  Few-layer black phosphorus possesses unique electronic properties giving rise
to distinct quantum phenomena and thus offers a fertile platform to explore the
emergent correlation phenomena in low dimensions. A great progress has been
demonstrated in improving the quality of hole-doped few-layer black phosphorus
and its quantum transport studies, whereas the same achievements are rather
modest for electron-doped few-layer black phosphorus. Here, we report the
ambipolar quantum transport in few-layer black phosphorus exhibiting
undoubtedly the quantum Hall effect for hole transport and showing clear
signatures of the quantum Hall effect for electron transport. By bringing the
spin-resolved Landau levels of the electron-doped black phosphorus to the
coincidence, we measure the spin susceptibility $\chi_s=m^\ast
g^\ast=1.1\pm0.03$. This value is larger than for hole-doped black phosphorus
and illustrates an energetically equidistant arrangement of spin-resolved
Landau levels. Evidently, the n-type black phosphorus offers a unique platform
with equidistant sequence of spin-up and spin-down states for exploring the
quantum spintronic.
