We propose the first example of a FF translation-invariant spin-1 chain that has a unique highly entangled ground state and exhibits some signatures of a critical behavior. The ground state can be viewed as the uniform superposition of balanced strings of left and right parentheses separated by empty spaces. Entanglement entropy of one half of the chain scales as log(n)/2 + O(1), where n is the number of spins. We prove that the energy gap above the ground state is polynomial in 1/n. The proof relies on a new result concerning statistics of Dyck paths which might be of independent interest.

quant-ph↗

arXiv2012

Condition Numbers of Indefinite Rank 2 Ghost Wishart Matrices

We define an indefinite Wishart matrix as a matrix of the form A=W^{T}WΣ, where Σis an indefinite diagonal matrix and W is a matrix of independent standard normals. We focus on the case where W is L by 2 which has engineering applications. We obtain the distribution of the ratio of the eigenvalues of A. This distribution can be "folded" to give the distribution of the condition number. We calculate formulas for W real (β=1), complex (β=2), quaternionic (β=4) or any ghost 0<β<\infty. We then corroborate our work by comparing them against numerical experiments.

math.ST↗

arXiv2012

Isotropic Entanglement

The method of "Isotropic Entanglement" (IE), inspired by Free Probability Theory and Random Matrix Theory, predicts the eigenvalue distribution of quantum many-body (spin) systems with generic interactions. At the heart is a "Slider", which interpolates between two extrema by matching fourth moments. The first extreme treats the non-commuting terms classically and the second treats them isotropically. Isotropic means that the eigenvectors are in generic positions. We prove Matching Three Moments and Slider Theorems and further prove that the interpolation is universal, i.e., independent of the choice of local terms. Our examples show that IE provides an accurate picture well beyond what one expects from the first four moments alone.

quant-ph↗

arXiv2012

Error analysis of free probability approximations to the density of states of disordered systems

Theoretical studies of localization, anomalous diffusion and ergodicity breaking require solving the electronic structure of disordered systems. We use free probability to approximate the ensemble- averaged density of states without exact diagonalization. We present an error analysis that quantifies the accuracy using a generalized moment expansion, allowing us to distinguish between different approximations. We identify an approximation that is accurate to the eighth moment across all noise strengths, and contrast this with the perturbation theory and isotropic entanglement theory.

cond-mat.dis-nn↗

arXiv2011

Density of States of Quantum Spin Systems from Isotropic Entanglement

We propose a method which we call "Isotropic Entanglement" (IE), that predicts the eigenvalue distribution of quantum many body (spin) systems (QMBS) with generic interactions. We interpolate between two known approximations by matching fourth moments. Though, such problems can be QMA-complete, our examples show that IE provides an accurate picture of the spectra well beyond what one expects from the first four moments alone. We further show that the interpolation is universal, i.e., independent of the choice of local terms.

cond-mat.str-el↗

arXiv2011

Unfrustrated Qudit Chains and their Ground States

We investigate chains of 'd' dimensional quantum spins (qudits) on a line with generic nearest neighbor interactions without translational invariance. We find the conditions under which these systems are not frustrated, i.e. when the ground states are also the common ground states of all the local terms in the Hamiltonians. The states of a quantum spin chain are naturally represented in the Matrix Product States (MPS) framework. Using imaginary time evolution in the MPS ansatz, we numerically investigate the range of parameters in which we expect the ground states to be highly entangled and find them hard to approximate using our MPS method.

quant-ph↗

arXiv2008

Bitemporal Dynamics Sinai Divergence, An Energetic Analog to Boltzmann's Entropy?

Sinai chaos is characterized by exponential divergence between neighboring trajectories of a point billiard. If the repulsive potential of the finite-diameter fixed particle in the middle of the table is made smooth, the Sinai divergence persists with finite measure. So it does if the smooth potential is made attractive. So it still does if the potential is in addition made time-dependent (periodic). Then a systematic decrease in energy of the moving particle can be predicted to occur in both time directions for a long time. If so, classical entropy acquires an analog in real space.

physics.class-ph↗

arXiv2006

On The Geometrical Description of Dynamical Stability II

Geometrization of dynamics using (non)-affine parametization of arc length with time is investigated. The two archetypes of such parametrizations, the Eisenhart and the Jacobi metrics, are applied to a system of linear harmonic oscillators. Application of the Jacobi metric results in positive values of geometrical lyapunov exponent. The non-physical instabilities are shown to be due to a non-affine parametrization. In addition the degree of instability is a monotonically increasing function of the fluctuations in the kinetic energy. We argue that the Jacobi metric gives equivalent results as Eisenhart metric for ergodic systems at equilibrium, where number of degrees of freedom $N\to\infty$. We conclude that, in addition to being computationally more expensive, geometrization using the Jacobi metric is meaningful only when the kinetic energy of the system is a positive constant.

math-ph↗