qiskit qaoa.compute_minimum_eigenvalue

Question

I am having some trouble running the compute_minimum_eigenvalue method from qiskit's QAOA: the documentation states that one need to pass H (" Qubit operator of the Observable" as OperatorBase). My question is, given that I implemented H as QuantumCircuit, how can I convert it to the right format? Thanks a lot!

Answer 1

I had suggested in my comment above how you might do this. Operator is not an opflow OperatorBase (it's a BaseOperator - different type) but can be used to create an opflow MatrixOp which is. Given your code sample above, I did the following based on it, which runs and prints values.

import numpy as np
from qiskit import BasicAer, QuantumCircuit
from qiskit.algorithms import QAOA
from qiskit.algorithms.optimizers import COBYLA
from qiskit.quantum_info.operators import Operator
from qiskit.opflow import MatrixOp

nqubits = 4
H = QuantumCircuit(nqubits)
for i in range(nqubits):
    H.z(i)
    H_op = MatrixOp(Operator(H))
    qaoa = QAOA(optimizer= COBYLA(), reps=1, mixer=H,
                initial_point=np.array([1.0]),
                quantum_instance=BasicAer.get_backend('statevector_simulator'))
    print(qaoa.compute_minimum_eigenvalue(H_op))

Note: you should use an optimizer from the qiskit.algorithms.optimizers package - the qiskit.optimization is part of the now deprecated and unsupported Qiskit Aqua, and that specific module you had used (CobylaOptimizer), in any case, had a different purpose.

Answer 2

I'm not sure if I'm interpreting the scope of the question correctly, but you can convert a QuantumCircuit to a compatible OperatorBase type for compute_minimum_eigenvalue as follows:

from qiskit import QuantumCircuit
from qiskit.algorithms import QAOA
from qiskit.opflow import MatrixOp
from qiskit.quantum_info.operators import Operator

circuit = QuantumCircuit(1)
circuit.h(0)

op = MatrixOp(Operator(circuit)) # convert circuit to operator

qaoa = QAOA(...) # specifications of your problem

qaoa.compute_minimum_eigenvalue(op, ...)