Use only discrete-basis translations in GateDirection

qiskit/transpiler/passes/utils/gate_direction.py

@@ -21,7 +21,9 @@
 from qiskit.circuit import QuantumRegister, ControlFlowOp
 from qiskit.dagcircuit import DAGCircuit, DAGOpNode
 from qiskit.circuit.library.standard_gates import (
-    RYGate,
+    SGate,
+    SdgGate,
+    SXGate,
     HGate,
     CXGate,
     CZGate,
@@ -49,11 +51,14 @@ class GateDirection(TransformationPass):
         q_1: ┤ X ├      q_1: ┤ H ├──■──┤ H ├
              └───┘           └───┘     └───┘
 
-             ┌──────┐          ┌───────────┐┌──────┐┌───┐
-        q_0: ┤0     ├     q_0: ┤ RY(-pi/2) ├┤1     ├┤ H ├
-             │  ECR │  =       └┬──────────┤│  ECR │├───┤
-        q_1: ┤1     ├     q_1: ─┤ RY(pi/2) ├┤0     ├┤ H ├
-             └──────┘           └──────────┘└──────┘└───┘
+
+                          global phase: 3π/2
+             ┌──────┐           ┌───┐ ┌────┐┌─────┐┌──────┐┌───┐
+        q_0: ┤0     ├     q_0: ─┤ S ├─┤ √X ├┤ Sdg ├┤1     ├┤ H ├
+             │  ECR │  =       ┌┴───┴┐├────┤└┬───┬┘│  Ecr │├───┤
+        q_1: ┤1     ├     q_1: ┤ Sdg ├┤ √X ├─┤ S ├─┤0     ├┤ H ├
+             └──────┘          └─────┘└────┘ └───┘ └──────┘└───┘
+
 
              ┌──────┐          ┌───┐┌──────┐┌───┐
         q_0: ┤0     ├     q_0: ┤ H ├┤1     ├┤ H ├
@@ -90,11 +95,19 @@ def __init__(self, coupling_map, target=None):
         self._cx_dag.apply_operation_back(HGate(), [qr[0]], [])
         self._cx_dag.apply_operation_back(HGate(), [qr[1]], [])
 
+        # This is done in terms of less-efficient S/SX/Sdg gates instead of the more natural
+        # `RY(pi /2)` so we have a chance for basis translation to keep things in a discrete basis
+        # during resynthesis, if that's what's being asked for.
         self._ecr_dag = DAGCircuit()
         qr = QuantumRegister(2)
+        self._ecr_dag.global_phase = -pi / 2
         self._ecr_dag.add_qreg(qr)
-        self._ecr_dag.apply_operation_back(RYGate(-pi / 2), [qr[0]], [])
-        self._ecr_dag.apply_operation_back(RYGate(pi / 2), [qr[1]], [])
+        self._ecr_dag.apply_operation_back(SGate(), [qr[0]], [])
+        self._ecr_dag.apply_operation_back(SXGate(), [qr[0]], [])
+        self._ecr_dag.apply_operation_back(SdgGate(), [qr[0]], [])
+        self._ecr_dag.apply_operation_back(SdgGate(), [qr[1]], [])
+        self._ecr_dag.apply_operation_back(SXGate(), [qr[1]], [])
+        self._ecr_dag.apply_operation_back(SGate(), [qr[1]], [])
         self._ecr_dag.apply_operation_back(ECRGate(), [qr[1], qr[0]], [])
         self._ecr_dag.apply_operation_back(HGate(), [qr[0]], [])
         self._ecr_dag.apply_operation_back(HGate(), [qr[1]], [])

releasenotes/notes/discrete-basis-gatedirection-bdffad3b47c1c532.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    The :class:`.GateDirection` transpiler pass will now use discrete-basis translations rather than
+    relying on a continuous :class:`.RYGate`, which should help make some discrete-basis-set targets
+    slightly more reliable.  In general, :func:`.transpile` only has partial support for basis sets
+    that do not contain a continuously-parametrised operation, and so it may not always succeed in
+    these situations, and will almost certainly not produce optimal results.

test/python/compiler/test_transpiler.py

@@ -51,16 +51,20 @@
 from qiskit.circuit.library import (
     CXGate,
     CZGate,
+    ECRGate,
     HGate,
     RXGate,
     RYGate,
     RZGate,
+    SGate,
     SXGate,
+    SXdgGate,
+    SdgGate,
     U1Gate,
     U2Gate,
     UGate,
     XGate,
-    SGate,
+    ZGate,
 )
 from qiskit.circuit.measure import Measure
 from qiskit.compiler import transpile
@@ -417,6 +421,28 @@ def test_transpile_bell(self):
         circuits = transpile(qc, backend)
         self.assertIsInstance(circuits, QuantumCircuit)
 
+    def test_transpile_bell_discrete_basis(self):
+        """Test that it's possible to transpile a very simple circuit to a discrete stabiliser-like
+        basis.  In general, we do not make any guarantees about the possibility or quality of
+        transpilation in these situations, but this is at least useful as a check that stuff that
+        _could_ be possible remains so."""
+
+        target = Target(num_qubits=2)
+        for one_q in [XGate(), SXGate(), SXdgGate(), SGate(), SdgGate(), ZGate()]:
+            target.add_instruction(one_q, {(0,): None, (1,): None})
+        # This is only in one direction, and not the direction we're going to attempt to lay it out
+        # onto, so we can test the basis translation.
+        target.add_instruction(ECRGate(), {(1, 0): None})
+
+        qc = QuantumCircuit(2)
+        qc.h(0)
+        qc.cx(0, 1)
+
+        # Try with the initial layout in both directions to ensure we're dealing with the basis
+        # having only a single direction.
+        self.assertIsInstance(transpile(qc, target=target, initial_layout=[0, 1]), QuantumCircuit)
+        self.assertIsInstance(transpile(qc, target=target, initial_layout=[1, 0]), QuantumCircuit)
+
     def test_transpile_one(self):
         """Test transpile a single circuit.
 

test/python/transpiler/test_gate_direction.py

@@ -172,12 +172,16 @@ def test_ecr_flip(self):
         #       ├─────────┴┐│  Ecr │├───┤
         # qr_1: ┤ Ry(-π/2) ├┤1     ├┤ H ├
         #       └──────────┘└──────┘└───┘
-        expected = QuantumCircuit(qr)
-        expected.ry(pi / 2, qr[0])
-        expected.ry(-pi / 2, qr[1])
-        expected.ecr(qr[0], qr[1])
-        expected.h(qr[0])
-        expected.h(qr[1])
+        expected = QuantumCircuit(qr, global_phase=-pi / 2)
+        expected.s(1)
+        expected.sx(1)
+        expected.sdg(1)
+        expected.sdg(0)
+        expected.sx(0)
+        expected.s(0)
+        expected.ecr(0, 1)
+        expected.h(0)
+        expected.h(1)
 
         pass_ = GateDirection(coupling)
         after = pass_.run(dag)
@@ -405,8 +409,13 @@ def test_coupling_map_control_flow(self):
             expected.h([0, 1])
             expected.cx(0, 1)
             with expected.if_test((circuit.clbits[0], True)) as else_:
-                expected.ry(pi / 2, 2)
-                expected.ry(-pi / 2, 3)
+                expected.global_phase -= pi / 2
+                expected.sdg(2)
+                expected.sx(2)
+                expected.s(2)
+                expected.s(3)
+                expected.sx(3)
+                expected.sdg(3)
                 expected.ecr(2, 3)
                 expected.h([2, 3])
             with else_:
@@ -442,8 +451,13 @@ def test_target_control_flow(self):
             expected.h([0, 1])
             expected.cx(0, 1)
             with expected.if_test((circuit.clbits[0], True)) as else_:
-                expected.ry(pi / 2, 2)
-                expected.ry(-pi / 2, 3)
+                expected.global_phase -= pi / 2
+                expected.sdg(2)
+                expected.sx(2)
+                expected.s(2)
+                expected.s(3)
+                expected.sx(3)
+                expected.sdg(3)
                 expected.ecr(2, 3)
                 expected.h([2, 3])
             with else_: