Implement unitary AQC

qiskit_addon_aqc_tensor/objective.py

@@ -85,7 +85,45 @@ def target(self) -> TensorNetworkState:
         return self._target_tensornetwork
 
 
+class MaximizeProcessFidelity:
+    """Maximize process fidelity."""
+
+    def __init__(self, target, ansatz: QuantumCircuit, settings: TensorNetworkSimulationSettings):
+        """Initialize the objective function.
+
+        Args:
+            ansatz: Parametrized ansatz circuit.
+            target: Target state in tensor-network representation.
+            settings: Tensor network simulation settings.
+        """
+        if ansatz is not None:
+            from .ansatz_generation import AnsatzBlock
+
+            ansatz = ansatz.decompose(AnsatzBlock)
+        self._ansatz = ansatz
+        self._simulation_settings = settings
+        self._target_tensornetwork = target
+        if settings is not None:
+            from .simulation.abstract import _preprocess_for_gradient
+
+            self._preprocessed = _preprocess_for_gradient(self, settings)
+
+    def loss_function(self, x: np.ndarray) -> tuple[float, np.ndarray]:
+        """Evaluate ``(objective_value, gradient)`` of function at point ``x``."""
+        from .simulation.abstract import _compute_objective_and_gradient
+
+        return _compute_objective_and_gradient(
+            self, self._simulation_settings, self._preprocessed, x
+        )
+
+    @property
+    def target(self) -> TensorNetworkState:
+        """Target tensor network."""
+        return self._target_tensornetwork
+
+
 # Reminder: update the RST file in docs/apidocs when adding new interfaces.
 __all__ = [
     "OneMinusFidelity",
+    "MaximizeProcessFidelity",
 ]

qiskit_addon_aqc_tensor/simulation/quimb/__init__.py

@@ -12,6 +12,7 @@
 
 """Quimb as a tensor network backend."""
 
+# ruff: noqa: F811
 from __future__ import annotations
 
 import logging
@@ -27,7 +28,7 @@
 from wrapt import register_post_import_hook
 
 from ...ansatz_generation import AnsatzBlock
-from ...objective import OneMinusFidelity
+from ...objective import MaximizeProcessFidelity, OneMinusFidelity
 from ..abstract import TensorNetworkSimulationSettings
 from ..explicit_gradient import (
     compute_gradient_of_tensornetwork_overlap,
@@ -405,6 +406,39 @@ def oneminusfidelity_loss_fn(
     return 1 - fidelity
 
 
+@dispatch
+def tnoptimizer_objective_kwargs(objective: MaximizeProcessFidelity, /) -> dict[str, Any]:
+    """Return keyword arguments for use with :func:`~quimb.tensor.TNOptimizer`.
+
+    - ``loss_fn``
+    - ``loss_kwargs``
+    """
+    import quimb.tensor as qtn
+
+    target = objective.target
+    if isinstance(target, qtn.Circuit):
+        target = target.psi
+    return {
+        "loss_fn": maximizeprocessfidelity_loss_fn,
+        "loss_kwargs": {"target": target},
+    }
+
+
+def maximizeprocessfidelity_loss_fn(
+    circ: quimb.tensor.Circuit,
+    /,
+    *,
+    target: quimb.tensor.TensorNetworkGenVector,
+    optimize="auto-hq",
+):
+    import autoray as ar
+
+    return (
+        1
+        - ar.do("abs", (circ.uni.H & target).contract(all, optimize=optimize)) / 2.0**target.nsites
+    )
+
+
 # Reminder: update the RST file in docs/apidocs when adding new interfaces.
 __all__ = [
     "is_quimb_available",