proj_sym.m

量子信息科学使用的源码以及量子进化算法

% proj_sym  Gives the projector to the symmetric subspace
%    proj_sym(n)  Gives the projector to the symmetric subspace
%    of n qubits. proj_sym(n,d) does the same for qudits with dimension d.
%    Only n=2 is implemented.

% Copyright (C) 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 P=proj_sym(N,varargin)

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

if N~=2,
   error('Only the two-qudit case is implemented.');
end %if

P=eye(d^2,d^2)-proj_asym(N,d);