Foldability check includes a check for inverse (unitaryfund#939)

* cirq inverse * inverse ry test * format * Update mitiq/zne/scaling/folding.py Co-authored-by: Andrea Mari <[email protected]> * terminal measurements Co-authored-by: Andrea Mari <[email protected]>
ryanhill1 · Oct 6, 2021 · 11c6c68 · 11c6c68
1 parent 0f57a04
commit 11c6c68
Show file tree

Hide file tree

Showing 2 changed files with 89 additions and 6 deletions.
diff --git a/mitiq/zne/scaling/folding.py b/mitiq/zne/scaling/folding.py
@@ -67,10 +67,14 @@ def _check_foldable(circuit: Circuit) -> None:
         )
 
     if not has_unitary(circuit):
-        raise UnfoldableCircuitError(
-            "Circuit contains non-unitary channels which are not terminal "
-            "measurements and cannot be folded."
-        )
+        circ_measurements = _pop_measurements(circuit)
+        if inverse(circuit, default=None) is None:
+            raise UnfoldableCircuitError(
+                "Circuit contains non-invertible channels which are not"
+                "terminal measurements and cannot be folded."
+            )
+        else:
+            _append_measurements(circuit, circ_measurements)
 
 
 def _squash_moments(circuit: Circuit) -> Circuit:

diff --git a/mitiq/zne/scaling/tests/test_folding.py b/mitiq/zne/scaling/tests/test_folding.py
@@ -25,6 +25,8 @@
     equal_up_to_global_phase,
     InsertStrategy,
     testing,
+    ry,
+    rz,
 )
 from qiskit import ClassicalRegister, QuantumCircuit, QuantumRegister
 from qiskit.quantum_info.operators import Operator
@@ -51,6 +53,7 @@
     _create_fold_mask,
     _apply_fold_mask,
 )
+from sympy import Symbol
 
 
 def test_squash_moments_two_qubits():
@@ -407,14 +410,90 @@ def test_fold_with_intermediate_measurements_raises_error(fold_method):
 )
 def test_fold_with_channels_raises_error(fold_method):
     """Tests local folding functions raise an error on circuits with
-    non-unitary channels (which are not measurements).
+    non-invertible channels (which are not measurements).
     """
     qbit = LineQubit(0)
     circ = Circuit(
         ops.H.on(qbit), ops.depolarize(p=0.1).on(qbit), ops.measure(qbit)
     )
     with pytest.raises(
-        UnfoldableCircuitError, match="Circuit contains non-unitary channels"
+        UnfoldableCircuitError, match="Circuit contains non-invertible"
+    ):
+        fold_method(circ, scale_factor=3.0)
+
+
+@pytest.mark.parametrize(
+    "fold_method",
+    [
+        fold_gates_from_left,
+        fold_gates_from_right,
+        fold_gates_at_random,
+        fold_global,
+    ],
+)
+def test_parametrized_circuit_folding(fold_method):
+    """Checks if the circuit is folded as expected when the circuit operations
+    have a valid inverse.
+    """
+    theta = Symbol("theta")
+    q = LineQubit(0)
+    ansatz_circ = Circuit(ry(theta).on(q))
+    folded_circ = fold_method(ansatz_circ, scale_factor=3.0)
+    expected_circ = Circuit(ry(theta).on(q), ry(-theta).on(q), ry(theta).on(q))
+    assert _equal(folded_circ, expected_circ)
+
+
+@pytest.mark.parametrize(
+    "fold_method",
+    [
+        fold_gates_from_left,
+        fold_gates_from_right,
+        fold_gates_at_random,
+        fold_global,
+    ],
+)
+def test_parametrized_circuit_folding_terminal_measurement(fold_method):
+    """Checks if the circuit with a terminal measurement is folded as expected
+    when the circuit operations have a valid inverse.
+    """
+    theta = Symbol("theta")
+    q = LineQubit(0)
+    ansatz_circ = Circuit(ry(theta).on(q), ops.measure(q))
+    folded_circ = fold_method(ansatz_circ, scale_factor=3.0)
+    expected_circ = Circuit(
+        ry(theta).on(q), ry(-theta).on(q), ry(theta).on(q), ops.measure(q)
+    )
+    assert _equal(folded_circ, expected_circ)
+
+
+@pytest.mark.parametrize(
+    "fold_method",
+    [
+        fold_gates_from_left,
+        fold_gates_from_right,
+        fold_gates_at_random,
+        fold_global,
+    ],
+)
+def test_errors_raised_parametrized_circuits(fold_method):
+    """Checks if proper error is raised in a symbolic circuit when it cannot
+    be folded.
+    """
+    theta = Symbol("theta")
+    q = LineQubit(0)
+    ansatz_circ = Circuit(ry(theta).on(q), ops.measure(q), rz(theta).on(q))
+    with pytest.raises(
+        UnfoldableCircuitError,
+        match="Circuit contains intermediate measurements",
+    ):
+        fold_method(ansatz_circ, scale_factor=3.0)
+
+    qbit = LineQubit(0)
+    circ = Circuit(
+        ry(theta).on(q), ops.depolarize(p=0.1).on(qbit), ops.measure(qbit)
+    )
+    with pytest.raises(
+        UnfoldableCircuitError, match="Circuit contains non-invertible"
     ):
         fold_method(circ, scale_factor=3.0)