  Quantum batteries harness the unique properties of quantum mechanics to
enhance energy storage compared to conventional batteries. In particular, they
are predicted to undergo superextensive charging, where batteries with larger
capacity actually take less time to charge. Up until now however, they have not
been experimentally demonstrated, due to the challenges in quantum coherent
control. Here we implement an array of two-level systems coupled to a photonic
mode to realise a Dicke quantum battery. Our quantum battery is constructed
with a microcavity formed by two dielectric mirrors enclosing a thin film of a
fluorescent molecular dye in a polymer matrix. We use ultrafast optical
spectroscopy to time resolve the charging dynamics of the quantum battery at
femtosecond resolution. We experimentally demonstrate superextensive increases
in both charging power and storage capacity, in agreement with our theoretical
modelling. We find that decoherence plays an important role in stabilising
energy storage, analogous to the role that dissipation plays in photosynthesis.
This experimental proof-of-concept is a major milestone towards the practical
application of quantum batteries in quantum and conventional devices. Our work
opens new opportunities for harnessing collective effects in light-matter
coupling for nanoscale energy capture, storage, and transport technologies,
including the enhancement of solar cell efficiencies.
