Fix cirq-to-stim conversion not adding TICKs for multi-moment singl…

…e-rep `CircuitOperation`s (#728)

glue/cirq/stimcirq/_cirq_to_stim.py

@@ -409,7 +409,9 @@ def __init__(self):
 
     def process_circuit_operation_into_repeat_block(self, op: cirq.CircuitOperation) -> None:
         if self.flatten or op.repetitions == 1:
-            self.process_operations(cirq.decompose_once(op))
+            moments = cirq.unroll_circuit_op(cirq.Circuit(op), deep=False, tags_to_check=None).moments
+            self.process_moments(moments)
+            self.out = self.out[:-1] # Remove a trailing TICK (to avoid double TICK)
             return
 
         child = CirqToStimHelper()

glue/cirq/stimcirq/_cirq_to_stim_test.py

@@ -339,6 +339,31 @@ def test_on_loop():
     result = stimcirq.StimSampler().run(c)
     assert result.measurements.keys() == {'0:a', '0:b', '1:a', '1:b', '2:a', '2:b'}
 
+
+def test_multi_moment_circuit_operation():
+    q0 = cirq.LineQubit(0)
+    cc = cirq.Circuit(
+        cirq.CircuitOperation(
+            cirq.FrozenCircuit(
+                cirq.Moment(cirq.H(q0)),
+                cirq.Moment(cirq.H(q0)),
+                cirq.Moment(cirq.H(q0)),
+                cirq.Moment(cirq.H(q0)),
+            )
+        )
+    )
+    assert stimcirq.cirq_circuit_to_stim_circuit(cc) == stim.Circuit("""
+        H 0
+        TICK
+        H 0
+        TICK
+        H 0
+        TICK
+        H 0
+        TICK
+    """)
+
+
 def test_on_tagged_loop():
     a, b = cirq.LineQubit.range(2)
     c = cirq.Circuit(