Pennylane statevector results different from qiskit statevector? issue

[josh]

Thanks @Amandeep!

I had a look, and this is to be expected. It is because PennyLane and Qiskit use different conventions for the underlying statevector ordering.

For more details, see here:

Probability() usage

Hi @Miguel_Fernandez, as odd as it looks, I believe PennyLane and Qiskit are returning the exact same result. The disagreement results from Qiskit not implementing the same convention as commonly seen in quantum information papers and textbook; typically, the top qubit (labelled 0) in a circuit is the leftmost integer within the ket; |q_0 q_1 \cdots q_{n-1} q_n\rangle. For example, for a 2-qubit system originally in the ground state |00\rangle, applying an X gate to qubit 0 in this convention…

In this particular case, if you want to get the Qiskit results to match the PennyLane results, you can simply reshape and transpose the statevector:

def f_statevector(theta):
    qc = construct_circuit(theta, 0)
    statevector = backend_statevector.run(qc).result().get_statevector(qc)
    statevector = statevector.reshape([2] * qubits).T.flatten()