From 053ee324eef0b872e21e1d5e8816ecc0e3c4057f Mon Sep 17 00:00:00 2001
From: Shelly Garion <46566946+ShellyGarion@users.noreply.github.com>
Date: Wed, 18 Dec 2024 16:36:26 +0200
Subject: [PATCH] `StabilizerState.expectation_value` can also accept
 `SparsePauliOp` (#13539)

* update ecpectation_value function

* add a test

* add release notes

* revision following code review
---
 qiskit/quantum_info/states/stabilizerstate.py | 31 +++++++++++++++++--
 ...expval-sparsepauliop-3e32a871d8e908ce.yaml |  5 +++
 .../states/test_stabilizerstate.py            | 14 ++++++++-
 3 files changed, 47 insertions(+), 3 deletions(-)
 create mode 100644 releasenotes/notes/stabilizerstate-expval-sparsepauliop-3e32a871d8e908ce.yaml

diff --git a/qiskit/quantum_info/states/stabilizerstate.py b/qiskit/quantum_info/states/stabilizerstate.py
index b6bc8243b40..dfedb285fff 100644
--- a/qiskit/quantum_info/states/stabilizerstate.py
+++ b/qiskit/quantum_info/states/stabilizerstate.py
@@ -24,7 +24,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info.operators.op_shape import OpShape
 from qiskit.quantum_info.operators.operator import Operator
-from qiskit.quantum_info.operators.symplectic import Clifford, Pauli, PauliList
+from qiskit.quantum_info.operators.symplectic import Clifford, Pauli, PauliList, SparsePauliOp
 from qiskit.quantum_info.operators.symplectic.clifford_circuits import _append_x
 from qiskit.quantum_info.states.quantum_state import QuantumState
 from qiskit.circuit import QuantumCircuit, Instruction
@@ -259,7 +259,34 @@ def evolve(
         ret._data = self.clifford.compose(other.clifford, qargs=qargs)
         return ret
 
-    def expectation_value(self, oper: Pauli, qargs: None | list = None) -> complex:
+    def expectation_value(self, oper: Pauli | SparsePauliOp, qargs: None | list = None) -> complex:
+        """Compute the expectation value of a Pauli or SparsePauliOp operator.
+
+        Args:
+            oper: A Pauli or SparsePauliOp operator to evaluate the expectation value.
+            qargs: Subsystems to apply the operator on.
+
+        Returns:
+            The expectation value.
+
+        Raises:
+            QiskitError: if oper is not a Pauli or SparsePauliOp operator.
+        """
+        if isinstance(oper, Pauli):
+            return self._expectation_value_pauli(oper, qargs)
+
+        if isinstance(oper, SparsePauliOp):
+            return sum(
+                coeff * self._expectation_value_pauli(Pauli((z, x)), qargs)
+                for z, x, coeff in zip(oper.paulis.z, oper.paulis.x, oper.coeffs)
+            )
+
+        raise QiskitError(
+            "Operator for expectation value is not a Pauli or SparsePauliOp operator, "
+            f"but {type(oper)}."
+        )
+
+    def _expectation_value_pauli(self, oper: Pauli, qargs: None | list = None) -> complex:
         """Compute the expectation value of a Pauli operator.
 
         Args:
diff --git a/releasenotes/notes/stabilizerstate-expval-sparsepauliop-3e32a871d8e908ce.yaml b/releasenotes/notes/stabilizerstate-expval-sparsepauliop-3e32a871d8e908ce.yaml
new file mode 100644
index 00000000000..135156272dc
--- /dev/null
+++ b/releasenotes/notes/stabilizerstate-expval-sparsepauliop-3e32a871d8e908ce.yaml
@@ -0,0 +1,5 @@
+---
+features_quantum_info:
+  - |
+    The method :meth:`.StabilizerState.expectation_value` can now accept an operator of type
+    :class:`.SparsePauliOp`.
diff --git a/test/python/quantum_info/states/test_stabilizerstate.py b/test/python/quantum_info/states/test_stabilizerstate.py
index 4e1659ff699..f76df1134e8 100644
--- a/test/python/quantum_info/states/test_stabilizerstate.py
+++ b/test/python/quantum_info/states/test_stabilizerstate.py
@@ -25,7 +25,7 @@
 from qiskit.quantum_info.random import random_clifford, random_pauli
 from qiskit.quantum_info.states import StabilizerState, Statevector
 from qiskit.circuit.library import IGate, XGate, HGate
-from qiskit.quantum_info.operators import Clifford, Pauli, Operator
+from qiskit.quantum_info.operators import Clifford, Pauli, Operator, SparsePauliOp
 from test import combine  # pylint: disable=wrong-import-order
 from test import QiskitTestCase  # pylint: disable=wrong-import-order
 
@@ -1101,6 +1101,18 @@ def test_expval_random_subsystem(self, num_qubits):
             target = Statevector(qc).expectation_value(op, qargs)
             self.assertAlmostEqual(exp_val, target)
 
+    def test_expval_sparsepauliop(self):
+        """Test expectation_value method of SparsePauliOp"""
+        cliff = random_clifford(num_qubits=3, seed=1234)
+        stab = StabilizerState(cliff)
+        labels = ["XXX", "IXI", "YYY", "III"]
+        coeffs = [3.0, 5.5, -1j, 23]
+        spp_op = SparsePauliOp.from_list(list(zip(labels, coeffs)))
+        expval = stab.expectation_value(spp_op)
+        qc = cliff.to_circuit()
+        target = Statevector(qc).expectation_value(spp_op)
+        self.assertAlmostEqual(expval, target)
+
     def test_stabilizer_bell_equiv(self):
         """Test that two circuits produce the same stabilizer group."""