Writing and verifying a Quantum optimizing compiler (keynote)

Abstract

As quantum computing hardware evolves, it will continue to face four key limitations: low qubit counts, limited connectivity, high error rates, and short coherence times. Quantum compilers play a key role in addressing these issues, reducing the number of qubits needed to perform a computation, mapping those qubits to the desired hardware, and minimizing the number of costly operations, both in terms of error rates and execution time. However, we cannot afford for compilers to become another source of bugs: Quantum computing is an inherently probabilistic and error-prone process and any additional sources of error are unlikely to be properly diagnosed. To address this, we present VOQC, a verified optimizing compiler for quantum circuits. VOQC heavily optimizes quantum programs while guaranteeing that the output is quantum-mechanically indistinguishable from the input program, up to permutation of qubits. This ensures that compilation produces an equivalent program that is executable on the given hardware. In this talk, we will address the key differences between classical and quantum compilation and the challenges unique to the latter. We will discuss the design decisions that underlie VOQC and how they enable its most powerful optimizations. Finally, we will discuss the developments since VOQC was first published, both within the VOQC toolchain and competing compilers, verified and unverified.