Initial commit.

CMakeLists.txt

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+cmake_minimum_required(VERSION 2.8)
+project(Scrambling)
+
+add_definitions(-std=gnu++11)
+
+add_executable(scrambling scrambling.cpp)

scrambling.cpp

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+/*************************************************************************
+ *  Copyright (C) 2015 by Andrius Štikonas <andrius@stikonas.eu>         *
+ *                                                                       *
+ *  This program is free software; you can redistribute it and/or modify *
+ *  it under the terms of the GNU General Public License as published by *
+ *  the Free Software Foundation; either version 3 of the License, or    *
+ *  (at your option) any later version.                                  *
+ *                                                                       *
+ *  This program is distributed in the hope that it will be useful,      *
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of       *
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
+ *  GNU General Public License for more details.                         *
+ *                                                                       *
+ *  You should have received a copy of the GNU General Public License    *
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.*
+ *************************************************************************/
+
+// Calculation of mutual information in the setup of http://arxiv.org/abs/1503.08161
+
+#include <cmath>
+#include <complex>
+#include <iostream>
+#include <iomanip>
+
+static double alpha, beta;
+std::complex<double> crossRatio (std::complex<double>, std::complex<double>, std::complex<double>, std::complex<double>);
+double Fitzpatrick(std::complex<double>, std::complex<double>, double);
+
+int main()
+{
+	// Parameters that can be changed:
+	double tPlus = 0;  // time on the left boundary
+	double tMinus = 0; // time on the right boundary
+	alpha = 0.4; // encodes the conformal dimention of local operator h_Psi
+	double y = 1; // endpoint of interval A
+	double L = 5; // length of interval A
+	double epsilon = 0.01; // smearing parameter
+	beta = 10; // inverse temperature
+	double tOmega = 5.4418; // thermal state is perturbed by operator inserted at time -tOmega
+	double c = 600; // central charge. Must be large in our approximation
+	// End of parameters
+
+	// Operator insertion points: Left boundary
+	std::complex<double> x1 (0, -epsilon), x4 (0, epsilon), x1bar, x4bar;
+	x1bar = conj(x1);
+	x4bar = conj(x4);
+	double x2    = y - tOmega - tMinus;
+	double x2bar = y + tOmega + tMinus;
+	double x3    = L + x2;
+	double x3bar = L + x2bar;
+
+	// Operator insertion points: Right boundary
+	std::complex<double> x6    (y - tPlus - tOmega, beta/2);
+	std::complex<double> x6bar (y + tPlus + tOmega, -beta/2);
+	std::complex<double> x5    = L + x6;
+	std::complex<double> x5bar = L + x6bar;
+
+	// Cross-ratios for S_A
+	std::complex<double> zA    = crossRatio(x1, x2, x3, x4);
+	std::complex<double> zAbar = crossRatio(x1bar, x2bar, x3bar, x4bar);
+
+	// Cross-ratios for S_B
+	std::complex<double> zB    = crossRatio(x1, x5, x6, x4);
+	std::complex<double> zBbar = crossRatio(x1bar, x5bar, x6bar, x4bar);
+
+	// Cross-ratios for S_{A union B}
+	std::complex<double> z2    = crossRatio(x1, x2, x6, x4);
+	std::complex<double> z2bar = crossRatio(x1bar, x2bar, x6bar, x4bar);
+	std::complex<double> z5    = crossRatio(x1, x5, x3, x4);
+	std::complex<double> z5bar = crossRatio(x1bar, x5bar, x3bar, x4bar);
+
+	// Now we calculate entanglement entropies using Fitzpatrick, Kaplan, Walters formula.
+	double S_A = c/6 * log(Fitzpatrick(zA, zAbar, 2*M_PI));
+	double S_B = c/6 * log(Fitzpatrick(zB, zBbar, 0));
+	double S_union = c/6 * log(Fitzpatrick(z2, z2bar, 2*M_PI) * Fitzpatrick(z5, z5bar, 0));
+	double S_thermal = 2*c/3 * log(sinh(M_PI*L/beta)/cosh(M_PI/beta*(tMinus-tPlus)));
+	double I = S_A + S_B - S_union + S_thermal;
+	std::cout << I << std::endl;
+	return 0;
+}
+
+std::complex<double> crossRatio (std::complex<double> x1, std::complex<double> x2, std::complex<double> x3, std::complex<double> x4)
+{
+	double pb = M_PI/beta;
+	return sinh(pb*(x1-x2))*sinh(pb*(x3-x4))/sinh(pb*(x1-x3))/sinh(pb*(x2-x4));
+}
+
+double Fitzpatrick(std::complex<double> z, std::complex<double> zbar, double phase)
+{
+	// phase is necessary to take into account nontrivial monodromy
+	std::complex<double> c1(1, 0); // one as a complex number
+	std::complex<double> i(0,1); // imaginary unit
+	double alphaExpr = 0.5-alpha/2;
+	std::complex<double> exponent1 = exp(phase*i*alphaExpr);
+	std::complex<double> exponent2 = exp(phase*i*alpha);
+	return real(exponent1 * pow(z, alphaExpr) * pow(zbar, alphaExpr)
+			* (c1 - exponent2 * pow(z, alpha)) * (c1 - pow(zbar, alpha))
+			/ ( alpha*alpha * (c1-z) * (c1-zbar) ) );
+}