Post-Cut Metadata Inference Attacks on Quantum Circuit Cutting Pipelines

Abstract

Quantum cloud providers can identify a user's algorithm and secret problem structure without ever seeing actual quantum data, simply by analyzing routine metadata collected for billing and system management. Existing confidentiality tools such as blind quantum computation and quantum homomorphic encryption protect the quantum payload itself, but they do not protect this classical orchestration metadata. This leaves an unexplored security risk in the logs generated when a large quantum program is split into smaller pieces to fit onto limited hardware, a process known as circuit cutting. These fragments leak sensitive information through what we term the topological transpilation penalty: the unavoidable depth and gate inflation added when a compiler reorganizes a program for a restricted hardware topology. Tests on a 156-qubit production Quantum Processing Unit (QPU) show that traditional timing side-channels fail in this setting, since hardware control-plane delays mask actual quantum execution time. The unique shape of the transpilation penalty acts instead as a persistent structural fingerprint for the hidden workload. Using 12,000 circuit fragments across eight algorithm families, our attack recovers algorithm family and Hamiltonian k-locality with near-perfect accuracy, achieving instance-disjoint AUC = 1.000 for both. This leakage persists under size-holdout evaluation on unseen circuit scales, with AUC = 0.987 and 0.986 respectively. The cutting mechanism is inferred with AUC = 0.991, and hardware topology is recovered well above chance with AUC = 0.818. These results show that circuit cutting exposes algorithmic intent, and potentially proprietary problem structure, through metadata alone, without any need to observe quantum data.