Hi all,

I'm trying to perform a quantum dynamics using trajectories. At each step I have N MPS corresponding each one to a given trajectory.

I need to calculate the entanglement entropy of the state as an average over quantum trajectories.

Turns out, obviously, that considering

$$

|\psi> =\frac{1}{N}\sum*i |\psi*i>

$$

where @@|\psi*i>@@ is the i-th MPS and computing the entanglement entropy, is different than considering
$$
\rho =\frac{1}{N}\sum*i |\psi

*i><\psi*i|

$$

and computing the entanglement entropy again.

My problem is that while in the first case I can perform an SVD and get the singular values in given QN sectors, in the second case I don't know how to approach to the calculation.

In summary, I'm not sure how to proceed to calculate the entanglement entropy starting from a density matrix (Written as MPO) instead of a state (written as an MPS), in a given symmetry sector.

Thank you for your time.

Best,

Vittorio