Add warning when converting global phase operator to OpenQASM 2.0

cirq-core/cirq/circuits/qasm_output.py

@@ -293,7 +293,12 @@ def output(text):
                 output(f'creg {meas_id}[{len(meas.qubits)}];\n')
             else:
                 output(f'creg {meas_id}[{len(meas.qubits)}];  // Measurement: {comment}\n')
-        output_line_gap(2)
+        # In OpenQASM 2.0, the transformation of global phase gates is ignored.
+        # Therefore, no newline is created when the operations contained in a circuit consist only of global phase gates.
+        if not (
+            len(self.operations) == 1 and isinstance(self.operations[0].gate, ops.GlobalPhaseGate)
+        ):
+            output_line_gap(2)
 
         # Operations
         self._write_operations(self.operations, output, output_line_gap)

cirq-core/cirq/circuits/qasm_output_test.py

@@ -191,6 +191,19 @@ def test_h_gate_with_parameter():
     )
 
 
+def test_qasm_global_pahse():
+    output = cirq.QasmOutput((cirq.global_phase_operation(np.exp(1j * 5))), ())
+    assert (
+        str(output)
+        == """OPENQASM 2.0;
+include "qelib1.inc";
+
+
+// Qubits: []
+"""
+    )
+
+
 def test_precision():
     (q0,) = _make_qubits(1)
     output = cirq.QasmOutput((cirq.X(q0) ** 0.1234567,), (q0,), precision=3)

cirq-core/cirq/ops/gate_operation.py

@@ -15,6 +15,7 @@
 """Basic types defining qubits, gates, and operations."""
 
 import re
+import warnings
 from typing import (
     AbstractSet,
     Any,
@@ -35,7 +36,7 @@
 
 import numpy as np
 
-from cirq import protocols, value
+from cirq import ops, protocols, value
 from cirq.ops import raw_types, gate_features, control_values as cv
 from cirq.type_workarounds import NotImplementedType
 
@@ -348,6 +349,14 @@ def __rmul__(self, other: Any) -> Any:
         return self.gate._rmul_with_qubits(self._qubits, other)
 
     def _qasm_(self, args: 'protocols.QasmArgs') -> Optional[str]:
+        if isinstance(self.gate, ops.GlobalPhaseGate):
+            warnings.warn(
+                "OpenQASM 2.0 does not support global phase."
+                "Since the global phase does not affect the measurement results, "
+                "the conversion to QASM is disregarded."
+            )
+            return ""
+
         return protocols.qasm(self.gate, args=args, qubits=self.qubits, default=None)
 
     def _equal_up_to_global_phase_(

cirq-core/cirq/testing/consistent_qasm.py

@@ -42,6 +42,7 @@ def assert_qasm_is_consistent_with_unitary(val: Any):
     if isinstance(val, ops.Operation):
         qubits: Sequence[ops.Qid] = val.qubits
         op = val
+        gate = val.gate
     elif isinstance(val, ops.Gate):
         qid_shape = protocols.qid_shape(val)
         remaining_shape = list(qid_shape)
@@ -52,9 +53,14 @@ def assert_qasm_is_consistent_with_unitary(val: Any):
                     remaining_shape.pop(i)
         qubits = devices.LineQid.for_qid_shape(remaining_shape)
         op = val.on(*qubits)
+        gate = val
     else:
         raise NotImplementedError(f"Don't know how to test {val!r}")
 
+    if isinstance(gate, ops.GlobalPhaseGate):
+        # OpenQASM 2.0 does not support global phase gates.
+        return
+
     args = protocols.QasmArgs(qubit_id_map={q: f'q[{i}]' for i, q in enumerate(qubits)})
     qasm = protocols.qasm(op, args=args, default=None)
     if qasm is None: