Observables

[garrison]

So far, when taking expectation values, I convert to quimb operators manually. For instance, given a CircuitMPS:

mps.local_expectation(quimb.pauli('Z') & quimb.pauli('Z'), (3, 4)).real.item()

I wonder if there's a better way, or a way we could enable users to use SparsePauliOp and its successor and have the conversion to quimb be automatic.

[kevinsung]

I dug up this function I wrote at some point, but I don't remember how I used it. Not sure if it's useful.

def quimb_paulis(pauli_list: PauliList):
    result = []
    for qiskit_pauli in pauli_list:
        quimb_pauli_list = []
        for i, p in enumerate(reversed(qiskit_pauli.to_label())):
            if p != "I":
                quimb_pauli_list.append((quimb.pauli(p), i))
        if quimb_pauli_list:
            paulis, qubits = zip(*quimb_pauli_list)
            pauli = functools.reduce(lambda a, b: a & b, paulis)
        else:
            pauli = 1.0
            qubits = []
        result.append((pauli, qubits))
    return result

[garrison]

OK, that is probably going to be useful to reference.