Upper bounds on charging power and tangible advantage in quantum batteries

A quantum battery is expected to outperform its classical counterpart due to quantum effects. Usually, in a quantum battery made of N cells, quantum advantage is demonstrated through super-extensive scaling of the upper bound to the charging power with N. In this work, we show that potential quantum advantage as measured by the power bounds need not translate to tangible advantage in practice. We demonstrate this by considering an all-to-all coupled spin-chain model of a quantum battery with 2-local interactions. It exhibits super-extensive charging when analyzed using the upper bound derived from the uncertainty principle. Unlike the previously studied models, the contribution to this apparent quantum advantage is twofold—arising from both the battery and the charger. The model is also experimentally friendly, as it does not require global couplings and yet generates genuine multipartite entanglement. However, we demonstrate that the potential quantum advantage in this scenario is not tangible by employing a tighter upper bound on power. Additionally, we show that even this tighter bound can fail in a range of physical situations and indicate a quantum enhancement that is intangible in practice. Hence, we argue that actual power transferred must be evaluated along with proper characterization of the resources before claiming quantum advantage.