CutBackdoor: A Circuit Cut Triggered Backdoor Attack on Variational Quantum Algorithms

2026-07-20

AI Summary

This work addresses the lack of effective security mechanisms in variational quantum algorithms concerning parameter supply chains and circuit cutting by proposing a novel covert backdoor attack. The method uniquely exploits the quantum circuit cutting process itself as a trigger, enabling significant amplification of reconstruction errors along cut paths—increasing energy errors by 1.3–2.9×—without altering the circuit architecture or requiring any intervention at the deployment end, while preserving near-ideal performance on the full uncut circuit. Leveraging the CutQC framework and integrating variational quantum eigensolver (VQE), variational quantum deflation (VQD), and quantum approximate optimization algorithm (QAOA), the attack’s efficacy and robustness are validated on IBM noisy quantum hardware, with zero-noise extrapolation further demonstrating its resilience against detection.

Abstract

Variational Quantum Algorithms (VQAs) are a leading paradigm for near-term quantum computing, combining parameterized quantum circuits with classical optimization across quantum chemistry, combinatorial optimization, and quantum machine learning. Since real-world VQA deployments routinely require circuits that exceed available hardware capacity, quantum circuit cutting has become an indispensable execution strategy, and pre-trained parameters are increasingly distributed through public repositories, introducing supply-chain security risks that have received little attention. Prior quantum backdoor attacks either introduce detectable circuit modifications or depend on device-specific noise, and none consider circuit cutting as an attack surface. We present CutBackdoor, the first parameter-supply-chain backdoor that uses cut circuit execution from CutQC as the deployment-time trigger against VQAs. Under noisy finite-shot circuit-cut execution, poisoned parameters preserve full-circuit validation performance while substantially increasing cut-path reconstruction error, without any circuit modification. The trigger activates when a resource-limited victim responds to a qubit-capacity mismatch by invoking the cutting workflow, requiring no attacker presence at deployment. We provide a theoretical analysis and empirically validate it across varying shot budgets. Evaluation across multiple VQA benchmarks on IBM quantum backends demonstrates cut-path energy amplification of $1.3\times$ to $2.9\times$ \revA{over clean baselines on the VQE and VQD benchmarks while maintaining small stealthiness error on the full-circuit path. The cut-path gap persists across the evaluated backends and cut placements under matched compilation; Zero-Noise Extrapolation provides only partial mitigation, and the diagonal-cost QAOA benchmark delineates the attack's structural boundary

Problem

• backdoor attack
• variational quantum algorithms
• quantum circuit cutting
• supply-chain security
• parameter poisoning

Innovation

• backdoor attack
• variational quantum algorithms
• quantum circuit cutting
• parameter supply chain
• stealthiness