Fix phase management in StabilizerState.expectation_value (#7460) (#…

…7675)

* fix a bug in expectation value. Handle Pauli phases.

* extend the test to include Pauli expectation vals with phases.

* add release notes

* add more 1-qubit and 2-qubit tests for exp_val=-1,i,-i

* more ddt

* Fix typo in error message

* Add comment on phase

Co-authored-by: Ikko Hamamura <ikkoham@users.noreply.github.com>
Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: Jake Lishman <jake@binhbar.com>
Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
(cherry picked from commit 6da136c)

Co-authored-by: Shelly Garion <46566946+ShellyGarion@users.noreply.github.com>

qiskit/quantum_info/states/stabilizerstate.py

@@ -196,15 +196,21 @@ def evolve(self, other, qargs=None):
         return ret
 
     def expectation_value(self, oper, qargs=None):
-        """Compute the expectation value of an operator.
+        """Compute the expectation value of a Pauli operator.
 
         Args:
-            oper (BaseOperator): an operator to evaluate expval.
+            oper (Pauli): a Pauli operator to evaluate expval.
             qargs (None or list): subsystems to apply the operator on.
 
         Returns:
-            complex: the expectation value (only 0 or 1 or -1).
+            complex: the expectation value (only 0 or 1 or -1 or i or -i).
+
+        Raises:
+            QiskitError: if oper is not a Pauli operator.
         """
+        if not isinstance(oper, Pauli):
+            raise QiskitError("Operator for expectation value is not a Pauli operator.")
+
         num_qubits = self.clifford.num_qubits
         if qargs is None:
             qubits = range(num_qubits)
@@ -214,19 +220,12 @@ def expectation_value(self, oper, qargs=None):
         # Construct Pauli on num_qubits
         pauli = Pauli(num_qubits * "I")
         phase = 0
+        pauli_phase = (-1j) ** oper.phase if oper.phase else 1
 
         for pos, qubit in enumerate(qubits):
-            pauli_pos = (oper.to_label())[len(oper) - 1 - pos]
-            if pauli_pos == "X":
-                pauli.x[qubit] = 1
-            elif pauli_pos == "Y":
-                pauli.x[qubit] = 1
-                pauli.z[qubit] = 1
-                phase += 1
-            elif pauli_pos == "Z":
-                pauli.z[qubit] = 1
-            else:
-                pass
+            pauli.x[qubit] = oper.x[pos]
+            pauli.z[qubit] = oper.z[pos]
+            phase += pauli.x[qubit] & pauli.z[qubit]
 
         # Check if there is a stabilizer that anti-commutes with an odd number of qubits
         # If so the expectation value is 0
@@ -258,10 +257,11 @@ def expectation_value(self, oper, qargs=None):
             phase += 2 * np.count_nonzero(pauli_z & stab.X[p])
             pauli_z = pauli_z ^ stab.Z[p]
 
+        # For valid stabilizers, `phase` can only be 0 (= 1) or 2 (= -1) at this point.
         if phase % 4 != 0:
-            return -1
+            return -pauli_phase
 
-        return 1
+        return pauli_phase
 
     def probabilities(self, qargs=None, decimals=None):
         """Return the subsystem measurement probability vector.

releasenotes/notes/fix_stabilizerstate_expval-2556c5ee916f5327.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix a bug in :class:`~qiskit.quantum_info.StabilizerState` expectation value calculation,
+    that occured when the Pauli operator has a non-trivial phase.