Add conversion support for Qiskit's NoiseModels

pennylane_qiskit/converter.py

@@ -15,15 +15,19 @@
 This module contains functions for converting Qiskit QuantumCircuit objects
 into PennyLane circuit templates.
 """
-from typing import Dict, Any, Iterable, Sequence, Union
 import warnings
 from functools import partial, reduce
+from typing import Any, Dict, Iterable, Sequence, Union
 
 import numpy as np
+import pennylane as qml
+import pennylane.ops as pennylane_ops
 import qiskit.qasm2
+from pennylane.noise.conditionals import WiresIn, _rename
+from pennylane.operation import AnyWires
 from qiskit import QuantumCircuit
+from qiskit.circuit import Barrier, Clbit, ControlFlowOp, Measure
 from qiskit.circuit import Parameter, ParameterExpression, ParameterVector
-from qiskit.circuit import Measure, Barrier, ControlFlowOp, Clbit
 from qiskit.circuit.classical import expr
 from qiskit.circuit.controlflow.switch_case import _DefaultCaseType
 from qiskit.circuit.library import GlobalPhaseGate
@@ -32,10 +36,10 @@
 from qiskit.quantum_info import SparsePauliOp
 from sympy import lambdify
 
-import pennylane as qml
-import pennylane.ops as pennylane_ops
 from pennylane_qiskit.qiskit_device import QISKIT_OPERATION_MAP
 
+from .noise_models import _build_noise_model_map
+
 # pylint: disable=too-many-instance-attributes
 
 inv_map = {v.__name__: k for k, v in QISKIT_OPERATION_MAP.items()}
@@ -1042,3 +1046,56 @@ def _expr_eval_clvals(clbits, clvals, expr_func, bitwise=False):
         condition_res = expr_func(meas1, clreg2)
 
     return condition_res
+
+
+def load_noise_model(noise_model, **kwargs) -> qml.NoiseModel:
+    """Loads a PennyLane ``NoiseModel`` from a Qiskit `NoiseModel
+    <https://qiskit.github.io/qiskit-aer/stubs/qiskit_aer.noise.NoiseModel.html>`_.
+
+    Args:
+        noise_model (qiskit_aer.noise.NoiseModel): A Qiskit noise model object
+        kwargs: Optional keyword arguments for conversion of the noise model.
+
+    Keyword Arguments:
+        gate_times (Dict[str, float]): gate times for building thermal relaxation error.
+            If not provided, the default value of ``1.0`` will be used for construction.
+        decimals (int): number of decimal places to round the Kraus matrices for errors to.
+            If not provided, the default value of ``10`` is used.
+        atol (float): the relative tolerance parameter. Default value is ``1e-05``.
+        rtol (float): the absolute tolernace parameters. Defualt value is ``1e-08``.
+        optimize (bool): controls if intermediate optimization is used while transforming Kraus
+            operators to a Choi matrix, wherever required. Default is ``False``.
+        kraus_shape (bool): use shape of the Kraus operators to display ``qml.QubitChannel``
+            instead of the complete list of matrices. Default is ``True``.
+
+    Returns:
+        qml.NoiseModel: An equivalent noise model constructed in PennyLane
+
+    Raises:
+        ValueError: When an encountered quantum error cannoted be converted.
+
+    .. note::
+
+        Currently, PennyLane noise models does not support readout errors, so those will be skipped during conversion.
+    """
+
+    qerror_dmap, _ = _build_noise_model_map(noise_model, **kwargs)
+    model_map = {}
+    for error, wires_map in qerror_dmap.items():
+        conditions = []
+        cwires = []
+        for wires, operations in wires_map.items():
+            cond = qml.noise.op_in(operations)
+            if wires != AnyWires:
+                cond &= WiresIn(wires)
+                cwires.append(wires)
+            conditions.append(cond)
+        fcond = reduce(lambda cond1, cond2: cond1 | cond2, conditions)
+
+        noise = qml.noise.partial_wires(error)
+        if isinstance(error, qml.QubitChannel) and kwargs.get("kraus_shape", True):
+            noise = _rename(f"QubitChannel(Klist=Tensor{qml.math.shape(error.data)})")(noise)
+
+        model_map[fcond] = noise
+
+    return qml.NoiseModel(model_map)