function c = concurrence(p)
% CONCURRENCE
% requires: nothing
% author: Toby Cubitt
% license: GPL2
%
% C = CONCURRENCE(RHO) returns the concurrence of 2x2 density
% matrix RHO.
%% Copyright (C) 2004-2009 Toby Cubitt
%%
%% 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 2
%% 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, write to the Free Software
%% Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
%% MA 02110-1301, USA.
switch any(size(p) == 1)
case true
c = 2*abs(p(1)*p(4) - p(2)*p(3));
case false
e = real(sqrt(sort(eig(p*flip(p)))));
c = max(0,[-1,-1,-1,1]*e);
end