Single-component gradient rules for variational quantum algorithms

TL;DR

This paper unifies and generalizes gradient evaluation methods for variational quantum algorithms, introducing a new parameter shift rule that encompasses existing methods and offers insights into their limitations and potential extensions.

Contribution

It presents a generalized parameter shift rule for quantum gradient estimation, unifying existing methods and analyzing their fundamental constraints.

Findings

Unified family of gradient rules for variational quantum algorithms.

Proved the non-existence of a single-evaluation parameter shift rule.

Introduced a new perspective for developing future gradient rules.

Abstract

Many near-term quantum computing algorithms are conceived as variational quantum algorithms, in which parameterized quantum circuits are optimized in a hybrid quantum-classical setup. Examples are variational quantum eigensolvers, quantum approximate optimization algorithms as well as various algorithms in the context of quantum-assisted machine learning. A common bottleneck of any such algorithm is constituted by the optimization of the variational parameters. A popular set of optimization methods work on the estimate of the gradient, obtained by means of circuit evaluations. We will refer to the way in which one can combine these circuit evaluations as gradient rules. This work provides a comprehensive picture of the family of gradient rules that vary parameters of quantum gates individually. The most prominent known members of this family are the parameter shift rule and the finite differences method. To unite this family, we propose a generalized parameter shift rule that expresses all members of the aforementioned family as special cases, and discuss how all of these can be seen as providing access to a linear combination of exact first- and second-order derivatives. We further prove that a parameter shift rule with one non-shifted evaluation and only one shifted circuit evaluation can not exist does not exist, and introduce a novel perspective for approaching new gradient rules.