Leakage reduces device coherence demands for molecular simulations with pulse-level VQE

Variational Quantum Eigensolver (VQE) algorithms have generally involved the optimization of a series of parameterized quantum gates designed to approximate the quantum state of a molecule. While this has many attractive features such as robustness to noise, the limited coherence times and frequent gate errors limit the number of these gates in near-term quantum devices. Our team's proposed method (ctrl-VQE) uses a pulse shaping routine to prepare the final state by variationally optimizing the pulse parameters directly instead of using a parameterized circuit. Through numerical simulations, we have observed reductions in the coherence time required of multiple orders of magnitude. Consequently, ctrl-VQE may extend the applicability of quantum algorithms for near-term quantum devices to highly entangled systems, such as strongly correlated molecules. In this talk, I will discuss recent results related to the behaviour of ctrl-VQE at the minimum time of evolution. When constraining the pulse to within some experimentally convenient bounds, we reveal the emergence of bang-bang shapes of optimal pulses, consistent with Pontryagin's Principle. Interestingly, we observe that leakage outside computational space using qudits with access to more than two states (something that is usually considered disadvantageous) actually speeds up the state preparation, which ultimately further reduces device coherence time demands.