New answers tagged density-matrix
1
The fundamental problem is that you've transposed the matrix of eigenvectors compared to what you need. Yes, as you've written it, the eigenvectors are the columns of the matrix. But I think you must be using the rows in your actual code.
One of the basic checks that I did (and this is crucial whenever you code something) is to be able to do simple test ...
2
$\mathcal{H}_A \neq \mathcal{H}_B$, they are two distinct physical systems (even if they have the same dimension).
That formula for a reduced density matrix is a shortcut for
$$
\rho_A = \sum_j \big(I_A \otimes \langle j |_B\big) \cdot | \Psi\rangle \langle\Psi| \cdot \big(I_A \otimes |j\rangle_B \big)
$$
You can check the dimensions. If $d_A = \text{dim}...
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