fix symplectic matrix + test

src/qibo/backends/clifford.py

@@ -173,7 +173,11 @@ def execute_circuit(self, circuit, initial_state=None, nshots: int = 1000):
             for gate in circuit.queue:
                 circuit_stim.append(gate.__class__.__name__, list(gate.qubits))
 
-            symplectic_matrix = self._stim.Tableau.from_circuit(circuit_stim)
+            quad_1, quad_2, quad_3, quad_4, x_phases, z_phases = (
+                self._stim.Tableau.from_circuit(circuit_stim).to_numpy()
+            )
+            symplectic_matrix = np.block([[quad_1, quad_2], [quad_3, quad_4]])
+            symplectic_matrix = np.c_[symplectic_matrix, np.r_[x_phases, z_phases]]
 
             return Clifford(
                 symplectic_matrix,

tests/test_backends_clifford.py

@@ -226,17 +226,20 @@ def test_set_backend(backend):
 def test_noise_channels(backend):
     clifford_bkd = construct_clifford_backend(backend)
 
-    nqubits = 3
-    c = random_clifford(nqubits, backend=backend)
-    c.density_matrix = True
-    c_copy = c.copy()
-    c.add(gates.M(*range(nqubits)))
-    c_copy.add(gates.M(*range(nqubits)))
     noise = NoiseModel()
     noise.add(PauliError([("X", 0.5)]), gates.X)
     noise.add(DepolarizingError(0.1), gates.CZ)
+
+    nqubits = 3
+
+    c = random_clifford(nqubits, density_matrix=True, backend=backend)
+    c.add(gates.M(*range(nqubits)))
+
+    c_copy = c.copy()
+
     c = noise.apply(c)
     c_copy = noise.apply(c_copy)
+
     numpy_result = numpy_bkd.execute_circuit(c)
     clifford_result = clifford_bkd.execute_circuit(c_copy)
 
@@ -245,3 +248,18 @@ def test_noise_channels(backend):
         clifford_result.probabilities(),
         atol=1e-1,
     )
+
+
+def test_stim(backend):
+    clifford_bkd = construct_clifford_backend(backend)
+    clifford_stim = CliffordBackend(engine="stim")
+
+    nqubits = 3
+    circuit = random_clifford(nqubits, backend=backend)
+
+    result_qibo = clifford_bkd.execute_circuit(circuit)
+    result_stim = clifford_stim.execute_circuit(circuit)
+
+    backend.assert_allclose(
+        result_stim.symplectic_matrix, result_qibo.symplectic_matrix
+    )