104 řádky
4.9 KiB
C++
104 řádky
4.9 KiB
C++
/*************************************************************************
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* Copyright (C) 2015 by Andrius Štikonas <andrius@stikonas.eu> *
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* *
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* This program is free software; you can redistribute it and/or modify *
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* it under the terms of the GNU General Public License as published by *
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* the Free Software Foundation; either version 3 of the License, or *
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* (at your option) any later version. *
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* *
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* This program is distributed in the hope that it will be useful, *
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* but WITHOUT ANY WARRANTY; without even the implied warranty of *
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the *
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* GNU General Public License for more details. *
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* *
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* You should have received a copy of the GNU General Public License *
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* along with this program. If not, see <http://www.gnu.org/licenses/>.*
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*************************************************************************/
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// Calculation of mutual information in the setup of http://arxiv.org/abs/1503.08161
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#include <cmath>
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#include <complex>
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#include <iostream>
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static double alpha, beta;
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std::complex<double> crossRatio (std::complex<double>, std::complex<double>, std::complex<double>, std::complex<double>);
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double Fitzpatrick(std::complex<double>, std::complex<double>, double);
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int main()
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{
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// Parameters that can be changed:
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double tPlus = 0; // time on the left boundary
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double tMinus = 0; // time on the right boundary
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alpha = 0.4; // encodes the conformal dimention of local operator h_Psi
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double y = 3; // endpoint of interval A
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double L = 15; // length of interval A
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double epsilon = 0.01; // smearing parameter
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beta = 10; // inverse temperature
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// double tOmega = /*5.4418*/; // thermal state is perturbed by operator inserted at time -tOmega
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double c = 600; // central charge. Must be large in our approximation
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// End of parameters
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for(unsigned int i = 0; i < 1000; ++i)
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{
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double tOmega = i*2*L/1000;
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// Operator insertion points: Left boundary
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std::complex<double> x1 (0, -epsilon), x4 (0, epsilon), x1bar, x4bar;
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x1bar = conj(x1);
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x4bar = conj(x4);
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double x2 = y - tOmega - tMinus;
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double x2bar = y + tOmega + tMinus;
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double x3 = L + x2;
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double x3bar = L + x2bar;
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// Operator insertion points: Right boundary
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std::complex<double> x6 (y - tPlus - tOmega, beta/2);
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std::complex<double> x6bar (y + tPlus + tOmega, -beta/2);
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std::complex<double> x5 = L + x6;
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std::complex<double> x5bar = L + x6bar;
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// Cross-ratios for S_A
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std::complex<double> zA = crossRatio(x1, x2, x3, x4);
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std::complex<double> zAbar = crossRatio(x1bar, x2bar, x3bar, x4bar);
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// Cross-ratios for S_B
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std::complex<double> zB = crossRatio(x1, x5, x6, x4);
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std::complex<double> zBbar = crossRatio(x1bar, x5bar, x6bar, x4bar);
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// Cross-ratios for S_{A union B}
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std::complex<double> z2 = crossRatio(x1, x2, x6, x4);
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std::complex<double> z2bar = crossRatio(x1bar, x2bar, x6bar, x4bar);
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std::complex<double> z5 = crossRatio(x1, x5, x3, x4);
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std::complex<double> z5bar = crossRatio(x1bar, x5bar, x3bar, x4bar);
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// Now we calculate entanglement entropies using Fitzpatrick, Kaplan, Walters formula.
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double S_A = c/6 * log(Fitzpatrick(zA, zAbar, y < tMinus + tOmega && tMinus + tOmega < y + L ? 2*M_PI : 0));
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double S_B = c/6 * log(Fitzpatrick(zB, zBbar, 0));
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double S_union = c/6 * log(Fitzpatrick(z2, z2bar, y < tMinus + tOmega ? 2*M_PI : 0) * Fitzpatrick(z5, z5bar, y +L < tMinus + tOmega ? -2*M_PI : 0));
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double S_union_analytic = c/3 * log(beta/M_PI/epsilon * sin(M_PI*alpha)/alpha) + c/6* log(sinh(M_PI/beta*(tMinus + tOmega - y)) * cosh(M_PI/beta*(tPlus + tOmega - y)) * sinh(M_PI/beta*(tMinus + tOmega - y - L)) * cosh(M_PI/beta*(tPlus + tOmega - y - L)));
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double S_thermal = 2*c/3 * log(sinh(M_PI*L/beta)/cosh(M_PI/beta*(tMinus-tPlus)));
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double I = S_A + S_B - S_union + S_thermal;
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std::cout << tOmega << "\t" << I << std::endl;
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}
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return 0;
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}
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std::complex<double> crossRatio (std::complex<double> x1, std::complex<double> x2, std::complex<double> x3, std::complex<double> x4)
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{
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double pb = M_PI/beta;
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return sinh(pb*(x1-x2))*sinh(pb*(x3-x4))/sinh(pb*(x1-x3))/sinh(pb*(x2-x4));
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}
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double Fitzpatrick(std::complex<double> z, std::complex<double> zbar, double phase)
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{
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// phase is necessary to take into account nontrivial monodromy
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std::complex<double> one = 1; // one as a complex number
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std::complex<double> i(0,1); // imaginary unit
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double alphaExpr = 0.5-alpha/2;
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std::complex<double> exponent1 = exp(phase*i*alphaExpr);
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std::complex<double> exponent2 = exp(phase*i*alpha);
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return real(exponent1 * pow(z, alphaExpr) * pow(zbar, alphaExpr)
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* (one - exponent2 * pow(z, alpha)) * (one - pow(zbar, alpha))
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/ ( alpha*alpha * (one-z) * (one-zbar) ) );
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}
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