Recent years have witnessed an explosion of interest in quantum devices for the production, storage, and transfer of energy. This Colloquium concentrates on the field of quantum energy storage by reviewing recent theoretical and experimental progress in quantum batteries. Provided first is a theoretical background discussing the advantages that quantum batteries offer with respect to their classical analogs. The existing quantum many-body battery models are then reviewed and a thorough discussion of important issues related to their “open nature” is presented. The Colloquium concludes with a discussion of promising experimental implementations, preliminary results available in the literature, and perspectives.

In a major step toward the development of a quantum battery, superabsorption has been achieved in an organic microcavity.

We introduce an open quantum-battery protocol using dark states to achieve both superextensive capacity and power density, with noninteracting spins coupled to a reservoir. Further, our power density actually scales with the number of spins $N$ in the battery. We show that the enhanced capacity and power are correlated with entanglement. While connected to the charger, the charged state of the battery is a steady state, stabilized through quantum interference in the open system.

We derive the gravitonic Casimir effect with nonidealized boundary conditions. This allows the quantification of the gravitonic contribution to the Casimir effect from real bodies. We quantify the meagerness of the gravitonic Casimir effect in ordinary matter. We also quantify the enhanced effect produced by the speculated Heisenberg-Couloumb (HC) effect in superconductors, thereby providing a test for the validity of the HC theory, and, consequently, the existence of gravitons.

We study entanglement entropy (EE) as a signature of quantum chaos in ergodic and nonergodic systems. In particular we look at the quantum kicked top and kicked rotor as multispin systems and investigate the single-spin EE which characterizes bipartite entanglement of this spin with the rest of the system. We study the correspondence of the Kolmogorov-Sinai entropy of the classical kicked systems with the EE of their quantum counterparts. We find that EE is a signature of global chaos in ergodic systems and local chaos in nonergodic systems. In particular, we show that EE can be maximized even when systems are highly nonergodic, when the corresponding classical system is locally chaotic. In contrast, we find evidence that the quantum analog of Kolmogorov-Arnol'd-Moser (KAM) tori are tori of low entanglement entropy. We conjecture that entanglement should play an important role in any quantum KAM theory.