Quantum phase transitions take place at zero temperature when a parameter of the Hamiltonian describing the system is tuned to a critical value. The quantum phase transition of superconductor-to-metal type in 2D is believed to be driven by quantum phase fluctuations, which can be realized by tuning conductance of the 2D film. However, so far, a direct experimental proof is missing, due to the lack of a 2D metal substrate with tunable conductivity. Graphene is then naturally a best candidate for this study: its exposed and inert 2D surface allows the intimate coupling of superconductors, giving rise to superconducting proximity effect. At the same time, gate-tunability of the electron density opens the possibility to study the stability of the superconducting phase against quantum fluctuations. Here, we show that an
array of superconducting islands deposited on graphene can be gate-tuned from a superconducting ground state to a new metallic state. Below a critical conductance, the ensuing metallic state emanates from quantum phase fluctuations of individual superconducting islands. Our results therefore provide evidence for a superconducting quantum fluctuations-induced metallic state at very low temperatures, and may provide a hint to the understanding of long-standing issue of "zero-temperature" bosonic metallic state observed earlier in a many systems.