Support dynamic circuit in BackendEstimator (#9700)

* Support dynamic circuit in Estimator

* fix lint

* revert the validation condition

* fix test

* add reno

* separate measurements

* lint

* add test and bug fix reno

* fix reno

* revise tests and fix typo

---------

Co-authored-by: Takashi Imamichi <[email protected]>

Author: ikkoham

qiskit/primitives/backend_estimator.py

@@ -22,8 +22,9 @@
 
 import numpy as np
 
-from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import ClassicalRegister, QuantumCircuit, QuantumRegister
 from qiskit.compiler import transpile
+from qiskit.exceptions import QiskitError
 from qiskit.providers import BackendV1, BackendV2, Options
 from qiskit.quantum_info import Pauli, PauliList
 from qiskit.quantum_info.operators.base_operator import BaseOperator
@@ -216,10 +217,18 @@ def _transpile(self):
             transpiled_circuits = []
             for diff_circuit in diff_circuits:
                 transpiled_circuit_copy = transpiled_circuit.copy()
-                for creg in diff_circuit.cregs:
-                    if creg not in transpiled_circuit_copy.cregs:
-                        transpiled_circuit_copy.add_register(creg)
-                transpiled_circuit_copy.compose(diff_circuit, inplace=True)
+                # diff_circuit is supposed to have a classical register whose name is different from
+                # those of the transpiled_circuit
+                clbits = diff_circuit.cregs[0]
+                for creg in transpiled_circuit_copy.cregs:
+                    if clbits.name == creg.name:
+                        raise QiskitError(
+                            "Classical register for measurements conflict with those of the input "
+                            f"circuit: {clbits}. "
+                            "Recommended to avoid register names starting with '__'."
+                        )
+                transpiled_circuit_copy.add_register(clbits)
+                transpiled_circuit_copy.compose(diff_circuit, clbits=clbits, inplace=True)
                 transpiled_circuit_copy.metadata = diff_circuit.metadata
                 transpiled_circuits.append(transpiled_circuit_copy)
             self._transpiled_circuits += transpiled_circuits
@@ -298,7 +307,9 @@ def _measurement_circuit(num_qubits: int, pauli: Pauli):
         qubit_indices = np.arange(pauli.num_qubits)[pauli.z | pauli.x]
         if not np.any(qubit_indices):
             qubit_indices = [0]
-        meas_circuit = QuantumCircuit(num_qubits, len(qubit_indices))
+        meas_circuit = QuantumCircuit(
+            QuantumRegister(num_qubits, "q"), ClassicalRegister(len(qubit_indices), f"__c_{pauli}")
+        )
         for clbit, i in enumerate(qubit_indices):
             if pauli.x[i]:
                 if pauli.z[i]:
@@ -432,7 +443,8 @@ def _pauli_expval_with_variance(counts: Counts, paulis: PauliList) -> tuple[np.n
     expvals = np.zeros(size, dtype=float)
     denom = 0  # Total shots for counts dict
     for bin_outcome, freq in counts.items():
-        outcome = int(bin_outcome, 2)
+        split_outcome = bin_outcome.split(" ", 1)[0] if " " in bin_outcome else bin_outcome
+        outcome = int(split_outcome, 2)
         denom += freq
         for k in range(size):
             coeff = (-1) ** _parity(diag_inds[k] & outcome)

releasenotes/notes/primitives-dynamic-circuit-b489f82df47af138.yaml

@@ -0,0 +1,3 @@
+---
+fixes:
+    Fix the bug :class:`.BackendEstimator` fails if circuits have classical registers.

test/python/primitives/test_backend_estimator.py

@@ -113,12 +113,12 @@ def test_estimator_run_no_params(self, backend):
         self.assertIsInstance(result, EstimatorResult)
         np.testing.assert_allclose(result.values, [-1.284366511861733], rtol=0.05)
 
-    @combine(backend=BACKENDS)
-    def test_run_1qubit(self, backend):
+    @combine(backend=BACKENDS, creg=[True, False])
+    def test_run_1qubit(self, backend, creg):
         """Test for 1-qubit cases"""
         backend.set_options(seed_simulator=123)
-        qc = QuantumCircuit(1)
-        qc2 = QuantumCircuit(1)
+        qc = QuantumCircuit(1, 1) if creg else QuantumCircuit(1)
+        qc2 = QuantumCircuit(1, 1) if creg else QuantumCircuit(1)
         qc2.x(0)
 
         op = SparsePauliOp.from_list([("I", 1)])
@@ -141,12 +141,12 @@ def test_run_1qubit(self, backend):
         self.assertIsInstance(result, EstimatorResult)
         np.testing.assert_allclose(result.values, [-1], rtol=0.1)
 
-    @combine(backend=BACKENDS)
-    def test_run_2qubits(self, backend):
+    @combine(backend=BACKENDS, creg=[True, False])
+    def test_run_2qubits(self, backend, creg):
         """Test for 2-qubit cases (to check endian)"""
         backend.set_options(seed_simulator=123)
-        qc = QuantumCircuit(2)
-        qc2 = QuantumCircuit(2)
+        qc = QuantumCircuit(2, 1) if creg else QuantumCircuit(2)
+        qc2 = QuantumCircuit(2, 1) if creg else QuantumCircuit(2, 1)
         qc2.x(0)
 
         op = SparsePauliOp.from_list([("II", 1)])
@@ -191,8 +191,6 @@ def test_run_errors(self, backend):
         est = BackendEstimator(backend=backend)
         with self.assertRaises(ValueError):
             est.run([qc], [op2], [[]]).result()
-        with self.assertRaises(ValueError):
-            est.run([qc2], [op], [[]]).result()
         with self.assertRaises(ValueError):
             est.run([qc], [op], [[1e4]]).result()
         with self.assertRaises(ValueError):
@@ -270,10 +268,6 @@ def max_circuits(self):
 
         backend = FakeNairobiLimitedCircuits()
         backend.set_options(seed_simulator=123)
-        qc = QuantumCircuit(1)
-        qc2 = QuantumCircuit(1)
-        qc2.x(0)
-        backend.set_options(seed_simulator=123)
         qc = RealAmplitudes(num_qubits=2, reps=2)
         op = SparsePauliOp.from_list([("IZ", 1), ("XI", 2), ("ZY", -1)])
         k = 5
@@ -385,6 +379,45 @@ def test_layout(self, backend):
             else:
                 self.assertEqual(value, -1)
 
+    @unittest.skipUnless(optionals.HAS_AER, "qiskit-aer is required to run this test")
+    def test_circuit_with_measurement(self):
+        """Test estimator with a dynamic circuit"""
+        from qiskit_aer import AerSimulator
+
+        bell = QuantumCircuit(2)
+        bell.h(0)
+        bell.cx(0, 1)
+        bell.measure_all()
+        observable = SparsePauliOp("ZZ")
+
+        backend = AerSimulator()
+        backend.set_options(seed_simulator=15)
+        estimator = BackendEstimator(backend, skip_transpilation=True)
+        estimator.set_transpile_options(seed_transpiler=15)
+        result = estimator.run(bell, observable).result()
+        self.assertAlmostEqual(result.values[0], 1, places=1)
+
+    @unittest.skipUnless(optionals.HAS_AER, "qiskit-aer is required to run this test")
+    def test_dynamic_circuit(self):
+        """Test estimator with a dynamic circuit"""
+        from qiskit_aer import AerSimulator
+
+        qc = QuantumCircuit(2, 1)
+        with qc.for_loop(range(5)):
+            qc.h(0)
+            qc.cx(0, 1)
+            qc.measure(1, 0)
+            qc.break_loop().c_if(0, True)
+
+        observable = SparsePauliOp("IZ")
+
+        backend = AerSimulator()
+        backend.set_options(seed_simulator=15)
+        estimator = BackendEstimator(backend, skip_transpilation=True)
+        estimator.set_transpile_options(seed_transpiler=15)
+        result = estimator.run(qc, observable).result()
+        self.assertAlmostEqual(result.values[0], 0, places=1)
+
 
 if __name__ == "__main__":
     unittest.main()

test/python/primitives/test_estimator.py

@@ -225,8 +225,6 @@ def test_run_errors(self):
         est = Estimator()
         with self.assertRaises(ValueError):
             est.run([qc], [op2], [[]]).result()
-        with self.assertRaises(ValueError):
-            est.run([qc2], [op], [[]]).result()
         with self.assertRaises(ValueError):
             est.run([qc], [op], [[1e4]]).result()
         with self.assertRaises(ValueError):