Three Qubit Decomposition

I wanted to ask if there is a tool in PennyLane that can be used for three-qubit decomposition. Specifically, I have an 8x8 matrix and I would like to decompose it into single-qubit gates using Quantum Shannon Decomposition (QSD) or any other decomposition method.

Hi @Aaqib_Ali ! Pennylane currently has no way to decompose matrices that are larger than 4x4. You could try to create a variational circuit and approximate it as a first approach :slight_smile:

Thanks Guillermo for your reply
Can you please elaborate on what the cost function (Fidelity) should be in the context of the circuit?

Good question :slight_smile: Assuming you can access the circuit matrix, you could use this:

import numpy as np

A = np.array([[1, 2], [3, 4]])
B = np.array([[1, 2], [3, 5]])

norm_diff = np.linalg.norm(A - B)

There are several definitions of norm that you can choose from by setting the ord parameter inside the np.linalg.norm function

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