from_polynomial_tensor constructor creates 0d array instead of scalar for constant coefficient

[grossardt]

Environment

• Qiskit Nature version: 0.7.0
• Python version: 3.10.12
• Operating system: Linux

What is happening?

from_polynomial_tensor constructor for

• BosonicOp
• FermionicOp
• SpinOp
• VibrationalOp

produces constant part (empty string key) with coefficient a 0d numpy array (should be number, as for the rest of coefficients)

How can we reproduce the issue?

from qiskit_nature.second_q.operators import BosonicOp, PolynomialTensor
import numpy as np
data = {'': 1, '+': np.array([1,0])}
t = PolynomialTensor(data)
b = BosonicOp.from_polynomial_tensor(t)
print(b._data)

Output:

{'': array(1), '+_0': 1}

What should happen?

Output should be:

{'': 1, '+_0': 1}

Any suggestions?

In line 246 of bosonic_op.py (and equivalent lines for fermionic_op.py, spin_op.py, vibrational_op.py):

data[""] = tensor[key]

cast rhs to scalar, e.g. by applying sum method to 0d tensor:

data[""] = tensor[key].sum()