Ultrafast critical ground state preparation via bang-bang protocols

TL;DR

This paper demonstrates that bang-bang protocols enable rapid, high-fidelity preparation of critical ground states in quantum systems, outperforming traditional optimal control methods in speed and computational efficiency.

Contribution

It introduces bang-bang control as an effective and computationally efficient method for critical ground state preparation in quantum systems, especially near phase transitions.

Findings

High-fidelity ground state achieved faster than standard methods

Reduced computational complexity of control optimization

Successful benchmarking on Landau-Zener and Lipkin-Meshkov-Glick models

Abstract

The fast and faithful preparation of the ground state of quantum systems is a challenging task but crucial for several applications in the realm of quantum-based technologies. Decoherence poses a limit to the maximum time-window allowed to an experiment to faithfully achieve such desired states. This is of particular significance in critical systems, where the vanishing energy gap challenges an adiabatic ground state preparation. We show that a bang-bang protocol, consisting of a time evolution under two different values of an externally tunable parameter, allows for a high-fidelity ground state preparation in evolution times no longer than those required by the application of standard optimal control techniques, such as the chopped-random basis quantum optimization. In addition, owing to their reduced number of variables, such bang-bang protocols are very well suited to optimization purposes, reducing the increasing computational cost of other optimal control protocols. We benchmark the performance of such approach through two paradigmatic models, namely the Landau-Zener and the Lipkin-Meshkov-Glick model. Remarkably, the critical ground state of the latter model can be prepared with a high fidelity in a total evolution time that scales slower than the inverse of the vanishing energy gap.