  The influence of static disorder on a quantum phase transition (QPT) is a
fundamental issue in condensed matter physics. As a prototypical example of a
disorder-tuned QPT, the superconductor-insulator transition (SIT) has been
investigated intensively over the past three decades, but as yet without a
general consensus on its nature. A key element is good control of disorder.
Here, we present an experimental study of the SIT based on precise in-situ
tuning of disorder in dual-gated bilayer graphene proximity-coupled to two
superconducting electrodes through electrical and reversible control of the
band gap and the charge carrier density. In the presence of a static disorder
potential, Andreev-paired carriers formed close to the Fermi level in bilayer
graphene constitute a randomly distributed network of proximity-induced
superconducting puddles. The landscape of the network was easily tuned by
electrical gating to induce percolative clusters at the onset of
superconductivity. This is evidenced by scaling behavior consistent with the
classical percolation in transport measurements. At lower temperatures, the
solely electrical tuning of the disorder-induced landscape enables us to
observe, for the first time, a crossover from classical to quantum percolation
in a single device, which elucidates how thermal dephasing engages in
separating the two regimes.
