Source code for pennylane.templates.embeddings.amplitude

# Copyright 2018-2020 Xanadu Quantum Technologies Inc.

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Contains the ``AmplitudeEmbedding`` template.
# pylint: disable-msg=too-many-branches,too-many-arguments,protected-access
import math
import numpy as np
from pennylane.templates.decorator import template
from pennylane.ops import QubitStateVector
from pennylane.templates.utils import (
from pennylane.variable import Variable

# tolerance for normalization

[docs]@template def AmplitudeEmbedding(features, wires, pad=None, normalize=False): r"""Encodes :math:`2^n` features into the amplitude vector of :math:`n` qubits. By setting ``pad`` to a real or complex number, ``features`` is automatically padded to dimension :math:`2^n` where :math:`n` is the number of qubits used in the embedding. To represent a valid quantum state vector, the L2-norm of ``features`` must be one. The argument ``normalize`` can be set to ``True`` to automatically normalize the features. If both automatic padding and normalization are used, padding is executed *before* normalizing. .. note:: On some devices, ``AmplitudeEmbedding`` must be the first operation of a quantum node. .. warning:: ``AmplitudeEmbedding`` calls a circuit that involves non-trivial classical processing of the features. The ``features`` argument is therefore **not differentiable** when using the template, and gradients with respect to the features cannot be computed by PennyLane. Args: features (array): input array of shape ``(2^n,)`` wires (Sequence[int] or int): :math:`n` qubit indices that the template acts on pad (float or complex): if not None, the input is padded with this constant to size :math:`2^n` normalize (Boolean): controls the activation of automatic normalization Raises: ValueError: if inputs do not have the correct format .. UsageDetails:: Amplitude embedding encodes a normalized :math:`2^n`-dimensional feature vector into the state of :math:`n` qubits: .. code-block:: python import pennylane as qml from pennylane.templates import AmplitudeEmbedding dev = qml.device('default.qubit', wires=2) @qml.qnode(dev) def circuit(f=None): AmplitudeEmbedding(features=f, wires=range(2)) return qml.expval(qml.PauliZ(0)) circuit(f=[1/2, 1/2, 1/2, 1/2]) Checking the final state of the device, we find that it is equivalent to the input passed to the circuit: >>> dev._state [0.5+0.j 0.5+0.j 0.5+0.j 0.5+0.j] **Passing features as positional arguments to a quantum node** The ``features`` argument of ``AmplitudeEmbedding`` can in principle also be passed to the quantum node as a positional argument: .. code-block:: python @qml.qnode(dev) def circuit(f): AmplitudeEmbedding(features=f, wires=range(2)) return qml.expval(qml.PauliZ(0)) However, due to non-trivial classical processing to construct the state preparation circuit, the features argument is **not differentiable**. >>> g = qml.grad(circuit, argnum=0) >>> g([1,1,1,1]) ValueError: Cannot differentiate wrt parameter(s) {0, 1, 2, 3}. **Normalization** The template will raise an error if the feature input is not normalized. One can set ``normalize=True`` to automatically normalize it: .. code-block:: python @qml.qnode(dev) def circuit(f=None): AmplitudeEmbedding(features=f, wires=range(2), normalize=True) return qml.expval(qml.PauliZ(0)) circuit(f=[15, 15, 15, 15]) The re-normalized feature vector is encoded into the quantum state vector: >>> dev._state [0.5 + 0.j, 0.5 + 0.j, 0.5 + 0.j, 0.5 + 0.j] **Padding** If the dimension of the feature vector is smaller than the number of amplitudes, one can automatically pad it with a constant for the missing dimensions using the ``pad`` option: .. code-block:: python from math import sqrt @qml.qnode(dev) def circuit(f=None): AmplitudeEmbedding(features=f, wires=range(2), pad=0.) return qml.expval(qml.PauliZ(0)) circuit(f=[1/sqrt(2), 1/sqrt(2)]) >>> dev._state [0.70710678 + 0.j, 0.70710678 + 0.j, 0.0 + 0.j, 0.0 + 0.j] **Operations before the embedding** On some devices, ``AmplitudeEmbedding`` must be the first operation in the quantum node. For example, ``'default.qubit'`` complains when running the following circuit: .. code-block:: python dev = qml.device('default.qubit', wires=2) @qml.qnode(dev) def circuit(f=None): qml.Hadamard(wires=0) AmplitudeEmbedding(features=f, wires=range(2)) return qml.expval(qml.PauliZ(0)) >>> circuit(f=[1/2, 1/2, 1/2, 1/2]) pennylane._device.DeviceError: Operation QubitStateVector cannot be used after other Operations have already been applied on a default.qubit device. """ ############# # Input checks check_no_variable(pad, msg="'pad' cannot be differentiable") check_no_variable(normalize, msg="'normalize' cannot be differentiable") wires = check_wires(wires) n_amplitudes = 2 ** len(wires) expected_shape = (n_amplitudes,) if pad is None: shape = check_shape( features, expected_shape, msg="'features' must be of shape {}; got {}. Use the 'pad' " "argument for automated padding." "".format(expected_shape, get_shape(features)), ) else: shape = check_shape( features, expected_shape, bound="max", msg="'features' must be of shape {} or smaller " "to be padded; got {}" "".format(expected_shape, get_shape(features)), ) check_type( pad, [float, complex, type(None)], msg="'pad' must be a float or complex; got {}".format(pad), ) check_type(normalize, [bool], msg="'normalize' must be a boolean; got {}".format(normalize)) ############### ############# # Preprocessing # pad n_features = shape[0] if pad is not None and n_amplitudes > n_features: features = np.pad( features, (0, n_amplitudes - n_features), mode="constant", constant_values=pad ) # normalize if isinstance(features[0], Variable): feature_values = [s.val for s in features] norm = np.sum(np.abs(feature_values) ** 2) else: norm = np.sum(np.abs(features) ** 2) if not np.isclose(norm, 1.0, atol=TOLERANCE): if normalize or pad: features = features / math.sqrt(norm) else: raise ValueError( "'features' must be a vector of length 1.0; got length {}." "Use 'normalization=True' to automatically normalize.".format(norm) ) ############### features = np.array(features) QubitStateVector(features, wires=wires)