Optimal VQA convergence is obtained in the ideal noise-free scenario (orange). In reality, VQA is affected by static and transient noise, and estimates can be much worse than ideal (red). QISMET (green) attempts to avoid significant transient error and thereby reaches close to the otherwise unrealistic blue line with only static noise. Prior error miti- gation proposals can help bring the blue (and green) closer to the orange.

In the Noisy Intermediate Scale Quantum (NISQ) era, the dynamic nature of quantum systems causes noise sources to constantly vary over time. Transient errors from the dynamic NISQ noise landscape are challenging to comprehend and are especially detrimental to classes of applications that are iterative and/or long-running, and therefore their timely mitigation is important for quantum advantage in real-world applications. The most popular examples of iterative long-running quantum applications are variational quantum algorithms (VQAs). Iteratively, VQA’s classical optimizer evaluates circuit candidates on an objective function and picks the best circuits towards achieving the application’s target. Noise fluctuation can cause a significant transient impact on the objective function estimation of the VQA iterations’ tuning candidates. This can severely affect VQA tuning and, by extension, its accuracy and convergence. This paper proposes QISMET: Quantum Iteration Skipping to Mitigate Error Transients, to navigate the dynamic noise landscape of VQAs. QISMET actively avoids instances of high fluctuating noise which are predicted to have a significant transient error impact on specific VQA iterations. To achieve this, QISMET estimates transient error in VQA iterations and designs a controller to keep the VQA tuning faithful to the transient-free scenario. By doing so, QISMET efficiently mitigates a large portion of the transient noise impact on VQAs and is able to improve the fidelity by 1.3x-3x over a traditional VQA baseline, with 1.6-2.4x improvement over alternative approaches, across different applications and machines. DOI 10.1145/3575693.3575739

Gokul Subramanian Ravi, Kaitlin Smith, Jonathan M. Baker, Tejas Kannan, Nathan Earnest, Ali Javadi-Abhari, Henry Hoffmann, and Frederic T. Chong