pauli_list.insert(..., qubit=True) with length-1 pauli_list sets phase to wrong shape

[aeddins-ibm]

Environment

• Qiskit version: 1.3.1
• Python version: 3.11
• Operating system: macos

What is happening?

I believe the phase attribute of any PauliList should always be a 1D numpy array.

For instance, this creates a PauliList where the phase attribute is a 1D array of length 1:

plist = PauliList(['XYZ'])

However, this code:

plist = plist.insert(1, PauliList(['X']), qubit=True)

makes plist.phase a 2D numpy array (shape = 1 x 1).

This causes problems when further manipulating the object. For example, trying to pass it to SparsePauliOp() fails.

How can we reproduce the issue?

import qiskit.quantum_info as qi

plist1 = qi.PauliList(['XX'])

plist2 = qi.PauliList(['X'])
plist2 = plist2.insert(1, qi.PauliList(['X']), qubit=True)

# qiskit thinks the two PauliLists are equivalent:
print(f'{(plist1 == plist2) = }') # np.True_

# but their phase attributes are different shapes:
print(f'{plist1.phase.shape = }') # (1,)
print(f'{plist2.phase.shape = }') # (1, 1)

# and plist1 can be used to construct a SparsePauliOp:
qi.SparsePauliOp(plist1)

# whereas plist2 cannot:
qi.SparsePauliOp(plist2) # raises an error

What should happen?

The phase attribute of the output of insert should be a 1D array, not a 2D array.
Perhaps the PauliList.from_symplectic() constructor (called at the end of insert) should also verify/coerce that the phase argument is 1D.

(If that's somehow actually the correct behavior, then maybe we'd instead need to fix the SparsePauliOp constructor to handle that input.)

Any suggestions?

The issue may be how these lines in PauliList.insert() treat phase on the same footing as x and z, which are 2D arrays:

qiskit/qiskit/quantum_info/operators/symplectic/pauli_list.py

Lines 448 to 452 in e33da2a

	if len(value) == 1:
	# Pad blocks to correct size
	value_x = np.vstack(size * [value.x])
	value_z = np.vstack(size * [value.z])
	value_phase = np.vstack(size * [value.phase])