arXiv subjects

Itai Arad

Publications and source records attributed to Itai Arad.

44 records · Page 3Linked to original sources

arXiv2010

Quantum computation and the evaluation of tensor networks

We present a quantum algorithm that additively approximates the value of a tensor network to a certain scale. When combined with existing results, this provides a complete problem for quantum computation. The result is a simple new way of looking at quantum computation in which unitary gates are replaced by tensors and time is replaced by the order in which the tensor-network is "swallowed". We use this result to derive new quantum algorithms that approximate the partition function of a variety of classical statistical mechanics models, including the Potts model.

quant-ph↗

arXiv2008

The Detectability Lemma and Quantum Gap Amplification

The quantum analogue of a constraint satisfaction problem is a sum of local Hamiltonians - each local Hamiltonian specifies a local constraint whose violation contributes to the energy of the given quantum state. Formalizing the intuitive connection between the ground (minimal) energy of the Hamiltonian and the minimum number of violated constraints is problematic, since the number of constraints being violated is not well defined when the terms in the Hamiltonian do not commute. The detectability lemma proved in this paper provides precisely such a quantitative connection. We apply the lemma to derive a quantum analogue of a basic primitive in classical complexity: amplification of probabilities by random walks on expander graphs. It holds under the restriction that the interaction graph of the local Hamiltonian is an expander. Our proofs are based on a novel structure imposed on the Hilbert space that we call the $XY$ decomposition, which enables a reduction from the quantum non-commuting case to the commuting case (where many classical arguments go through). The results may have several interesting implications. First, proving a quantum analogue to the PCP theorem is one of the most important challenges in quantum complexity theory. Our quantum gap amplification lemma may be viewed as the quantum analogue of the first of the three main steps in Dinur's PCP proof. Quantum gap amplification may also be related to spectral gap amplification, and in particular, to fault tolerance of adiabatic computation. Finally, the detectability lemma, and the $XY$ decomposition provide a handle on the structure of local Hamiltonians and their ground states.

quant-ph↗

arXiv2007

Polynomial Quantum Algorithms for Additive approximations of the Potts model and other Points of the Tutte Plane

In the first 36 pages of this paper, we provide polynomial quantum algorithms for additive approximations of the Tutte polynomial, at any point in the Tutte plane, for any planar graph. This includes as special cases the AJL algorithm for the Jones polynomial, the partition function of the Potts model for any weighted planer graph at any temperature, and many other combinatorial graph properties. In the second part of the paper we prove the quantum universality of many of the problems for which we provide an algorithm, thus providing a large set of new quantum-complete problems. Unfortunately, we do not know that this holds for the Potts model case; this is left as an important open problem. The main progress in this work is in our ability to handle non-unitary representations of the Temperley Lieb algebra, both when applying them in the algorithm, and, more importantly, in the proof of universality, when encoding quantum circuits using non-unitary operators. To this end we develop many new tools, that allow proving density and applying the Solovay Kitaev theorem in the case of non-unitary matrices. We hope that these tools will open up new possibilities of using non-unitary reps in other quantum computation contexts.

quant-ph↗

arXiv2004

Phase-Space Structure & Substructure of Dark Halos

A method is presented for computing the 6-D phase-space density f(x,v) and its PDF v(f) in an N-body system. It is based on Delaunay tessellation, yielding v(f) with a fixed smoothing window over a wide f range, independent of the sampling resolution. It is found that in a gravitationally relaxed halo built by hierarchical clustering, v(f) is a robust power law, v(f) f^{-2.5 \pm 0.05}, over more than 4 decades in f, from its virial level to the current resolution limit. This is valid for halos of different sizes in the LCDM cosmology, indicating insensitivity to the initial-fluctuation power spectrum as long as the small-scale fluctuations were not completely suppressed. By mapping f in position space, we find that the high-f contributions to v(f) come from the "cold" subhalos within the parent halo rather than the halo central region and its global spherical profile. The f in subhalos near the halo virial radius is more than 100 times higher than at the halo center, and it decreases gradually with decreasing radius. This indicates phase mixing due to mergers and tidal effects involving puffing up and heating. The phase-space structure provides a sensitive tool for studying the evolution of subhalos during the buildup of halos. One wishes to understand why the substructure adds up to the universal power law in v(f). It seems that the f^{-2.5} behavior is related to the hierarchical clustering process and is not a general result of violent relaxation.

astro-ph↗

arXiv2003

Dark-Halo Cusp: Asymptotic Convergence

We propose a model for how the buildup of dark halos by merging satellites produces a characteristic inner cusp, of a density profile ρ\prop r^-a with a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of a<1 exerts tidal compression which prevents local deposit of satellite material; the satellite sinks intact into the halo center thus causing a rapid steepening to a>1. Using merger N-body simulations, we learn that this cusp is stable under a sequence of mergers, and derive a practical tidal mass-transfer recipe in regions where the local slope of the halo profile is a>1. According to this recipe, the ratio of mean densities of halo and initial satellite within the tidal radius equals a given function psi(a), which is significantly smaller than unity (compared to being 1 according to crude resonance criteria) and is a decreasing function of a. This decrease makes the tidal mass transfer relatively more efficient at larger a, which means steepening when a is small and flattening when a is large, thus causing converges to a stable solution. Given this mass-transfer recipe, linear perturbation analysis, supported by toy simulations, shows that a sequence of cosmological mergers with homologous satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1. The slope depends only weakly on the fluctuation power spectrum, in agreement with cosmological simulations. During a long interim period the profile has an NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough compact satellite remnants make it intact into the inner halo. In order to maintain a flat core, satellites must be disrupted outside the core, possibly as a result of a modest puffing up due to baryonic feedback.

astro-ph↗

arXiv2002

Galactic Halo Cusp versus Core: Tidal Effects in Mergers

We show how the buildup of halos by merging satellites forces an inner cusp, with a density profile ρ\propto r^{-α} where α\to α_a \gsim 1$. Our analysis is based on a new prescription for tidal stripping as a function of α(r), using a simple toy model which matches N-body simulations. In a core of α< 1 there is tidal compression rather than stripping and the satellites sink towards the halo center, causing a rapid steepening of the profile to α> 1. Where α> 1, the stripping of each satellite shell is preceded by gradual puffing up, which makes the stripping more efficient at larger α, causing flattening where αis large enough. Therefore, we can show using linear perturbation analysis that a sequence of mergers slowly leads to a fixed point α(r)=α_a. This result implies that a cusp is enforced as long as enough satellite material makes it into the inner halo and is deposited there. We conclude that in order to maintain a flat core, as indicated by observations, satellites must be disrupted outside the core, e.g., because of puffing up due to baryonic feedback effects.

astro-ph↗

arXiv1998

Correlation functions in isotropic and anisotropic turbulence: the role of the symmetry group

The theory of fully developed turbulence is usually considered in an idealized homogeneous and isotropic state. Real turbulent flows exhibit the effects of anisotropic forcing. The analysis of correlation functions and structure functions in isotropic and anisotropic situations is facilitated and made rational when performed in terms of the irreducible representations of the relevant symmetry group which is the group of all rotations SO(3). In this paper we firstly consider the needed general theory and explain why we expect different (universal) scaling exponents in the different sectors of the symmetry group. We exemplify the theory context of isotropic turbulence (for third order tensorial structure functions) and in weakly anisotropic turbulence (for the second order structure function). The utility of the resulting expressions for the analysis of experimental data is demonstrated in the context of high Reynolds number measurements of turbulence in the atmosphere.

chao-dyn↗

arXiv1998

The Extraction of Anisotropic Contributions in Turbulent Flows

We analyze turbulent velocity signals measured by two probes in the atmosphere, both at the height of 35 meters but displaced by 40 cm nominally orthogonal to the mean wind. Choosing a suitable coordinate system with respect to that of the mean wind, we derive theoretical forms for second order structure functions, and fit them to experimental data. We show that the effect of flow anisotropy is small on the longitudinal component but significant on the transverse component. The data provide an estimate of a universal exponent from among a hierarchy that governs the decay of flow anisotropy with the scale-size.

chao-dyn↗