% ising   Hamiltonian for the Ising model with non-periodic boundary%         condition.%   ising(B,N) gives the Hamiltonian for the %   ferromagnetic Ising model in transverse field for N qubits, if the %   field strength is B, the coefficient %   of the nearest neighbor coupling is 1.%   The coupling is z-z and the direction of the field is x. %   That is, the Hamiltonian is H= - sum_k z(k) z(k+1) + B*sum_k x(k), %   where x and z denote Pauli spin matrices.%   For the Hamiltonian non-periodic boundary condition is used.%   If argument n is omitted then the default%   is taken to be the value of global variable N.% Copyright (C) 2005  Geza Toth    E.mail: toth@alumni.nd.edu%% 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; see gpl.txt% of this subroutine package.%% 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, write to the Free Software% Foundation, Inc., 51 Franklin Street, Fifth Floor, % Boston, MA  02110-1301, USA.function H=ising(BField,varargin)if isempty(varargin),     global N;else    if length(varargin)~=1,        error('Wrong number of input arguments.')    end %if    N=varargin{1};end %if   x=[0 1;1 0];   y=[0 -i;i 0];    z=[1 0; 0 -1];   e=[1 0; 0 1];   % Using routines from the library to make the Ising Hamiltonian   H=-nnchain(z,z,N)+BField*nnchain(x,e,N);