Verifiying Noisy Quantum Devices at Scale

ended

Quantum computing is undergoing a phase transition, as the field shifts from asking “what if” (quantum computers existed) to ``how do'' (we leverage the power of emerging quantum devices). Progress in the design of experimental quantum systems raises a unique challenge: given that their size already precludes direct classical simulation, and given that quantum states are perturbed by observation, how does one test and certify the new devices? This difficulty is starkly evidenced by the ongoing race for demonstrating a quantum computational advantage. Given that the task cannot be replicated classically, can it be verified? The main goals of this project are to develop effective means to characterize, certify and harness complex quantum states and devices. To achieve this we employ the framework of interactive proofs from classical complexity theory. We use this to model interactions as varied as demonstrations of quantumness, the delegation of a quantum computation, or cryptographic tasks such as quantum key distribution. The major challenges that we address are scalability, noise tolerance, and security. To achieve scalability we build on complexity-theoretic techniques such as the notion of probabilistically checkable proofs. We focus on the design of protocols that successfully complete even when the quantum device is slightly noisy. The security notions that we seek encompass device independence (no a priori trust is placed on the quantum equipment) and side information (privacy should be guaranteed with respect to any external party). Large-scale experimental demonstrations of quantum networks are currently being planned in many countries, including a leading European effort (EuroQCI). Our work lays the theoretical groundwork for scalable, secure and trustworthy interactions in such networks. It paves the way to making the power of quantum devices for computation and communication available to a wider public remotely and through classical means.