maxsymsep.m

量子信息科学使用的源码以及量子进化算法

% maxsymsep   Maximum for symmetric multi-qubit product states. 
%   maxsymsep(op) gives the maximum of op for symmetric seperable multiqubit 
%   states. Maxsymsep(op,d) makes it possible to handle qudits
%   with dimension d. Uses simple numerical search.
%   maxsymsep(op,d,par) makes it possible to set parameters.
%   par is a three-element vector. Defaults value
%   [10000 20000 0.005 ]. First element: Number of
%   random trials in the first phase. Second element:
%   Number of random trials in the second phase.
%   Third element: Constant deternmining accuracy.
%   Faster than maxsep. The form [maximum,phi]=maxsymsep(op)
%   gives back also the state giving the maximum.
%   [To be more precise, the maximum is given by the state
%   mkron(phi,phi,phi,...,phi).]

% Copyright (C) 2005-2007  Geza Toth    E.mail: [e-mail]
%
% 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 [m,fa0]=maxsymsep(op,varargin)

% Parameters for the simulation
Delta=0.005;
Nit1=10000;
Nit2=20000;

if isempty(varargin),
   d=2;
elseif length(varargin)==1,
   d=varargin{1};  
elseif length(varargin)==2,
   d=varargin{1};  
   par=varargin{2};
   Nit1=par(1);
   Nit2=par(2);
   Delta=par(3);
else
        error('Wrong number of input arguments.');
end %if

[sx,sy]=size(op);
N=floor(log(sx)/log(d)+0.5);

rmax=-Inf;

fa=zeros(d,N);
famax=fa;
for n=1:Nit1
     %%if mod(n,100)==0,  randn('state',sum(100*clock));  end %if
     f=randn(d,1)+i*randn(d,1);
     fa=f;
     for n=1:N-1
        f=kron(f,fa);
     end %for
     r=real(trace(op*f*f')/(f'*f));
     if r>rmax,
         rmax=r;
	     famax=fa;
     end %if
end %for

fa0=famax;
r0=rmax;

% Second phase of the search
for n=1:Nit2
     %%if mod(n,100)==0,  randn('state',sum(100*clock));  end %if
     f=randn(d,1)+i*randn(d,1);;
     fa=fa0+Delta*f;
     f=fa;
     for n=1:N-1
        f=kron(f,fa);
     end %for
     r=real(trace(op*f*f')/(f'*f));
     if r>r0,
         r0=r;
         fa0=fa;        
     end %if
end %for
 
m=r0;