Fix the calculation for identity operator's expectation value

[yjh-bill]

Summary

Fix a bug in qiskit.quantum_info in the calculation of expectation value of the identity operator.

Fixes #13029

Details and comments

Currently, qiskit.quantum_info yields incorrect results for the expectation value of an identity operator for a given state vector. It returns the norm of the state vector; however, it ought to return the square of the norm.

Therefore, I am changing the following line

qiskit/qiskit/quantum_info/states/statevector.py

Lines 478 to 479 in 1be2c5a

	if x_mask + z_mask == 0:
	return pauli_phase * np.linalg.norm(self.data)

to

   if x_mask + z_mask == 0: 
       return pauli_phase * np.linalg.norm(self.data) ** 2

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[coveralls]

Pull Request Test Coverage Report for Build 11826979058

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2294 unchanged lines in 122 files lost coverage.
• Overall coverage increased (+0.4%) to 88.933%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/instructions/directives.py	1	97.14%
qiskit/circuit/library/boolean_logic/inner_product.py	1	96.0%
qiskit/transpiler/passes/scheduling/alignments/pulse_gate_validation.py	1	96.3%
qiskit/pulse/instructions/play.py	1	97.5%
crates/qasm2/src/expr.rs	1	94.02%
qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py	1	94.54%
qiskit/circuit/store.py	1	97.06%
qiskit/circuit/library/iqp.py	1	96.15%
qiskit/transpiler/passes/calibration/pulse_gate.py	1	95.45%
qiskit/circuit/library/boolean_logic/quantum_or.py	1	98.08%

Totals
Change from base Build 11411236768:	0.4%
Covered Lines:	79336
Relevant Lines:	89209

---

💛 - Coveralls

[Cryoris]

Thanks for opening a PR to fix this! Could you add a release note and a test, to verify it now works as expected? (See e.g. this PR for an example on how a bugfix PR should look like 🙂)

[yjh-bill]

Sure, I just added a release note and a test.

[Cryoris]

It seems like you need to run the formatter here, you can do that e.g. by calling make black (you might want to make sure the right version is installed before doing so, by running pip install -r requirements-dev.txt -c constraints.txt before)

[yjh-bill]

Got it. Thanks for the instruction.

[Cryoris]

Thanks for this fix!