ente.m

计算纠缠的程序

function ent = entanglement(psi,dim)

% ENTANGLEMENT  Entropy of entanglement of a bipartite pure state
% requires: TrX.m
% author: Toby Cubitt
%
%    ENTANGLEMENT(psi,dim) returns the entropy of entanglement of
%    bipartite pure state psi, divided into subsystems with
%    dimensions specified by two-component vector dim.
%    If only one dimension is specified, a dim(1) x dim(1)
%    system is assumed.
%    If two dimensions are specified, a dim(1) x dim(2) system is
%    assumed.
%    If three dimensions are specified, a dim(1) x dim (2) x dim(3)
%    system is assumed, and the entanglement is calculated with
%    respect to the partition (1 & 3)--(2).

if length(psi) ~= prod(dim)
  error('MATLAB:badopt',['Dimension of psi is not equal to product' ...
		    ' of dimensions of subsystems'])
end

switch length(dim)
 case 0
  n = 2;
  m = 2;
  l = 1;
 case 1
  n = dim(1);
  m = n;
  l = 1;
 case 2
  n = dim(1);
  m = dim(2);
  l = 1;
 case 3
  n = dim(1);
  m = dim(2);
  l = dim(3);    
end

rho = kron(psi,psi');
e = real(eig(TrX(rho,2,[n,m,l])));
indx = find((e ~= 0));
log_e = zeros(size(e));
log_e(indx) = log2(e(indx));
ent = -e'*log_e;