feat(folding_amplifier): add multi-qubit amplifier facade

test/noise_amplification/folding_amplifier/test_local_folding_amplifier.py

@@ -26,6 +26,7 @@
 from zne.noise_amplification import (
     CxAmplifier,
     LocalFoldingAmplifier,
+    MultiQubitAmplifier,
     TwoQubitAmplifier,
 )
 
@@ -460,8 +461,27 @@ class TestFacades:
             (TwoQubitAmplifier, {"gates_to_fold": 2}),
         ],
     )
-    def test_two_qubit_amplifier(self, cls, configs):
+    def test_amplifier_facade(self, cls, configs):
         amplifier = cls()
         assert isinstance(amplifier, LocalFoldingAmplifier)
         for key, value in configs.items():
             assert getattr(amplifier, key) == frozenset([value])
+
+    def test_multi_qubit_amplifier(self):
+        amplifier = MultiQubitAmplifier()
+        assert isinstance(amplifier, LocalFoldingAmplifier)
+        assert amplifier.gates_to_fold is None
+        circuit = QuantumCircuit(4)
+        circuit.h(0)
+        circuit.z(3)
+        circuit.cx(0, 1)
+        circuit.x(1)
+        circuit.ccx(0, 1, 2)
+        circuit.y(2)
+        circuit.ecr(2, 3)
+        circuit.measure_all()
+        for operation in circuit.data:
+            instruction = operation[0]
+            skipped_instructions = {"barrier", "measure"}
+            expected = instruction.num_qubits > 1 and instruction.name not in skipped_instructions
+            assert amplifier._check_gate_folds(operation) == expected

zne/noise_amplification/__init__.py

@@ -16,6 +16,7 @@
     CxAmplifier,
     GlobalFoldingAmplifier,
     LocalFoldingAmplifier,
+    MultiQubitAmplifier,
     TwoQubitAmplifier,
 )
 from .noise_amplifier import CircuitNoiseAmplifier, DAGNoiseAmplifier, NoiseAmplifier
@@ -27,6 +28,7 @@
         LocalFoldingAmplifier,
         CxAmplifier,
         TwoQubitAmplifier,
+        MultiQubitAmplifier,
     )
 }
 

zne/noise_amplification/folding_amplifier/__init__.py

@@ -16,6 +16,7 @@
 from .local_folding_amplifier import (
     CxAmplifier,
     LocalFoldingAmplifier,
+    MultiQubitAmplifier,
     TwoQubitAmplifier,
 )
 
@@ -24,4 +25,5 @@
     "LocalFoldingAmplifier",
     "CxAmplifier",
     "TwoQubitAmplifier",
+    "MultiQubitAmplifier",
 ]

zne/noise_amplification/folding_amplifier/local_folding_amplifier.py

@@ -325,3 +325,35 @@ def __init__(  # pylint: disable=too-many-arguments,duplicate-code
             noise_factor_relative_tolerance=noise_factor_relative_tolerance,
             warn_user=warn_user,
         )
+
+
+class MultiQubitAmplifier(LocalFoldingAmplifier):
+    """
+    Digital noise amplifier acting on multi-qubit gates exclusively.
+
+    Replaces each multi-qubit gate locally with as many gates as indicated by the noise_factor.
+    """
+
+    def __init__(  # pylint: disable=too-many-arguments,duplicate-code
+        self,
+        sub_folding_option: str = "from_first",
+        barriers: bool = True,
+        random_seed: int | None = None,
+        noise_factor_relative_tolerance: float = 1e-2,
+        warn_user: bool = True,
+    ) -> None:
+        super().__init__(
+            gates_to_fold=None,
+            sub_folding_option=sub_folding_option,
+            barriers=barriers,
+            random_seed=random_seed,
+            noise_factor_relative_tolerance=noise_factor_relative_tolerance,
+            warn_user=warn_user,
+        )
+
+    def _check_gate_folds(self, operation: CircuitInstruction) -> bool:
+        instruction, qargs, _ = operation
+        if instruction.name in {"barrier", "measure"}:
+            return False
+        num_qubits = len(qargs)
+        return num_qubits > 1