The role of reference states in pulse-level VQEs

Pulse-level control in a variational quantum eigensolver is the ultimate hardware-efficient ansatz, enabling ground state preparation in the least time physically possible on the quantum device. However, optimizing such time-optimal pulses is a challenge. In this talk, we will explore the role of the reference state in enabling efficient optimization of pulse-level VQEs. In particular, we show why the standard Jordan-Wigner mapping for the Hartree-Fock state is apparently incompatible with pulse-level gradient descent on transmon devices. We offer several solutions to this problem, and we compare their efficacy with both numerical simulation and hardware experiments.