Handle qudits in drop_terminal_measurements (#6879)

* Only apply X's, not I's, for invert_mask in drop_terminal_measurements

This allows drop_terminal_measurements to work for qudits, which failed on the `I` application, given invert_mask values are undefined for them anyway.

* Revive IdentityGate but with explicit dimension, to fix bug where the qubit was being removed completely if a terminal measurement was the only thing it contained.

Author: daxfohl

cirq-core/cirq/transformers/measurement_transformers.py

@@ -292,7 +292,20 @@ def drop_terminal_measurements(
     def flip_inversion(op: 'cirq.Operation', _) -> 'cirq.OP_TREE':
         if isinstance(op.gate, ops.MeasurementGate):
             return [
-                ops.X(q) if b else ops.I(q) for q, b in zip(op.qubits, op.gate.full_invert_mask())
+                (
+                    (ops.X if b else ops.I)
+                    if q.dimension == 2
+                    else (
+                        ops.MatrixGate(
+                            # Per SimulationState.measure(), swap 0,1 but leave other dims alone
+                            np.eye(q.dimension)[[1, 0, *range(2, q.dimension)]],
+                            qid_shape=(q.dimension,),
+                        )
+                        if b
+                        else ops.IdentityGate(qid_shape=(q.dimension,))
+                    )
+                ).on(q)
+                for q, b in zip(op.qubits, op.gate.full_invert_mask())
             ]
         return op
 

cirq-core/cirq/transformers/measurement_transformers_test.py

@@ -759,6 +759,41 @@ def test_drop_terminal():
     )
 
 
+def test_drop_terminal_qudit():
+    q0, q1 = cirq.LineQid.range(2, dimension=3)
+    circuit = cirq.Circuit(
+        cirq.CircuitOperation(cirq.FrozenCircuit(cirq.measure(q0, q1, key='m', invert_mask=[0, 1])))
+    )
+    dropped = cirq.drop_terminal_measurements(circuit)
+    expected_inversion_matrix = np.array([[0, 1, 0], [1, 0, 0], [0, 0, 1]])
+    cirq.testing.assert_same_circuits(
+        dropped,
+        cirq.Circuit(
+            cirq.CircuitOperation(
+                cirq.FrozenCircuit(
+                    cirq.IdentityGate(qid_shape=(3,)).on(q0),
+                    cirq.MatrixGate(expected_inversion_matrix, qid_shape=(3,)).on(q1),
+                )
+            )
+        ),
+    )
+    # Verify behavior equivalent to simulator (invert_mask swaps 0,1 but leaves 2 alone)
+    dropped.append(cirq.measure(q0, q1, key='m'))
+    sim = cirq.Simulator()
+    c0 = sim.simulate(circuit, initial_state=[0, 0])
+    d0 = sim.simulate(dropped, initial_state=[0, 0])
+    assert np.all(c0.measurements['m'] == [0, 1])
+    assert np.all(d0.measurements['m'] == [0, 1])
+    c1 = sim.simulate(circuit, initial_state=[1, 1])
+    d1 = sim.simulate(dropped, initial_state=[1, 1])
+    assert np.all(c1.measurements['m'] == [1, 0])
+    assert np.all(d1.measurements['m'] == [1, 0])
+    c2 = sim.simulate(circuit, initial_state=[2, 2])
+    d2 = sim.simulate(dropped, initial_state=[2, 2])
+    assert np.all(c2.measurements['m'] == [2, 2])
+    assert np.all(d2.measurements['m'] == [2, 2])
+
+
 def test_drop_terminal_nonterminal_error():
     q0, q1 = cirq.LineQubit.range(2)
     circuit = cirq.Circuit(