Fix QNN for input and weights ordering (#728)

* Fix QNN for input and weights ordering

* Black

* Lint

* Update QCNN tutorial

* Add reno

* Fix draw style per #725

---------

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

docs/tutorials/11_qcnn_initial_point.json

@@ -1 +1 @@
-[1.930057091422052, 0.2829424508139703, 0.35555636265939633, 0.1750006532903061, 0.3002103666790018, 0.6641911912373437, 1.3310981300850042, 0.5022717197547227, 0.44912874128880675, 0.40236963192983266, 0.3459537084665159, 0.9786311288435154, 0.48716712269991697, -0.007081389738930712, 0.21570815199311827, 0.07334182375267477, 0.6907887498355103, 0.21771166428570735, 1.087665977608006, 1.2571463700739218, 1.0866597360102666, 2.126145551821481, 0.8914518096731741, 1.5053260036617715, 0.44798876926441555, 0.9498701675467225, 0.15490304396579338, 0.1338674031994701, -0.6938374500039391, 0.029396385425104116, -0.09785818314088227, -0.31198441382224246, 0.20004568516690807, 1.848494069662786, -0.028371899054628447, -0.15229494459622284, 0.7653870524298326, 0.6881492316484289, 0.6759011152318357, 1.6028387103546868, 0.47711915171800057, -0.26162053028790294, -0.12898443497061718, 0.5281303751714184, 0.4957555866394333, 1.6095784010055925, 0.5685823964468215, 1.2812276175594062, 0.3032325725579015, 1.4291081956286258, 0.7081163438891277, 1.8291375321912147, -0.11047287562207528, 0.2751308409529747, 0.2834764252747557, 0.29668607404725605, 0.008300790063532154, 0.6707732056265118, 0.5325267632509095, 0.7240676576317691, 0.08123934531343553, -0.0038536767244725153, -0.1001165849018211]
+[1.68270961, 0.11605051, 0.34864916, 0.74675878, 1.87124355, 1.49219533, -0.38654013, 1.6794744, 1.46546974, 2.16547249, 1.05274095, 2.2565039, 0.31246977, -0.0977787, 0.26751274, -0.24319314, 0.28359516, 0.17431664, 0.86434056, 1.08183541, 1.64600062, 2.17350294, 0.17430376, 0.08381051, 0.30748524, 1.6671458, 0.23076889, 0.40720057, -0.38243368, -0.28842447, 0.08507067, 1.34472166, -0.08210173, 1.10931829, 0.15418569, 0.65755067, 3.09541972, 0.41647156, 1.12894435, 1.03898584, 0.64931532, -0.43442102, 0.24324246, 0.98370841, 0.57256531, 0.25832156, 0.42749823, 1.78949614, 0.27749909, 0.07237166, 0.05920573, 0.41896919, 0.66868785, 0.73035314, 0.00984019, 0.72243278, 1.10299638, 0.80821682, 0.39530007, 1.03814133, 0.41697893, 0.53016156, 1.13594375]

qiskit_machine_learning/neural_networks/estimator_qnn.py

@@ -148,7 +148,7 @@ def __init__(
         if estimator is None:
             estimator = Estimator()
         self.estimator = estimator
-        self._circuit = circuit
+        self._org_circuit = circuit
         if observables is None:
             observables = SparsePauliOp.from_list([("Z" * circuit.num_qubits, 1)])
         if isinstance(observables, BaseOperator):
@@ -173,10 +173,12 @@ def __init__(
             input_gradients=input_gradients,
         )
 
+        self._circuit = self._reparameterize_circuit(circuit, input_params, weight_params)
+
     @property
     def circuit(self) -> QuantumCircuit:
         """The quantum circuit representing the neural network."""
-        return copy(self._circuit)
+        return copy(self._org_circuit)
 
     @property
     def observables(self) -> Sequence[BaseOperator] | BaseOperator:

qiskit_machine_learning/neural_networks/neural_network.py

@@ -16,9 +16,11 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
+from typing import Sequence
 
 import numpy as np
 
+from qiskit.circuit import Parameter, ParameterVector, QuantumCircuit
 import qiskit_machine_learning.optionals as _optionals
 from ..exceptions import QiskitMachineLearningError
 
@@ -264,3 +266,54 @@ def _backward(
         self, input_data: np.ndarray | None, weights: np.ndarray | None
     ) -> tuple[np.ndarray | SparseArray | None, np.ndarray | SparseArray | None]:
         raise NotImplementedError
+
+    def _reparameterize_circuit(
+        self,
+        circuit: QuantumCircuit,
+        input_params: Sequence[Parameter] | None = None,
+        weight_params: Sequence[Parameter] | None = None,
+    ) -> QuantumCircuit:
+        # As the data (parameter values) for the primitive is ordered as inputs followed by weights
+        # we need to ensure that the parameters are ordered like this naturally too so the rewrites
+        # parameters to ensure this. "inputs" as a name comes before "weights" and within they are
+        # numerically ordered.
+        if input_params and self.num_inputs != len(input_params):
+            raise ValueError(
+                f"input_params length {len(input_params)}"
+                f" mismatch with num_inputs (self.num_inputs)"
+            )
+        if weight_params and self.num_weights != len(weight_params):
+            raise ValueError(
+                f"weight_params length {len(weight_params)}"
+                f" mismatch with num_weights (self.num_weights)"
+            )
+
+        parameters = circuit.parameters
+
+        if len(parameters) != (self.num_inputs + self.num_weights):
+            raise ValueError(
+                f"Number of circuit parameters {len(parameters)}"
+                f" mismatch with sum of num inputs and weights"
+                f" {self.num_inputs + self.num_weights}"
+            )
+
+        new_input_params = ParameterVector("inputs", self.num_inputs)
+        new_weight_params = ParameterVector("weights", self.num_weights)
+
+        new_parameters = {}
+        if input_params:
+            for i, param in enumerate(input_params):
+                if param not in parameters:
+                    raise ValueError(f"Input param `{param.name}` not present in circuit")
+                new_parameters[param] = new_input_params[i]
+
+        if weight_params:
+            for i, param in enumerate(weight_params):
+                if param not in parameters:
+                    raise ValueError(f"Weight param {param.name} `not present in circuit")
+                new_parameters[param] = new_weight_params[i]
+
+        if new_parameters:
+            circuit = circuit.assign_parameters(new_parameters)
+
+        return circuit

qiskit_machine_learning/neural_networks/sampler_qnn.py

@@ -182,9 +182,7 @@ def __init__(
             gradient = ParamShiftSamplerGradient(self.sampler)
         self.gradient = gradient
 
-        self._circuit = circuit.copy()
-        if len(self._circuit.clbits) == 0:
-            self._circuit.measure_all()
+        self._org_circuit = circuit
 
         if isinstance(circuit, QNNCircuit):
             self._input_params = list(circuit.input_parameters)
@@ -207,10 +205,15 @@ def __init__(
             input_gradients=self._input_gradients,
         )
 
+        if len(circuit.clbits) == 0:
+            circuit = circuit.copy()
+            circuit.measure_all()
+        self._circuit = self._reparameterize_circuit(circuit, input_params, weight_params)
+
     @property
     def circuit(self) -> QuantumCircuit:
         """Returns the underlying quantum circuit."""
-        return self._circuit
+        return self._org_circuit
 
     @property
     def input_params(self) -> Sequence[Parameter]:
@@ -274,7 +277,7 @@ def _compute_output_shape(
                     "No interpret function given, output_shape will be automatically "
                     "determined as 2^num_qubits."
                 )
-            output_shape_ = (2**self._circuit.num_qubits,)
+            output_shape_ = (2**self.circuit.num_qubits,)
 
         return output_shape_
 

releasenotes/notes/fix_qnn_binding_order-74caef8a49ecffe5.yaml

@@ -0,0 +1,13 @@
+---
+fixes:
+  - |
+    Fixes an issue for the Quantum Neural Networks where the binding order of the inputs
+    and weights might end up being incorrect. Though the params for the inputs and weights
+    are specified to the QNN, the code previously bound the inputs and weights in the order
+    given by the circuit.parameters. This would end up being the right order for the Qiskit
+    circuit library feature maps and ansatzes most often used, as the default parameter
+    names led to the order being as expected. However for custom names etc. this was not
+    always the case and then led to unexpected behavior. The sequences for the input and
+    weights parameters, as supplied, are now always used as the binding order, for the inputs
+    and weights respectively, such that the order of the parameters in the overall circuit
+    no longer matters.

test/neural_networks/test_estimator_qnn.py

@@ -18,7 +18,7 @@
 
 import numpy as np
 from qiskit.circuit import Parameter, QuantumCircuit
-from qiskit.circuit.library import ZZFeatureMap, RealAmplitudes
+from qiskit.circuit.library import ZZFeatureMap, RealAmplitudes, ZFeatureMap
 from qiskit.quantum_info import SparsePauliOp
 from qiskit_machine_learning.circuit.library import QNNCircuit
 
@@ -447,6 +447,26 @@ def test_qnn_qc_circui_construction(self):
             # Test if weights grad is identical
             np.testing.assert_array_almost_equal(backward_qc[1], backward_qnn_qc[1])
 
+    def test_binding_order(self):
+        """Test parameter binding order gives result as expected"""
+        qc = ZFeatureMap(feature_dimension=2, reps=1)
+        input_params = qc.parameters
+        weight = Parameter("weight")
+        for i in range(qc.num_qubits):
+            qc.rx(weight, i)
+
+        observable1 = SparsePauliOp.from_list([("Z" * qc.num_qubits, 1)])
+        estimator_qnn = EstimatorQNN(
+            circuit=qc, observables=observable1, input_params=input_params, weight_params=[weight]
+        )
+
+        estimator_qnn_weights = [3]
+        estimator_qnn_input = [2, 33]
+        res = estimator_qnn.forward(estimator_qnn_input, estimator_qnn_weights)
+        # When parameters were used in circuit order, before being assigned correctly, so inputs
+        # went to input params, weights to weight params, this gave 0.00613403
+        self.assertAlmostEqual(res[0][0], 0.00040017)
+
 
 if __name__ == "__main__":
     unittest.main()