  Higher-order topological phases give rise to new bulk and boundary physics,
as well as new classes of topological phase transitions. While the realization
of higher-order topological phases has been confirmed in many platforms by
detecting the existence of gapless boundary modes, a direct determination of
the higher-order topology and related topological phase transitions through the
bulk in experiments has still been lacking. To bridge the gap, in this work we
carry out the simulation of a two-dimensional second-order topological phase in
a superconducting qubit. Owing to the great flexibility and controllability of
the quantum simulator, we observe the realization of higher-order topology
directly through the measurement of the pseudo-spin texture in momentum space
of the bulk for the first time, in sharp contrast to previous experiments based
on the detection of gapless boundary modes in real space. Also through the
measurement of the evolution of pseudo-spin texture with parameters, we further
observe novel topological phase transitions from the second-order topological
phase to the trivial phase, as well as to the first-order topological phase
with nonzero Chern number. Our work sheds new light on the study of
higher-order topological phases and topological phase transitions.
