Fast charging of an Ising spin pair quantum battery using optimal control

TL;DR

This paper demonstrates how optimal control theory can be used to efficiently charge a quantum spin pair battery, achieving full energy storage through a specific bang-singular-bang pulse sequence.

Contribution

It introduces a novel optimal control approach for maximizing energy in a two-spin quantum battery, including explicit pulse sequences and conditions for complete charging.

Findings

Optimal control sequences enable full charging of the quantum battery.

Bang-singular-bang pulses are essential for achieving maximum energy storage.

Explicit transcendental equations determine pulse durations for optimal charging.

Abstract

We consider the problem of maximizing the stored energy for a given charging duration in a quantum battery composed of a pair of spins-$1/2$ with Ising coupling starting from the spin-down state, using bounded transverse field control. We map this problem to an optimal control problem on a single qubit and using optimal control theory we show that, although a single bang pulse can quickly achieve considerable charging levels for relatively large upper control bounds, higher levels of stored energy including complete charging are accomplished by a bang-singular-bang pulse-sequence, where the intermediate singular pulse is an Off pulse. If the control is restricted between zero and a maximum value, the initial and final bang pulses attain the maximum bound but have different durations, while if it is restricted between symmetric negative and positive boundaries, the bang pulses have the same duration but opposite boundary values. For both cases we provide transcendental equations from which the durations of the individual pulses in the optimal pulse-sequence can be calculated. For the case of full charging we surprisingly find that the three ``switching" functions for the equivalent qubit problem become zero while the adjoint ket does not, in consistency with optimal control theory.