Fix BackendSamplerV2 when used with an empty classical register (#1558

)

This is a port of Qiskit/qiskit#12055
to this repository, since this repository has its own fork of
`BackendSamplerV2` until Qiskit 1.1 is released (#1523).

qiskit_ibm_runtime/qiskit/primitives/backend_sampler_v2.py

@@ -187,7 +187,10 @@ def _analyze_circuit(circuit: QuantumCircuit) -> tuple[list[_MeasureInfo], int]:
     for creg in circuit.cregs:
         name = creg.name
         num_bits = creg.size
-        start = circuit.find_bit(creg[0]).index
+        if num_bits != 0:
+            start = circuit.find_bit(creg[0]).index
+        else:
+            start = 0
         meas_info.append(
             _MeasureInfo(
                 creg_name=name,

test/unit/qiskit/test_backend_sampler_v2.py

@@ -655,6 +655,21 @@ def test_no_cregs(self, backend):
         self.assertEqual(len(result), 1)
         self.assertEqual(len(result[0].data), 0)
 
+    @combine(backend=BACKENDS)
+    def test_empty_creg(self, backend):
+        """Test that the sampler works if provided a classical register with no bits."""
+        # Test case for issue #12043
+        q = QuantumRegister(1, "q")
+        c1 = ClassicalRegister(0, "c1")
+        c2 = ClassicalRegister(1, "c2")
+        qc = QuantumCircuit(q, c1, c2)
+        qc.h(0)
+        qc.measure(0, 0)
+
+        sampler = BackendSamplerV2(backend=backend, options=self._options)
+        result = sampler.run([qc], shots=self._shots).result()
+        self.assertEqual(result[0].data.c1.array.shape, (self._shots, 0))
+
 
 if __name__ == "__main__":
     unittest.main()