Add missing CH gate (#624)

`CH` is defined in OpenQASM standard library, but was omitted in #592.

---------

Co-authored-by: Mateo Uldemolins <m.uldemolins@gmail.com>

Author: thierry-martinez

.github/qasm-parser-requirements.txt

@@ -1 +1,2 @@
-graphix-qasm-parser @ git+https://github.com/TeamGraphix/graphix-qasm-parser@refs/pull/19/head
+# Keep in sync with noxfile.py
+graphix-qasm-parser @ git+https://github.com/TeamGraphix/graphix-qasm-parser@refs/pull/20/head

CHANGELOG.md

@@ -27,6 +27,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     - `E` command in patterns.
   - The pretty-printing (`repr`) of instructions and commands is updated to use notation without parentheses: `repr(E((0, 1))) == "E(0, 1)"`.
 
+- #694: CH gate is now supported (its support was missing as it is a standard OpenQASM gate).
+
 ### Changed
 
 - #587:

graphix/instruction.py

@@ -64,6 +64,7 @@ class InstructionKind(Enum):
     SX = enum.auto()
     SXDG = enum.auto()
     CY = enum.auto()
+    CH = enum.auto()
     P = enum.auto()
     U = enum.auto()
     CJ = enum.auto()
@@ -317,6 +318,36 @@ class CY(_KindChecker, ControlledSingleTargetInstruction):
     kind: ClassVar[Literal[InstructionKind.CY]] = field(default=InstructionKind.CY, init=False)
 
 
+@dataclass(repr=False)
+class CH(_KindChecker, ControlledSingleTargetInstruction):
+    r"""CH circuit instruction.
+
+    The CH gate applies the matrix
+
+    .. math::
+
+      \left[\begin{matrix}
+        1 & 0 & 0 & 0\\
+        0 & 1 & 0 & 0\\
+        0 & 0 & \frac 1 {\sqrt 2} & \frac 1 {\sqrt 2}\\
+        0 & 0 & \frac 1 {\sqrt 2} & -\frac 1 {\sqrt 2}
+      \end{matrix}\right]
+
+    in the computational basis. The basis states use big-endian
+    ordering, with the most significant qubit first. The qubits are
+    numbered in the order ``control``, ``target``.
+
+    Attributes
+    ----------
+    control : int
+        Index of the control qubit.
+    target : int
+        Index of the target qubit.
+    """
+
+    kind: ClassVar[Literal[InstructionKind.CH]] = field(default=InstructionKind.CH, init=False)
+
+
 @dataclass(repr=False)
 class CNOT(_KindChecker, ControlledSingleTargetInstruction):
     r"""CNOT circuit instruction.
@@ -1153,6 +1184,7 @@ def visit(self, visitor: InstructionVisitor, *, copy: bool = False) -> GPHASE:
     | CNOT
     | CY
     | CZ
+    | CH
     | CCX
     | RZZ
     | SWAP
@@ -1219,6 +1251,7 @@ class Instruction:
     CNOT: TypeAlias = CNOT
     CY: TypeAlias = CY
     CZ: TypeAlias = CZ
+    CH: TypeAlias = CH
     CCX: TypeAlias = CCX
     RZZ: TypeAlias = RZZ
     SWAP: TypeAlias = SWAP
@@ -1236,6 +1269,7 @@ def __init__(self) -> None:
         Instruction.CNOT
         | Instruction.CY
         | Instruction.CZ
+        | Instruction.CH
         | Instruction.CJ
         | Instruction.CCX
         | Instruction.CRX

graphix/pattern.py

@@ -1515,6 +1515,7 @@ def to_circuit(self) -> Circuit:
         To extract a unitary circuit from a pattern, see :meth:`OpenGraph.to_circuit`.
         """
         from graphix import Circuit  # noqa: PLC0415 # Avoid circular imports
+        from graphix.transpiler import OutputKind  # noqa: PLC0415 # Avoid circular imports
 
         width = len(self.input_nodes)
         all_nodes = [*self.input_nodes, *(cmd.node for cmd in self.__seq if cmd.kind == CommandKind.N)]
@@ -1589,7 +1590,7 @@ def to_circuit(self) -> Circuit:
 
         active_qubit_idx = 0
         for qubit in range(circuit.nqubit):
-            if qubit in circuit.active_qubits:
+            if circuit._visitor.output_kind[qubit] == OutputKind.Qubit:
                 idx = sorted_idx.index(qubit)
                 if active_qubit_idx != idx:
                     circuit.swap(qubit, sorted_idx[active_qubit_idx])

graphix/qasm3_exporter.py

@@ -172,6 +172,8 @@ def instruction_to_qasm3(instruction: InstructionType) -> Iterable[str]:
             yield qasm3_gate_call("cx", [qasm3_qubit(instruction.control), qasm3_qubit(instruction.target)])
         case InstructionKind.CY:
             yield qasm3_gate_call("cy", [qasm3_qubit(instruction.control), qasm3_qubit(instruction.target)])
+        case InstructionKind.CH:
+            yield qasm3_gate_call("ch", [qasm3_qubit(instruction.control), qasm3_qubit(instruction.target)])
         case InstructionKind.SWAP:
             yield qasm3_gate_call("swap", [qasm3_qubit(instruction.targets[i]) for i in (0, 1)])
         case InstructionKind.CSWAP: