Source code for pennylane.collections.sum

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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Contains the sum function, for summing QNode collections.
# pylint: disable=too-many-arguments,import-outside-toplevel

from .apply import apply

[docs]def sum(x): """Lazily sum the constituent QNodes of a :class:`QNodeCollection`. Args: x (QNodeCollection): a QNode collection of independent QNodes. .. seealso:: :func:`~.apply`, :func:`` **Example:** We can create a QNodeCollection using :func:``: >>> dev = qml.device("default.qubit", wires=2) >>> obs_list = [qml.PauliX(0) @ qml.PauliZ(1), qml.PauliZ(0) @ qml.PauliZ(1)] >>> qnodes =, obs_list, dev, interface="torch") For the cost function, we now sum the results of all QNodes in the collection: >>> cost = qml.sum(qnodes) This is a lazy summation --- no QNode evaluation has yet occured. Evaluation only occurs when the returned function ``cost`` is evaluated: >>> np.random.seed(42) >>> shape = qml.templates.StronglyEntanglingLayers.shape(n_layers=3, n_wires=2) >>> x = np.random.random(shape) >>> cost(x) tensor(0.9177, dtype=torch.float64) """ if hasattr(x, "interface") and x.interface is not None: if x.interface == "tf": import tensorflow as tf return apply(tf.reduce_sum, x) if x.interface == "torch": import torch return apply(torch.sum, x) if x.interface in ("autograd", "numpy"): from autograd import numpy as np return apply(np.sum, x) if x.interface == "jax": import jax.numpy as jnp return apply(jnp.sum, x) raise ValueError(f"Unknown interface {x.interface}") import numpy as np return apply(np.sum, x)