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function [ut1, st1, v1] = bloch_messiah(S, tol)
% Bloch-Messiah decomposition of a symplectic matrix.
%
% Args:
% S (matrix): symplectic matrix
% tol (double): tolerance for symplectic check (default: 1e-10)
%
% Returns:
% ut1, st1, v1 (matrices): Decomposition matrices such that S = ut1 * st1 * v1
if nargin < 2
tol = 1e-10;
end
[n, m] = size(S);
if n ~= m
error('The input matrix is not square');
end
if mod(n, 2) ~= 0
error('The input matrix must have an even number of rows/columns');
end
n = n / 2;
omega = sympmat(n);
if norm(S' * omega * S - omega) >= tol
error('The input matrix is not symplectic');
end
if norm(S' * S - eye(2*n)) >= tol
[u, sigma] = polardecomp(S, 'left');
[ss, uss] = takagi(sigma, tol);
% Apply permutation matrix
perm = [1:n, 2*n:-1:n+1];
pmat = eye(2*n);
pmat = pmat(perm, :);
ut = uss * pmat;
% Apply second permutation matrix
qomega = ut' * omega * ut;
st = pmat * diag(ss) * pmat;
% Identify degenerate subspaces
st_diag = diag(st);
[~, ~, ic] = unique(st_diag(1:n));
stop_is = cumsum(accumarray(ic, 1));
start_is = [0; stop_is(1:end-1)] + 1;
% Rotation matrices based on SVD
u_list = cell(1, length(start_is));
v_list = cell(1, length(start_is));
for i = 1:length(start_is)
start_i = start_is(i);
stop_i = stop_is(i);
x = real(qomega(start_i:stop_i, n+start_i:n+stop_i));
[u_svd, ~, v_svd] = svd(x);
u_list{i} = u_svd;
v_list{i} = v_svd';
end
pmat1 = blkdiag(u_list{:}, v_list{:});
st1 = pmat1' * pmat * diag(ss) * pmat * pmat1;
ut1 = uss * pmat * pmat1;
v1 = ut1' * u;
else
ut1 = S;
st1 = eye(2*n);
v1 = eye(2*n);
end
ut1 = real(ut1);
st1 = real(st1);
v1 = real(v1);
end
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