An Abstraction Hierarchy Toward Productive Quantum Programming

The computer industry, with over seven decades of experience to draw on, has shown that to thrive, there must be a community of software engineers that produce the applications users depend on. Supporting those software engineers so they can write code that performs well across multiple generations of hardware (since application software generally lasts longer than hardware) is fundamental to sustaining the computer industry. Today's quantum software developers must reason at a low level, close to the hardware, which is not sustainable. In this paper, we assert that quantum computing needs high-level abstractions that support a quantum computing applications ecosystem. A single abstraction that bridges from the mental models used by a programmer to the details of how qubits are realized in hardware is unlikely to work. We need a hierarchy of tightly coupled models that define a framework for reasoning about the software development process in quantum computing. We propose an abstraction hierarchy and then explore its utility with two approaches to the eigenvalue estimation problem: (1) a variational algorithm with error mitigation, and (2) phase estimation with error correction. We use our proposed abstraction hierarchy to pinpoint key differences between these approaches and demonstrate how an abstraction hierarchy helps us understand software development. This supports our central conclusion; that it is not enough to understand individual components of a software stack. To make progress, we need to think about an abstraction hierarchy holistically.