Backdoor Threats in Variational Quantum Circuits: Taxonomy, Attacks, and Defenses

TL;DR

This paper surveys backdoor threats in variational quantum circuits, detailing attack types, threat models, defenses, and highlighting the need for quantum-aware security solutions in NISQ-era quantum computing.

Contribution

It provides a comprehensive taxonomy and analysis of backdoor attacks in VQCs, formalizes threat models, and reviews existing defenses and their limitations.

Findings

Backdoor attacks can embed hidden malicious behaviors in VQCs.

Current detection methods have limitations against quantum-specific threats.

The survey highlights the need for developing quantum-aware defense strategies.

Abstract

Variational quantum algorithms (VQAs) are a central paradigm for noisy intermediate-scale (NISQ) quantum computing, yet their reliance on predesigned and pretrained variational quantum circuits (VQCs) introduces critical security vulnerabilities, particularly backdoor attacks. These attacks embed hidden malicious behaviors that remain dormant under normal conditions but are activated by specific triggers, leading to adversarial outcomes such as incorrect predictions or manipulated objective values. This paper presents a survey of backdoor attacks in VQCs, covering data-poisoning, compiler-level, and quantum-native mechanisms. We formalize key terminology and threat models, and review existing attack strategies along with their empirical characteristics. We also analyze current detection and defense approaches, highlighting their limitations, especially against quantum-specific threats. By synthesizing recent advances, this survey outlines the evolving security landscape of VQCs and identifies key challenges and future directions for developing robust, quantum-aware defenses in hybrid quantum-classical systems.