Source code for pennylane.optimize.momentum

# Copyright 2018 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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"""Momentum optimizer"""
from pennylane.utils import _flatten, unflatten
from .gradient_descent import GradientDescentOptimizer

[docs]class MomentumOptimizer(GradientDescentOptimizer): r"""Gradient-descent optimizer with momentum. The momentum optimizer adds a "momentum" term to gradient descent which considers the past gradients: .. math:: x^{(t+1)} = x^{(t)} - a^{(t+1)}. The accumulator term :math:`a` is updated as follows: .. math:: a^{(t+1)} = m a^{(t)} + \eta \nabla f(x^{(t)}), with user defined parameters: * :math:`\eta`: the step size * :math:`m`: the momentum Args: stepsize (float): user-defined hyperparameter :math:`\eta` momentum (float): user-defined hyperparameter :math:`m` """ def __init__(self, stepsize=0.01, momentum=0.9): super().__init__(stepsize) self.momentum = momentum self.accumulation = None
[docs] def apply_grad(self, grad, x): r"""Update the variables x to take a single optimization step. Flattens and unflattens the inputs to maintain nested iterables as the parameters of the optimization. Args: grad (array): The gradient of the objective function at point :math:`x^{(t)}`: :math:`\nabla f(x^{(t)})` x (array): the current value of the variables :math:`x^{(t)}` Returns: array: the new values :math:`x^{(t+1)}` """ grad_flat = _flatten(grad) x_flat = _flatten(x) if self.accumulation is None: self.accumulation = [self._stepsize * g for g in grad_flat] else: self.accumulation = [self.momentum * a + self._stepsize * g for a, g in zip(self.accumulation, grad_flat)] x_new_flat = [e-a for a, e in zip(self.accumulation, x_flat)] return unflatten(x_new_flat, x)
[docs] def reset(self): """Reset optimizer by erasing memory of past steps.""" self.accumulation = None