E-LoQ: Enhanced Locking for Quantum Circuit IP Protection

In recent years, quantum computing has started to demonstrate superior efficiency to classical computing. In quantum computing, quantum circuits that implement specific quantum algorithms are usually not directly executable on quantum computer hardware. Quantum circuit compilers decompose high-level quantum gates into the hardware's native gates and optimize the circuits for accuracy and performance. However, untrusted quantum compilers risk stealing original quantum designs (quantum circuits), leading to the theft of sensitive intellectual property (IP). In classical computing, logic locking is a family of techniques to secure integrated circuit (ICs) designs against reverse engineering and IP piracy. This technique involves inserting a keyed value into the circuit, ensuring the correct output is achieved only with the correct key. To address similar issues in quantum circuit protection, we propose an enhanced locking technique for quantum circuits (E-LoQ) where multiple key bits can be condensed into one key qubit. Compared to previous work that used one qubit for each key bit, our approach achieves higher security levels. We have demonstrated the practicality of our method through experiments on a set of benchmark quantum circuits. The effectiveness of E-LoQ was measured by assessing the divergence distance from the original circuit. Our results demonstrate that E-LoQ effectively conceals the function of the original quantum circuit, with an average fidelity degradation of less than 1 %.