Number of qubits does not match the number of qubits of the observable

Question

I have to compute $\langle\psi_1(\theta)|H_1|\psi_1(\theta)\rangle$

where $$H_1 = X_1Z_2 +3Y_0Y_4$$

psi1 = RealAmplitudes(5, reps=2)
H1 = SparsePauliOp.from_list([("XZ", 1),("YY",3)])
theta1 = np.linspace(0,1,5*3)

with Session(service=service, backend=backend):
    options = Options(simulator={"seed_simulator": 42}, resilience_level=0)
    estimator = Estimator(options=options)
    job = estimator.run(circuits=[psi1], 
                        parameter_values=[list(theta1)], 
                        observables=[H1])   
    result = job.result()

Though, I'm getting the below error:

Traceback (most recent call last):
  Input In [52] in <cell line: 3>
    job = estimator.run(circuits=[psi1],
  File /opt/conda/lib/python3.8/site-packages/qiskit_ibm_runtime/estimator.py:249 in run
    return super().run(
  File /opt/conda/lib/python3.8/site-packages/qiskit/primitives/base/base_estimator.py:227 in run
    self._cross_validate_circuits_observables(circuits, observables)
  File /opt/conda/lib/python3.8/site-packages/qiskit/primitives/base/base_estimator.py:283 in _cross_validate_circuits_observables
    raise ValueError(
ValueError: The number of qubits of the 0-th circuit (5) does not match the number of qubits of the 0-th observable (2).

Can anyone tell me what actually is causing this issue and how can this be solved?

Answer 1

The way you're using SparsePauliOp.from_list is wrong since it doesn't specify that there are $5$ qubits in your circuit. Instead, you should use it like this:

H1 = SparsePauliOp.from_list([("IIZXI", 1),("YIIIY",3)])

Because Qiskit uses little-endian notation the qubits are ordered from right to left.

Alternatively, you can utilize the method SparsePauliOp.from_sparse_list as shown here.