Faculty @ University of Siegen

Thursday June 30 – 15.45 BST

Coherent speed-up in the collisional charging of quantum batteries 

We study self-contained collisional models for the charging of a quantum battery by a stream of identical nonequilibrium qubit units, comparing the charging power for coherent and incoherent protocols on an initially empty battery. The battery can be an oscillator, a large spin, or any other linear energy ladder with a ground state, while the qubits are assumed to be resonant with the ladder, which obviates the need for additional work input as they exchange excitations with the battery. 
When the qubits are prepared in a population-inverted, incoherent mixture of energy eigenstates, the energy and ergotropy gain in the battery can be described by a generalized classical random walk process with level-dependent rates. We provide an upper bound on the charging power for any incoherent protocol, including adaptive charging strategies.  We show that this bound can be broken by non-adaptive protocols with qubits that contain quantum coherence, thus demonstrating a quantum speedup at the level of a single battery. 
In homogeneous ladder models with level-independent transition rates, the speedup can be attributed to quantum walk-like interference effects. In oscillator and spin batteries, the greatest speedup is reached in the limit when the charging process approximates a coherent Rabi drive. 
We show that the oscillator battery model could be realized in a state-of-the-art cavity micromaser setup, and the coherent speedup could be achieved in the presence of realistic cavity damping.