Fast quantum state preparation and bath dynamics using non-Gaussian variational ansatz and quantum optimal control

• URL: http://arxiv.org/abs/2306.01730v1
• Date: Fri, 2 Jun 2023 17:57:18 GMT
• Title: Fast quantum state preparation and bath dynamics using non-Gaussian variational ansatz and quantum optimal control
• Authors: Liam J. Bond, Arghavan Safavi-Naini, and Ji\v{r}\'i Min\'a\v{r}
• Abstract summary: We combine quantum optimal control with a variational ansatz based on non-Gaussian states for fast, non-adiabatic preparation of quantum many-body states. We show that the typical control quantity, the leakage from the variational manifold, provides only a loose bound on the state's fidelity. We apply the ansatz to the study of the out-of-time-order-correlator (OTOC) of the bath modes in a non-perturbative regime.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: We combine quantum optimal control with a variational ansatz based on non-Gaussian states for fast, non-adiabatic preparation of quantum many-body states. We demonstrate this on the example of the spin-boson model, and use a multi-polaron ansatz to prepare near-critical ground states. For one mode, we achieve a reduction in infidelity of up to $\approx 60$ ($\approx 20$) times compared to linear (optimised local adiabatic) ramps respectively; for many modes we achieve a reduction in infidelity of up to $\approx 5$ times compared to non-adiabatic linear ramps. Further, we show that the typical control quantity, the leakage from the variational manifold, provides only a loose bound on the state's fidelity. Instead, in analogy to the bond dimension of matrix product states, we suggest a controlled convergence criterion based on the number of polarons. Finally, motivated by the possibility of realizations in trapped ions, we study the dynamics of a system with bath properties going beyond the paradigm of (sub/super) Ohmic couplings. We apply the ansatz to the study of the out-of-time-order-correlator (OTOC) of the bath modes in a non-perturbative regime. The scrambling time is found to be a robust feature only weakly dependent on the details of the coupling between the bath and the spin.

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