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

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Nov 14, 2024
Merged

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

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86811c7
Fix the calculation for identity operator's expectation value
yjh-bill Oct 18, 2024
6138d25
Add release note and test
yjh-bill Nov 10, 2024
395abf0
reformatted `test_statevector.py` with black
yjh-bill Nov 13, 2024
bb961b7
Update releasenotes/notes/fix_identity_operator_9e2ec9770ac046a6.yaml
yjh-bill Nov 13, 2024
9bf6386
add a docstring to pass the pylint test
yjh-bill Nov 13, 2024
93c4bbf
Fix leading `.`
Cryoris Nov 13, 2024
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2 changes: 1 addition & 1 deletion qiskit/quantum_info/states/statevector.py
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Original file line number Diff line number Diff line change
Expand Up @@ -476,7 +476,7 @@ def _expectation_value_pauli(self, pauli, qargs=None):
pauli_phase = (-1j) ** pauli.phase if pauli.phase else 1

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

if x_mask == 0:
return pauli_phase * expval_pauli_no_x(self.data, self.num_qubits, z_mask)
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5 changes: 5 additions & 0 deletions releasenotes/notes/fix_identity_operator_9e2ec9770ac046a6.yaml
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@@ -0,0 +1,5 @@
---
fixes:
- |
Fixed a bug that caused :meth:`Statevector.expectation_value` to yield incorrect results
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for the identity operator when the statevector was not normalized.
25 changes: 25 additions & 0 deletions test/python/quantum_info/states/test_statevector.py
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Expand Up @@ -1152,6 +1152,31 @@ def test_expval_pauli_qargs(self, qubits):
expval = state.expectation_value(op, qubits)
self.assertAlmostEqual(expval, target)

def test_expval_identity(self):
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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)

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Got it. Thanks for the instruction.

"""Test whether the calculation for identity operator has been fixed"""

# 1 qubit case test
state_1 = Statevector.from_label("0")
state_1_n1 = 2 * state_1 # test the same state with different norms
state_1_n2 = (1 + 2j) * state_1
identity_op_1 = SparsePauliOp.from_list([("I", 1)])
expval_state_1 = state_1.expectation_value(identity_op_1)
expval_state_1_n1 = state_1_n1.expectation_value(identity_op_1)
expval_state_1_n2 = state_1_n2.expectation_value(identity_op_1)
self.assertAlmostEqual(expval_state_1, 1.0 + 0j)
self.assertAlmostEqual(expval_state_1_n1, 4 + 0j)
self.assertAlmostEqual(expval_state_1_n2, 5 + 0j)

# Let's try a multi-qubit case
n_qubits = 3
state_coeff = 3 - 4j
op_coeff = 2 - 2j
state_test = state_coeff * Statevector.from_label("0" * n_qubits)
op_test = SparsePauliOp.from_list([("I" * n_qubits, op_coeff)])
expval = state_test.expectation_value(op_test)
target = op_coeff * np.abs(state_coeff) ** 2
self.assertAlmostEqual(expval, target)

@data(*(qargs for i in range(4) for qargs in permutations(range(4), r=i + 1)))
def test_probabilities_qargs(self, qargs):
"""Test probabilities method with qargs"""
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