Merge pull request #1422 from qiboteam/unroller_cnot

Add `gates.CNOT` as a native gate in gate decomposition

src/qibo/transpiler/decompositions.py

@@ -387,6 +387,12 @@ def _u3_to_gpi2(t, p, l):
     lambda gate: two_qubit_decomposition(0, 1, gate.matrix(backend), backend=backend),
 )
 
+# temporary CNOT decompositions for CNOT, CZ, SWAP
+cnot_dec_temp = GateDecompositions()
+cnot_dec_temp.add(gates.CNOT, [gates.CNOT(0, 1)])
+cnot_dec_temp.add(gates.CZ, [gates.H(1), gates.CNOT(0, 1), gates.H(1)])
+cnot_dec_temp.add(gates.SWAP, [gates.CNOT(0, 1), gates.CNOT(1, 0), gates.CNOT(0, 1)])
+
 # register other optimized gate decompositions
 opt_dec = GateDecompositions()
 opt_dec.add(

src/qibo/transpiler/unroller.py

@@ -4,7 +4,14 @@
 from qibo.config import raise_error
 from qibo.models import Circuit
 from qibo.transpiler._exceptions import DecompositionError
-from qibo.transpiler.decompositions import cz_dec, gpi2_dec, iswap_dec, opt_dec, u3_dec
+from qibo.transpiler.decompositions import (
+    cnot_dec_temp,
+    cz_dec,
+    gpi2_dec,
+    iswap_dec,
+    opt_dec,
+    u3_dec,
+)
 
 
 class NativeGates(Flag):
@@ -22,6 +29,7 @@ class NativeGates(Flag):
         - :class:`qibo.gates.gates.U3`
         - :class:`qibo.gates.gates.CZ`
         - :class:`qibo.gates.gates.iSWAP`
+        - :class:`qibo.gates.gates.CNOT`
     """
 
     I = auto()
@@ -32,6 +40,7 @@ class NativeGates(Flag):
     U3 = auto()
     CZ = auto()
     iSWAP = auto()
+    CNOT = auto()  # For testing purposes
 
     @classmethod
     def default(cls):
@@ -240,6 +249,13 @@ def _translate_two_qubit_gates(gate: gates.Gate, native_gates: NativeGates):
                 iswap_decomposed.append(g_translated)
         return iswap_decomposed
 
+    # For testing purposes
+    # No CZ, iSWAP gates in the native gate set
+    # Decompose CNOT, CZ, SWAP gates into CNOT gates
+    if native_gates & NativeGates.CNOT:
+        return cnot_dec_temp(gate)
+
     raise_error(
-        DecompositionError, "Use only CZ and/or iSWAP as native gates"
+        DecompositionError,
+        "Use only CZ and/or iSWAP as native gates. CNOT is allowed in circuits where the two-qubit gates are limited to CZ, CNOT, and SWAP.",
     )  # pragma: no cover

tests/test_transpiler_unroller.py

@@ -121,3 +121,36 @@ def test_measurements_non_comp_basis():
     assert isinstance(transpiled_circuit.queue[2], gates.M)
     # After transpiling the measurement gate should be in the computational basis
     assert transpiled_circuit.queue[2].basis == []
+
+
+def test_temp_cnot_decomposition():
+    from qibo.transpiler.pipeline import Passes
+
+    circ = Circuit(2)
+    circ.add(gates.H(0))
+    circ.add(gates.CNOT(0, 1))
+    circ.add(gates.SWAP(0, 1))
+    circ.add(gates.CZ(0, 1))
+    circ.add(gates.M(0, 1))
+
+    glist = [gates.GPI2, gates.RZ, gates.Z, gates.M, gates.CNOT]
+    native_gates = NativeGates(0).from_gatelist(glist)
+
+    custom_pipeline = Passes([Unroller(native_gates=native_gates)])
+    transpiled_circuit, _ = custom_pipeline(circ)
+
+    # H
+    assert transpiled_circuit.queue[0].name == "z"
+    assert transpiled_circuit.queue[1].name == "gpi2"
+    # CNOT
+    assert transpiled_circuit.queue[2].name == "cx"
+    # SWAP
+    assert transpiled_circuit.queue[3].name == "cx"
+    assert transpiled_circuit.queue[4].name == "cx"
+    assert transpiled_circuit.queue[5].name == "cx"
+    # CZ
+    assert transpiled_circuit.queue[6].name == "z"
+    assert transpiled_circuit.queue[7].name == "gpi2"
+    assert transpiled_circuit.queue[8].name == "cx"
+    assert transpiled_circuit.queue[9].name == "z"
+    assert transpiled_circuit.queue[10].name == "gpi2"