Spectral invariance and maximality properties of the frequency spectrum of quantum neural networks

TL;DR

This paper investigates the frequency spectrum of Quantum Neural Networks (QNNs), revealing that the maximum spectrum depends only on the product of qubits and layers, and introduces algebraic and combinatorial methods to characterize and optimize it.

Contribution

It introduces a spectral invariance under area-preserving transformations and extends theoretical bounds on the maximum frequency spectrum of QNNs using novel combinatorial approaches.

Findings

Maximum frequency spectrum depends only on the area $A=RL$

Spectral invariance under area-preserving transformations

Extended bounds using Golomb rulers and the relaxed turnpike problem

Abstract

We analyze the frequency spectrum of Quantum Neural Networks (QNNs) using Minkowski sums, which yields a compact algebraic description and permits explicit computation. Using this description, we prove several maximality results for broad classes of QNN architectures. Under some mild technical conditions we establish a bijection between classes of models with the same area $A:=R\cdot L$ that preserves the frequency spectrum, where $R$ denotes the number of qubits and $L$ the number of layers, which we consequently call spectral invariance under area-preserving transformations. With this we explain the symmetry in $R$ and $L$ in the results often observed in the literature and show that the maximal frequency spectrum depends only on the area $A=RL$ and not on the individual values of $R$ and $L$. Moreover, we collect and extend existing results and specify the maximum possible frequency spectrum of a QNN with an arbitrary number of layers as a function of the spectrum of its generators. In the case of arbitrary dimensional generators, where our two introduced notions of maximality differ, we extend existing Golomb ruler based results and introduce a second novel approach based on a variation of the turnpike problem, which we call the relaxed turnpike problem. We clarify comprehensively how the generators of a QNN must be chosen in order to obtain a maximal frequency spectrum for a given area $A$, thereby contributing to a deeper theoretical understanding. However, our numerical experiments show that trainability depends not only on $A = RL$, but also on the choice of $(R,L)$, so that knowledge of the maximum frequency spectrum alone is not sufficient to ensure good trainability.