fix: Allow circuits containing CnZ gates to run on AerBackend (#369)

docs/changelog.rst

@@ -13,6 +13,7 @@ Unreleased
 * Add conversion of controlled unitary gates from qiskit to tket.
 * Initialize `TketAutoPass` with a `BackendV2`.
 * Update `TketBackend` to derive from `BackendV2`.
+* Fix to allow `AerBackend` to work with multi-controlled Z gates.
 
 0.55.0 (July 2024)
 ------------------

pytket/extensions/qiskit/qiskit_convert.py

@@ -758,7 +758,9 @@ def append_tk_command_to_qiskit(
     if optype == OpType.CnY:
         return qcirc.append(qiskit_gates.YGate().control(len(qargs) - 1), qargs)
     if optype == OpType.CnZ:
-        return qcirc.append(qiskit_gates.ZGate().control(len(qargs) - 1), qargs)
+        new_gate = qiskit_gates.ZGate().control(len(qargs) - 1)
+        new_gate.name = "mcz"
+        return qcirc.append(new_gate, qargs)
     if optype == OpType.CnRy:
         # might as well do a bit more checking
         assert len(op.params) == 1

tests/backend_test.py

@@ -1498,3 +1498,17 @@ def test_noisy_density_matrix_simulation() -> None:
     assert noisy_dm.shape == (8, 8)
     # Check purity to verify mixed state
     assert np.trace(noisy_dm**2).real < 0.99
+
+
+def test_mc_gate_on_aer() -> None:
+    """Test for cm gates support in aer simulators
+    https://github.com/CQCL/pytket-qiskit/issues/368"""
+    b = AerBackend()
+    c = Circuit(3, 3)
+    c.X(0).X(1)
+    c.H(2)
+    c.add_gate(OpType.CnZ, [0, 1, 2])
+    c.H(2)
+    c.measure_all()
+    r = b.run_circuit(c, n_shots=10)
+    assert r.get_counts() == Counter({(1, 1, 1): 10})

tests/qiskit_convert_test.py

@@ -828,7 +828,7 @@ def test_multicontrolled_gate_conversion() -> None:
     assert my_tkc.n_gates_of_type(OpType.CnRy) == 2
     my_new_qc = tk_to_qiskit(my_tkc)
     qiskit_ops = my_new_qc.count_ops()
-    assert qiskit_ops["c3y"] and qiskit_ops["c3z"] and qiskit_ops["c3ry"] == 2
+    assert qiskit_ops["c3y"] and qiskit_ops["mcz"] and qiskit_ops["c3ry"] == 2
     tcirc = qiskit_to_tk(my_new_qc)
     unitary_after = tcirc.get_unitary()
     assert compare_unitaries(unitary_before, unitary_after)
@@ -1034,7 +1034,7 @@ def test_ccz_conversion() -> None:
     assert tkc_ccz.n_gates_of_type(OpType.CnZ) == tkc_ccz.n_gates == 2
     # bidirectional CnZ conversion already supported
     qc_ccz2 = tk_to_qiskit(tkc_ccz)
-    assert qc_ccz2.count_ops()["ccz"] == 2
+    assert qc_ccz2.count_ops()["mcz"] == 2
     tkc_ccz2 = qiskit_to_tk(qc_ccz2)
     assert compare_unitaries(tkc_ccz.get_unitary(), tkc_ccz2.get_unitary())