  The currently established electronic phase diagram of cuprates is based on a
study of single- and double-layered compounds. These CuO$_2$ planes, however,
are directly contacted with dopant layers, thus inevitably disordered with an
inhomogeneous electronic state. Here, we solve this issue by investigating a
6-layered Ba$_2$Ca$_5$Cu$_6$O$_{12}$(F,O)$_2$ with inner CuO$_2$ layers, which
are clean with the extremely low disorder, by angle-resolved photoemission
spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi
pocket with a doping level less than 1% to exhibit well-defined quasiparticle
peaks which surprisingly lack the polaronic feature. This provides the first
evidence that the slightest amount of carriers is enough to turn a Mott
insulating state into a metallic state with long-lived quasiparticles. By
tuning hole carriers, we also find an unexpected phase transition from the
superconducting to metallic states at 4%. Our results are distinct from the
nodal liquid state with polaronic features proposed as an anomaly of the
heavily underdoped cuprates.
