Fix the calculation for identity operator's expectation value (backport #13345)

[mergify]

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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This is an automatic backport of pull request #13345 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 11828146073

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 88.931%

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Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/two_qubit_decompose.rs	1	92.12%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	5	92.23%

Totals
Change from base Build 11820794097:	0.02%
Covered Lines:	79061
Relevant Lines:	88902

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💛 - Coveralls