Add missing equivalences for XX{Plus,Minus}YYGate (#9017)

* Add missing equivalences for XX{Plus,Minus}YYGate

These are just copied from the `_define` method on the classes.  The
equivalence-library tests are updated (since `subTest` doesn't isolate
correctly with `testtools`) to print out all the names of the tests
being run, so it's clear that the existing test now tests these
definitions.

* Use default parameters to beat the scoping weirdness

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

qiskit/circuit/library/standard_gates/equivalence_library.py

@@ -64,6 +64,8 @@ from . import (
     ZGate,
     CZGate,
     CCZGate,
+    XXPlusYYGate,
+    XXMinusYYGate,
 )
 
 
@@ -1315,3 +1317,73 @@ h_to_rr.append(RGate(theta=pi / 2, phi=pi / 2), [q[0]])
 h_to_rr.append(RGate(theta=pi, phi=0), [q[0]])
 h_to_rr.global_phase = pi / 2
 _sel.add_equivalence(HGate(), h_to_rr)
+
+# XXPlusYYGate
+# ┌───────────────┐
+# ┤0              ├
+# │  {XX+YY}(θ,β) │
+# ┤1              ├
+# └───────────────┘
+#    ┌───────┐  ┌───┐            ┌───┐┌────────────┐┌───┐  ┌─────┐   ┌────────────┐
+#   ─┤ Rz(β) ├──┤ S ├────────────┤ X ├┤ Ry(-0.5*θ) ├┤ X ├──┤ Sdg ├───┤ Rz(-1.0*β) ├───────────
+# ≡ ┌┴───────┴─┐├───┴┐┌─────────┐└─┬─┘├────────────┤└─┬─┘┌─┴─────┴──┐└──┬──────┬──┘┌─────────┐
+#   ┤ Rz(-π/2) ├┤ √X ├┤ Rz(π/2) ├──■──┤ Ry(-0.5*θ) ├──■──┤ Rz(-π/2) ├───┤ √Xdg ├───┤ Rz(π/2) ├
+#   └──────────┘└────┘└─────────┘     └────────────┘     └──────────┘   └──────┘   └─────────┘
+q = QuantumRegister(2, "q")
+xxplusyy = QuantumCircuit(q)
+beta = Parameter("beta")
+theta = Parameter("theta")
+rules = [
+    (RZGate(beta), [q[0]], []),
+    (RZGate(-pi / 2), [q[1]], []),
+    (SXGate(), [q[1]], []),
+    (RZGate(pi / 2), [q[1]], []),
+    (SGate(), [q[0]], []),
+    (CXGate(), [q[1], q[0]], []),
+    (RYGate(-theta / 2), [q[1]], []),
+    (RYGate(-theta / 2), [q[0]], []),
+    (CXGate(), [q[1], q[0]], []),
+    (SdgGate(), [q[0]], []),
+    (RZGate(-pi / 2), [q[1]], []),
+    (SXdgGate(), [q[1]], []),
+    (RZGate(pi / 2), [q[1]], []),
+    (RZGate(-beta), [q[0]], []),
+]
+for instr, qargs, cargs in rules:
+    xxplusyy._append(instr, qargs, cargs)
+_sel.add_equivalence(XXPlusYYGate(theta, beta), xxplusyy)
+
+# XXMinusYYGate
+# ┌───────────────┐
+# ┤0              ├
+# │  {XX-YY}(θ,β) │
+# ┤1              ├
+# └───────────────┘
+#    ┌──────────┐ ┌────┐┌─────────┐      ┌─────────┐       ┌──────────┐ ┌──────┐┌─────────┐
+#   ─┤ Rz(-π/2) ├─┤ √X ├┤ Rz(π/2) ├──■───┤ Ry(θ/2) ├────■──┤ Rz(-π/2) ├─┤ √Xdg ├┤ Rz(π/2) ├
+# ≡ ┌┴──────────┴┐├───┬┘└─────────┘┌─┴─┐┌┴─────────┴─┐┌─┴─┐└─┬─────┬──┘┌┴──────┤└─────────┘
+#   ┤ Rz(-1.0*β) ├┤ S ├────────────┤ X ├┤ Ry(-0.5*θ) ├┤ X ├──┤ Sdg ├───┤ Rz(β) ├───────────
+#   └────────────┘└───┘            └───┘└────────────┘└───┘  └─────┘   └───────┘
+q = QuantumRegister(2, "q")
+xxminusyy = QuantumCircuit(q)
+beta = Parameter("beta")
+theta = Parameter("theta")
+rules = [
+    (RZGate(-beta), [q[1]], []),
+    (RZGate(-pi / 2), [q[0]], []),
+    (SXGate(), [q[0]], []),
+    (RZGate(pi / 2), [q[0]], []),
+    (SGate(), [q[1]], []),
+    (CXGate(), [q[0], q[1]], []),
+    (RYGate(theta / 2), [q[0]], []),
+    (RYGate(-theta / 2), [q[1]], []),
+    (CXGate(), [q[0], q[1]], []),
+    (SdgGate(), [q[1]], []),
+    (RZGate(-pi / 2), [q[0]], []),
+    (SXdgGate(), [q[0]], []),
+    (RZGate(pi / 2), [q[0]], []),
+    (RZGate(beta), [q[1]], []),
+]
+for instr, qargs, cargs in rules:
+    xxminusyy._append(instr, qargs, cargs)
+_sel.add_equivalence(XXMinusYYGate(theta, beta), xxminusyy)

releasenotes/notes/add-xxyy-equivalence-a941c9b9bc60747b.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Equivalence rules for :class:`.XXPlusYYGate` and :class:`.XXMinusYYGate` are
+    now available in the standard equivalence library shipped with Qiskit Terra.

test/python/circuit/test_gate_definitions.py

@@ -16,7 +16,7 @@
 import inspect
 
 import numpy as np
-from ddt import ddt, data, unpack
+from ddt import ddt, data, idata, unpack
 
 from qiskit import QuantumCircuit, QuantumRegister
 from qiskit.quantum_info import Operator
@@ -256,63 +256,57 @@ class TestStandardGates(QiskitTestCase):
             self.assertTrue(is_identity_matrix(Operator(gate).dot(gate.inverse().definition).data))
 
 
+@ddt
 class TestGateEquivalenceEqual(QiskitTestCase):
     """Test the decomposition of a gate in terms of other gates
     yields the same matrix as the hardcoded matrix definition."""
 
-    @classmethod
-    def setUpClass(cls):
-        super().setUpClass()
-        class_list = Gate.__subclasses__() + ControlledGate.__subclasses__()
-        exclude = {
-            "ControlledGate",
-            "DiagonalGate",
-            "UCGate",
-            "MCGupDiag",
-            "MCU1Gate",
-            "UnitaryGate",
-            "HamiltonianGate",
-            "MCPhaseGate",
-            "UCPauliRotGate",
-            "SingleQubitUnitary",
-            "MCXGate",
-            "VariadicZeroParamGate",
-            "ClassicalFunction",
-            "ClassicalElement",
-            "StatePreparation",
-            "LinearFunction",
-            "Commuting2qBlock",
-        }
-        cls._gate_classes = []
-        for aclass in class_list:
-            if aclass.__name__ not in exclude:
-                cls._gate_classes.append(aclass)
-
-    def test_equivalence_phase(self):
+    class_list = Gate.__subclasses__() + ControlledGate.__subclasses__()
+    exclude = {
+        "ControlledGate",
+        "DiagonalGate",
+        "UCGate",
+        "MCGupDiag",
+        "MCU1Gate",
+        "UnitaryGate",
+        "HamiltonianGate",
+        "MCPhaseGate",
+        "UCPauliRotGate",
+        "SingleQubitUnitary",
+        "MCXGate",
+        "VariadicZeroParamGate",
+        "ClassicalFunction",
+        "ClassicalElement",
+        "StatePreparation",
+        "LinearFunction",
+        "Commuting2qBlock",
+        "PauliEvolutionGate",
+    }
+    # Amazingly, Python's scoping rules for class bodies means that this is the closest we can get
+    # to a "natural" comprehension or functional iterable definition:
+    #   https://docs.python.org/3/reference/executionmodel.html#resolution-of-names
+    @idata(filter(lambda x, exclude=exclude: x.__name__ not in exclude, class_list))
+    def test_equivalence_phase(self, gate_class):
         """Test that the equivalent circuits from the equivalency_library
         have equal matrix representations"""
-        for gate_class in self._gate_classes:
-            with self.subTest(i=gate_class):
-                n_params = len(_get_free_params(gate_class))
-                params = [0.1 * i for i in range(1, n_params + 1)]
-                if gate_class.__name__ == "RXXGate":
-                    params = [np.pi / 2]
-                if gate_class.__name__ in ["MSGate"]:
-                    params[0] = 2
-                if gate_class.__name__ in ["PauliGate"]:
-                    params = ["IXYZ"]
-                if gate_class.__name__ in ["BooleanExpression"]:
-                    params = ["x | y"]
-                if gate_class.__name__ in ["PauliEvolutionGate", "PauliEvolutionGate"]:
-                    continue
+        n_params = len(_get_free_params(gate_class))
+        params = [0.1 * i for i in range(1, n_params + 1)]
+        if gate_class.__name__ == "RXXGate":
+            params = [np.pi / 2]
+        if gate_class.__name__ in ["MSGate"]:
+            params[0] = 2
+        if gate_class.__name__ in ["PauliGate"]:
+            params = ["IXYZ"]
+        if gate_class.__name__ in ["BooleanExpression"]:
+            params = ["x | y"]
 
-                gate = gate_class(*params)
-                equiv_lib_list = std_eqlib.get_entry(gate)
-                for ieq, equivalency in enumerate(equiv_lib_list):
-                    with self.subTest(msg=gate.name + "_" + str(ieq)):
-                        op1 = Operator(gate)
-                        op2 = Operator(equivalency)
-                        self.assertEqual(op1, op2)
+        gate = gate_class(*params)
+        equiv_lib_list = std_eqlib.get_entry(gate)
+        for ieq, equivalency in enumerate(equiv_lib_list):
+            with self.subTest(msg=gate.name + "_" + str(ieq)):
+                op1 = Operator(gate)
+                op2 = Operator(equivalency)
+                self.assertEqual(op1, op2)
 
 
 @ddt