Autograd on external library functions

PennyLane Help
1

Hi,
I am currently following this tutorial here https://pennylane.ai/qml/demos/tutorial_quantum_metrology.html, but replacing the phase_damp operations with the qutip simulation using mesolve for research purpose, then use the output density matrix from qutip to calculate the probabilities on each wire using projective POVMs (like |0\rangle \langle 0|) following the last parameterization. Unfortunately, it seems like doing this will prevent the autograd from calculating the gradient, thus no update for the parameters. A work around I have been tinkering with is trying to directly feed the mesolve output to pennylane’s quantum circuits again, which seems somewhat infeasible due to certain restrictions?

Thank you for any help!

2

Hi @trungkien!

Yes, unfortunately you’ve pinpointed exactly what is happening – since QuTiP is not written using an autodifferentiation library, it ‘breaks’ the autodifferentiation.

Currently, as far as I can tell, the only solution to preserve differentiability would be to port the mesolve algorithm to be written using Autograd (or alternatively, JAX/Torch/TF). Is this something that would be feasible?

Cheers,
Josh

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