Add noise amplification transformer (quantumlib#6665)

* Add noise amplification transformer

* add _gate_in_moment

* add copyright notice

* add raises documentation, remove import cirq, fix coverage

* transformer api

* * after `circuit` input

Co-authored-by: Noureldin <[email protected]>

* format

* update test

* Convert to a class

* types and lint

* suggestions from Nour

* lint

* suggestion from Nour

* add ()

* types

* lint

---------

Co-authored-by: Noureldin <[email protected]>

Author: eliottrosenberg

cirq/transformers/noise_adding.py

@@ -0,0 +1,114 @@
+# Copyright 2024 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from collections.abc import Mapping
+
+from typing import cast
+from cirq import ops, circuits
+from cirq.transformers import transformer_api
+import numpy as np
+
+
+def _gate_in_moment(gate: ops.Gate, moment: circuits.Moment) -> bool:
+    """Check whether `gate` is in `moment`."""
+    return any(op.gate == gate for op in moment)
+
+
+@transformer_api.transformer
+class DepolerizingNoiseTransformer:
+    """Add local depolarizing noise after two-qubit gates in a specified circuit. More specifically,
+    with probability p, append a random non-identity two-qubit Pauli operator after each specified
+    two-qubit gate.
+
+    Attrs:
+        p: The probability with which to add noise.
+        target_gate: Add depolarizing nose after this type of gate
+    """
+
+    def __init__(
+        self, p: float | Mapping[tuple[ops.Qid, ops.Qid], float], target_gate: ops.Gate = ops.CZ
+    ):
+        """Initialize the depolarizing noise transformer with some depolarizing probability and
+        target gate.
+
+        Args:
+            p: The depolarizing probability, either a single float or a mapping from pairs of qubits
+               to floats.
+           target_gate: The gate after which to add depolarizing noise.
+
+        Raises:
+            TypeError: If `p` is not either be a float or a mapping from sorted qubit pairs to
+                       floats.
+        """
+
+        if not isinstance(p, (Mapping, float)):
+            raise TypeError(  # pragma: no cover
+                "p must either be a float or a mapping from"  # pragma: no cover
+                + "sorted qubit pairs to floats"  # pragma: no cover
+            )  # pragma: no cover
+        self.p = p
+        self.p_func = (
+            (lambda _: p)
+            if isinstance(p, (int, float))
+            else (lambda pair: cast(Mapping, p).get(pair, 0.0))
+        )
+        self.target_gate = target_gate
+
+    def __call__(
+        self,
+        circuit: circuits.AbstractCircuit,
+        rng: np.random.Generator | None = None,
+        *,
+        context: transformer_api.TransformerContext | None = None,
+    ):
+        """Apply the transformer to the given circuit.
+
+        Args:
+            circuit: The circuit to add noise to.
+            context: Not used; to satisfy transformer API.
+
+        Returns:
+            The transformed circuit.
+        """
+        if rng is None:
+            rng = np.random.default_rng()
+        target_gate = self.target_gate
+
+        # add random Pauli gates with probability p after each of the specified gate
+        assert target_gate.num_qubits() == 2, "`target_gate` must be a two-qubit gate."
+        paulis = [ops.I, ops.X, ops.Y, ops.Z]
+        new_moments = []
+        for moment in circuit:
+            new_moments.append(moment)
+            if _gate_in_moment(target_gate, moment):
+                # add a new moment with the Paulis
+                target_pairs = {
+                    tuple(sorted(op.qubits)) for op in moment.operations if op.gate == target_gate
+                }
+                added_moment_ops = []
+                for pair in target_pairs:
+                    pair_sorted_tuple = (pair[0], pair[1])
+                    p_i = self.p_func(pair_sorted_tuple)
+                    apply = rng.choice([True, False], p=[p_i, 1 - p_i])
+                    if apply:
+                        choices = [
+                            (pauli_a(pair[0]), pauli_b(pair[1]))
+                            for pauli_a in paulis
+                            for pauli_b in paulis
+                        ][1:]
+                        pauli_to_apply = rng.choice(np.array(choices, dtype=object))
+                        added_moment_ops.append(pauli_to_apply)
+                if len(added_moment_ops) > 0:
+                    new_moments.append(circuits.Moment(*added_moment_ops))
+        return circuits.Circuit.from_moments(*new_moments)

cirq/transformers/noise_adding_test.py

@@ -0,0 +1,53 @@
+# Copyright 2024 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from cirq import ops, circuits, devices
+import cirq.transformers.noise_adding as na
+import numpy as np
+
+
+def test_noise_adding():
+    qubits = devices.LineQubit.range(4)
+    one_layer = circuits.Circuit(ops.CZ(*qubits[:2]), ops.CZ(*qubits[2:]))
+    circuit = one_layer * 10
+
+    # test that p=0 does nothing
+    transformed_circuit_p0 = na.DepolerizingNoiseTransformer(0.0)(circuit)
+    assert transformed_circuit_p0 == circuit
+
+    # test that p=1 doubles the circuit depth
+    transformed_circuit_p1 = na.DepolerizingNoiseTransformer(1.0)(circuit)
+    assert len(transformed_circuit_p1) == 20
+
+    # test that we get a deterministic result when using a specific rng
+    rng = np.random.default_rng(0)
+    transformed_circuit_p0_03 = na.DepolerizingNoiseTransformer(0.03)(circuit, rng=rng)
+    expected_circuit = (
+        one_layer * 2
+        + circuits.Circuit(ops.I(qubits[2]), ops.Z(qubits[3]))
+        + one_layer * 4
+        + circuits.Circuit(ops.Z(qubits[0]), ops.X(qubits[1]))
+        + one_layer * 4
+        + circuits.Circuit(ops.I(qubits[2]), ops.X(qubits[3]))
+    )
+    assert transformed_circuit_p0_03 == expected_circuit
+
+    # test that supplying a dictionary for p works
+    transformed_circuit_p_dict = na.DepolerizingNoiseTransformer(
+        {tuple(qubits[:2]): 1.0, tuple(qubits[2:]): 0.0}
+    )(circuit)
+    assert len(transformed_circuit_p_dict) == 20  # depth should be doubled
+    assert transformed_circuit_p_dict[1::2].all_qubits() == frozenset(
+        qubits[:2]
+    )  # no single-qubit gates get added to qubits[2:]