Endian is inconsistent between State and SpinOperator.to_matrix

[ikkoham]

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Describe the bug

Endian is inconsistent between State and SpinOperator.to_matrix.

Steps to reproduce the bug

@cudaq.kernel
def bell():
    qubits = cudaq.qvector(2)
    h(qubits[0])

observable = cudaq.SpinOperator.from_word("XZ")
res = cudaq.observe(bell, observable)
print(res.expectation())

0.9999999403953552

state = np.array(cudaq.get_state(bell))
print(state @ observable.to_matrix() @ state)

0j

Expected behavior

These two examples should return the same result.
In my understanding, CUDA-Q is big-endian, so observe's behavior is correct.

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA Quantum version: 0.7.1
• Python version: 3.10.12
• C++ compiler:
• Operating system: Ubuntu 22.04.4 LTS

Suggestions

No response

[ikkoham]

Workaround:

def reverse_endian(spin: cudaq.SpinOperator) -> cudaq.SpinOperator:
    return sum(
        term.get_coefficient() * SpinOperator.from_word(term.to_string(False)[::-1])
        for term in spin
    )

print(state @ reverse_endian(observable).to_matrix() @ state)

[ikkoham]

I think the endian of observable.to_matrix() is wrong. [should be confirmed]