Fix LinCombSamplerGradient to leverage AerSampler (#9258)

* fix LinCombSamplerGradient

* fix lib_comb circuit

* add reno

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/algorithms/gradients/lin_comb_sampler_gradient.py

@@ -114,8 +114,14 @@ def _run(
             n = 2 ** circuits[i].num_qubits
             grad_dists = np.zeros((len(metadata_[i]["parameters"]), n))
             for idx, coeff, dist in zip(result_indices_all[i], coeffs_all[i], result.quasi_dists):
-                grad_dists[idx][list(dist.keys())[:n]] += np.array(list(dist.values())[:n]) * coeff
-                grad_dists[idx][list(dist.keys())[:n]] -= np.array(list(dist.values())[n:]) * coeff
+                plus = {key: val for key, val in dist.items() if key < n}
+                minus = {key - n: val for key, val in dist.items() if key >= n}
+                grad_dists[idx][list(plus.keys())] += (
+                    np.fromiter(plus.values(), dtype=float) * coeff
+                )
+                grad_dists[idx][list(minus.keys())] -= (
+                    np.fromiter(minus.values(), dtype=float) * coeff
+                )
 
             gradient_ = []
             for grad_dist in grad_dists:

qiskit/algorithms/gradients/utils.py

@@ -318,10 +318,10 @@ def _make_lin_comb_gradient_circuit(
     ]
 
     circuit2 = transpile(circuit, basis_gates=supported_gates, optimization_level=0)
-    qr_aux = QuantumRegister(1, "aux")
-    cr_aux = ClassicalRegister(1, "aux")
+    qr_aux = QuantumRegister(1, "qr_aux")
+    cr_aux = ClassicalRegister(1, "cr_aux")
     circuit2.add_register(qr_aux)
-    circuit2.add_bits(cr_aux)
+    circuit2.add_register(cr_aux)
     circuit2.h(qr_aux)
     circuit2.data.insert(0, circuit2.data.pop())
     circuit2.sdg(qr_aux)

releasenotes/notes/fix-libcomb-sampler-gradient-d759d6b0e2659abe.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed an issue of :class:`~.LinCombSamplerGradient` that raises an error
+    with Aer Sampler.