Bug on expectation value calculated with Estimator primitive

[jvscursulim]

Environment

• Qiskit version: 1.2.4
• Python version: 3.11.10
• Operating system: Ubuntu 24.04.1 LTS

What is happening?

I was using Estimator to compute the expectation value of some observables, however I noticed that for some observables composed only by X, the result obtained with Estimator does not match with the result by the method expectation_value() from Statevector and calculations done by hand. The expectation value for some cases should be +1 or -1, but the results are equal to 0.

How can we reproduce the issue?

import numpy as np

from qiskit.circuit import QuantumCircuit, QuantumRegister
from qiskit.quantum_info import SparsePauliOp, Statevector
from qiskit_aer.primitives import EstimatorV2 as Estimator

stabilizers = [SparsePauliOp(data=["IZIZ"], coeffs=np.array([1.0])),
               SparsePauliOp(data=["ZIZI"], coeffs=np.array([1.0])),
               SparsePauliOp(data=["XXXX"], coeffs=np.array([1.0]))]

qubits = QuantumRegister(size=4, name="qubits")

qc = QuantumCircuit(qubits)
qc.h(qubit=qubits[0])
for i in range(1,4):
    qc.cx(control_qubit=qubits[0], target_qubit=qubits[i])

estimator = Estimator()
estimator_results = estimator.run(pubs=[(qc, stabilizer) for stabilizer in stabilizers]).result()

print("Noiseless experiment")
for idx, stabilizer in enumerate(stabilizers):
    print(f"Stabilizer - {stabilizer.paulis.to_labels()} - Expected value: {estimator_results[idx].data.evs}")

sv = Statevector(qc)
print(sv.expectation_value(stabilizers[-1]))

What should happen?

The expectation value of the SparsePauliOp(data=["XXXX"], coeffs=np.array([1.0])) should be equal to +1.0 when we calculate it with Estimator and with a quantum circuit that creates the following quantum state $\vert\psi\rangle = \frac{1}{\sqrt{2}}(\vert0000\rangle + \vert1111\rangle)$.

Any suggestions?

No response

[Cryoris]

What version of qiskit-aer are you using? For me the snippet above produces the following which seems correct (also on qiskit==1.2.4):

Noiseless experiment
Stabilizer - ['IZIZ'] - Expected value: 1.0
Stabilizer - ['ZIZI'] - Expected value: 1.0
Stabilizer - ['XXXX'] - Expected value: 1.0
(0.9999999999999998+0j)

[jvscursulim]

I'm using qiskit-aer-gpu==0.15.1.