  The exceptional point, known as the non-Hermitian degeneracy, has special
topological structure, leading to various counterintuitive phenomena and novel
applications, which are refreshing our cognition of quantum physics. One
particularly intriguing behavior is the mode switch phenomenon induced by
dynamically encircling an exceptional point in the parameter space. While these
mode switches have been explored in classical systems, the experimental
investigation in the quantum regime remains elusive due to the difficulty of
constructing time-dependent non-Hermitian Hamiltonians in a real quantum
system. Here we experimentally demonstrate dynamically encircling the
exceptional point with a single nitrogen-vacancy center in diamond. The
time-dependent non-Hermitian Hamiltonians are realized utilizing a dilation
method. Both the asymmetric and symmetric mode switches have been observed. Our
work reveals the topological structure of the exceptional point and paves the
way to comprehensively explore the exotic properties of non-Hermitian
Hamiltonians in the quantum regime.
