Adversarial Effects on Expressibility and Trainability in Distributed Variational Quantum Algorithms

TL;DR

This paper investigates how adversarial perturbations in distributed variational quantum algorithms can compromise their expressibility and trainability, introducing new metrics and analysis to understand these vulnerabilities.

Contribution

It develops a framework linking entanglement perturbations to gate noise, introduces Kraus expressibility, and analyzes the trade-off affecting quantum algorithm robustness.

Findings

Adversarial entanglement perturbations induce structured gate noise.

Kraus expressibility quantifies the impact of noise on quantum channels.

Adversaries can manipulate expressibility to bias optimization while avoiding barren plateaus.

Abstract

Distributed quantum algorithms offer a promising pathway to scale variational quantum algorithms beyond the constraints of noisy intermediate-scale quantum hardware. However, existing approaches implicitly assume a trusted entanglement-sharing layer across quantum processors. We show that this assumption introduces a fundamental vulnerability: adversarial perturbations of shared entanglement induce structured gate-level noise that directly impacts quantum learning. We develop a framework that maps entanglement-level perturbations to gate-level noise via an explicit Kraus representation. To quantify their impact, we introduce Kraus expressibility, a metric that generalizes unitary expressibility to noisy quantum channels. We then establish a trade-off between Kraus expressibility and trainability of noisy quantum circuits through gradient variance analysis. Our analysis reveals that an adversary can manipulate Kraus expressibility to maintain sufficiently large cost gradients (avoiding barren plateaus) while systematically biasing optimization toward incorrect solutions. We validate these findings through numerical simulations, demonstrating adversarial degradation of expressibility and trainability.