qml.qnn.KerasLayer

class KerasLayer(*args, **kwargs)

Bases: keras.engine.base_layer.Layer

Converts a QNode to a Keras Layer.

The result can be used within the Keras Sequential or Model classes for creating quantum and hybrid models.

Note

KerasLayer currently only supports Keras 2. If you are running the newest version of TensorFlow and Keras, you may automatically be using Keras 3. For instructions on running with Keras 2, instead, see the documentation on backwards compatibility .

Parameters

• qnode (qml.QNode) – the PennyLane QNode to be converted into a Keras Layer

• weight_shapes (dict[str, tuple]) – a dictionary mapping from all weights used in the QNode to their corresponding shapes

• output_dim (int) – the output dimension of the QNode

• weight_specs (dict[str, dict]) – An optional dictionary for users to provide additional specifications for weights used in the QNode, such as the method of parameter initialization. This specification is provided as a dictionary with keys given by the arguments of the add_weight() method and values being the corresponding specification.

• **kwargs – additional keyword arguments passed to the Layer base class

Example

First let’s define the QNode that we want to convert into a Keras Layer:

n_qubits = 2
dev = qml.device("default.qubit", wires=n_qubits)

@qml.qnode(dev)
def qnode(inputs, weights_0, weight_1):
    qml.RX(inputs[0], wires=0)
    qml.RX(inputs[1], wires=1)
    qml.Rot(*weights_0, wires=0)
    qml.RY(weight_1, wires=1)
    qml.CNOT(wires=[0, 1])
    return qml.expval(qml.Z(0)), qml.expval(qml.Z(1))

The signature of the QNode must contain an inputs named argument for input data, with all other arguments to be treated as internal weights. We can then convert to a Keras Layer with:

>>> weight_shapes = {"weights_0": 3, "weight_1": 1}
>>> qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2)

The internal weights of the QNode are automatically initialized within the KerasLayer and must have their shapes specified in a weight_shapes dictionary. It is then easy to combine with other neural network layers from the tensorflow.keras.layers module and create a hybrid:

>>> clayer = tf.keras.layers.Dense(2)
>>> model = tf.keras.models.Sequential([qlayer, clayer])

Usage Details

QNode signature

The QNode must have a signature that satisfies the following conditions:

• Contain an inputs named argument for input data.

• All other arguments must accept an array or tensor and are treated as internal weights of the QNode.

• All other arguments must have no default value.

• The inputs argument is permitted to have a default value provided the gradient with respect to inputs is not required.

• There cannot be a variable number of positional or keyword arguments, e.g., no *args or **kwargs present in the signature.

Output shape

If the QNode returns a single measurement, then the output of the KerasLayer will have shape (batch_dim, *measurement_shape), where measurement_shape is the output shape of the measurement:

def print_output_shape(measurements):
    n_qubits = 2
    dev = qml.device("default.qubit", wires=n_qubits, shots=100)

    @qml.qnode(dev)
    def qnode(inputs, weights):
        qml.templates.AngleEmbedding(inputs, wires=range(n_qubits))
        qml.templates.StronglyEntanglingLayers(weights, wires=range(n_qubits))
        if len(measurements) == 1:
            return qml.apply(measurements[0])
        return [qml.apply(m) for m in measurements]

    weight_shapes = {"weights": (3, n_qubits, 3)}
    qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=None)

    batch_dim = 5
    x = tf.zeros((batch_dim, n_qubits))
    return qlayer(x).shape

>>> print_output_shape([qml.expval(qml.Z(0))])
TensorShape([5])
>>> print_output_shape([qml.probs(wires=[0, 1])])
TensorShape([5, 4])
>>> print_output_shape([qml.sample(wires=[0, 1])])
TensorShape([5, 100, 2])

If the QNode returns multiple measurements, then the measurement results will be flattened and concatenated, resulting in an output of shape (batch_dim, total_flattened_dim):

>>> print_output_shape([qml.expval(qml.Z(0)), qml.probs(wires=[0, 1])])
TensorShape([5, 5])
>>> print_output_shape([qml.probs([0, 1]), qml.sample(wires=[0, 1])])
TensorShape([5, 204])

Initializing weights

The optional weight_specs argument of KerasLayer allows for a more fine-grained specification of the QNode weights, such as the method of initialization and any regularization or constraints. For example, the initialization method of the weights argument in the example above could be specified by:

weight_specs = {"weights": {"initializer": "random_uniform"}}

The values of weight_specs are dictionaries with keys given by arguments of the Keras add_weight() method. For the "initializer" argument, one can specify a string such as "random_uniform" or an instance of an Initializer class, such as tf.keras.initializers.RandomUniform.

If weight_specs is not specified, weights will be added using the Keras default initialization and without any regularization or constraints.

Model saving

The weights of models that contain KerasLayers can be saved using the usual tf.keras.Model.save_weights method:

clayer = tf.keras.layers.Dense(2, input_shape=(2,))
qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2)
model = tf.keras.Sequential([clayer, qlayer])
model.save_weights(SAVE_PATH)

To load the model weights, first instantiate the model like before, then call tf.keras.Model.load_weights:

clayer = tf.keras.layers.Dense(2, input_shape=(2,))
qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2)
model = tf.keras.Sequential([clayer, qlayer])
model.load_weights(SAVE_PATH)

Models containing KerasLayer objects can also be saved directly using tf.keras.Model.save. This method also saves the model architecture, weights, and training configuration, including the optimizer state:

clayer = tf.keras.layers.Dense(2, input_shape=(2,))
qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2)
model = tf.keras.Sequential([clayer, qlayer])
model.save(SAVE_PATH)

In this case, loading the model requires no knowledge of the original source code:

model = tf.keras.models.load_model(SAVE_PATH)

Note

Currently KerasLayer objects cannot be saved in the HDF5 file format. In order to save a model using the latter method above, the SavedModel file format (default in TensorFlow 2.x) should be used.

Additional example

The code block below shows how a circuit composed of templates from the Templates module can be combined with classical Dense layers to learn the two-dimensional moons dataset.

import pennylane as qml
import tensorflow as tf
import sklearn.datasets

n_qubits = 2
dev = qml.device("default.qubit", wires=n_qubits)

@qml.qnode(dev)
def qnode(inputs, weights):
    qml.templates.AngleEmbedding(inputs, wires=range(n_qubits))
    qml.templates.StronglyEntanglingLayers(weights, wires=range(n_qubits))
    return qml.expval(qml.Z(0)), qml.expval(qml.Z(1))

weight_shapes = {"weights": (3, n_qubits, 3)}

qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2)
clayer1 = tf.keras.layers.Dense(2)
clayer2 = tf.keras.layers.Dense(2, activation="softmax")
model = tf.keras.models.Sequential([clayer1, qlayer, clayer2])

data = sklearn.datasets.make_moons()
X = tf.constant(data[0])
Y = tf.one_hot(data[1], depth=2)

opt = tf.keras.optimizers.SGD(learning_rate=0.5)
model.compile(opt, loss='mae')

The model can be trained using:

>>> model.fit(X, Y, epochs=8, batch_size=5)
Train on 100 samples
Epoch 1/8
100/100 [==============================] - 9s 90ms/sample - loss: 0.3524
Epoch 2/8
100/100 [==============================] - 9s 87ms/sample - loss: 0.2441
Epoch 3/8
100/100 [==============================] - 9s 87ms/sample - loss: 0.1908
Epoch 4/8
100/100 [==============================] - 9s 87ms/sample - loss: 0.1832
Epoch 5/8
100/100 [==============================] - 9s 88ms/sample - loss: 0.1596
Epoch 6/8
100/100 [==============================] - 9s 87ms/sample - loss: 0.1637
Epoch 7/8
100/100 [==============================] - 9s 86ms/sample - loss: 0.1613
Epoch 8/8
100/100 [==============================] - 9s 87ms/sample - loss: 0.1474

Returning a state

If your QNode returns the state of the quantum circuit using state() or density_matrix(), you must immediately follow your quantum Keras Layer with a layer that casts to reals. For example, you could use tf.keras.layers.Lambda with the function lambda x: tf.abs(x). This casting is required because TensorFlow’s Keras layers require a real input and are differentiated with respect to real parameters.

Attributes

activity_regularizer	Optional regularizer function for the output of this layer.
compute_dtype	The dtype of the layer's computations.
dtype	The dtype of the layer weights.
dtype_policy	The dtype policy associated with this layer.
dynamic	Whether the layer is dynamic (eager-only); set in the constructor.
inbound_nodes	Return Functional API nodes upstream of this layer.
input	Retrieves the input tensor(s) of a layer.
input_arg	Name of the argument to be used as the input to the Keras Layer.
input_mask	Retrieves the input mask tensor(s) of a layer.
input_shape	Retrieves the input shape(s) of a layer.
input_spec	InputSpec instance(s) describing the input format for this layer.
losses	List of losses added using the add_loss() API.
metrics	List of metrics added using the add_metric() API.
name	Name of the layer (string), set in the constructor.
name_scope	Returns a tf.name_scope instance for this class.
non_trainable_variables	Sequence of non-trainable variables owned by this module and its submodules.
non_trainable_weights	List of all non-trainable weights tracked by this layer.
outbound_nodes	Return Functional API nodes downstream of this layer.
output	Retrieves the output tensor(s) of a layer.
output_mask	Retrieves the output mask tensor(s) of a layer.
output_shape	Retrieves the output shape(s) of a layer.
stateful
submodules	Sequence of all sub-modules.
supports_masking	Whether this layer supports computing a mask using compute_mask.
trainable
trainable_variables	Sequence of trainable variables owned by this module and its submodules.
trainable_weights	List of all trainable weights tracked by this layer.
updates
variable_dtype	Alias of Layer.dtype, the dtype of the weights.
variables	Returns the list of all layer variables/weights.
weights	Returns the list of all layer variables/weights.

input_arg

Name of the argument to be used as the input to the Keras Layer. Set to "inputs".

Methods

add_loss(losses, **kwargs)	Add loss tensor(s), potentially dependent on layer inputs.
add_metric(value[, name])	Adds metric tensor to the layer.
add_update(updates)	Add update op(s), potentially dependent on layer inputs.
add_variable(*args, **kwargs)	Deprecated, do NOT use! Alias for add_weight.
add_weight([name, shape, dtype, ...])	Adds a new variable to the layer.
build(input_shape)	Initializes the QNode weights.
call(inputs)	Evaluates the QNode on input data using the initialized weights.
compute_mask(inputs[, mask])	Computes an output mask tensor.
compute_output_shape(input_shape)	Computes the output shape after passing data of shape input_shape through the QNode.
compute_output_signature(input_signature)	Compute the output tensor signature of the layer based on the inputs.
construct(args, kwargs)	Constructs the wrapped QNode on input data using the initialized weights.
count_params()	Count the total number of scalars composing the weights.
finalize_state()	Finalizes the layers state after updating layer weights.
from_config(config)	Creates a layer from its config.
get_config()	Get serialized layer configuration
get_input_at(node_index)	Retrieves the input tensor(s) of a layer at a given node.
get_input_mask_at(node_index)	Retrieves the input mask tensor(s) of a layer at a given node.
get_input_shape_at(node_index)	Retrieves the input shape(s) of a layer at a given node.
get_output_at(node_index)	Retrieves the output tensor(s) of a layer at a given node.
get_output_mask_at(node_index)	Retrieves the output mask tensor(s) of a layer at a given node.
get_output_shape_at(node_index)	Retrieves the output shape(s) of a layer at a given node.
get_weights()	Returns the current weights of the layer, as NumPy arrays.
set_input_argument([input_name])	Set the name of the input argument.
set_weights(weights)	Sets the weights of the layer, from NumPy arrays.
with_name_scope(method)	Decorator to automatically enter the module name scope.

build(input_shape)

Initializes the QNode weights.

Parameters

input_shape (tuple or tf.TensorShape) – shape of input data; this is unused since the weight shapes are already known in the __init__ method.

call(inputs)

Evaluates the QNode on input data using the initialized weights.

Parameters

inputs (tensor) – data to be processed

Returns

output data

Return type

tensor

compute_output_shape(input_shape)

Computes the output shape after passing data of shape input_shape through the QNode.

Parameters

input_shape (tuple or tf.TensorShape) – shape of input data

Returns

shape of output data

Return type

tf.TensorShape

construct(args, kwargs)

Constructs the wrapped QNode on input data using the initialized weights.

This method was added to match the QNode interface. The provided args must contain a single item, which is the input to the layer. The provided kwargs is unused.

Parameters

• args (tuple) – A tuple containing one entry that is the input to this layer

• kwargs (dict) – Unused

get_config()

Get serialized layer configuration

Returns

layer configuration

Return type

dict

static set_input_argument(input_name='inputs')

Set the name of the input argument.

Parameters

input_name (str) – Name of the input argument

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