  A fundamental axiom of quantum mechanics requires the Hamiltonians to be
Hermitian which guarantees real eigen-energies and probability conservation.
However, a class of non-Hermitian Hamiltonians with Parity-Time
($\mathcal{PT}$) symmetry can still display entirely real spectra. The
Hermiticity requirement may be replaced by $\mathcal{PT}$ symmetry to develop
an alternative formulation of quantum mechanics. A series of experiments have
been carried out with classical systems including optics, electronics,
microwaves, mechanics and acoustics. However, there are few experiments to
investigate $\mathcal{PT}$ symmetric physics in quantum systems.Here we report
the first observation of the $\mathcal{PT}$ symmetry breaking in a single spin
system. We have developed a novel method to dilate a general $\mathcal{PT}$
symmetric Hamiltonian into a Hermitian one, which can be realized in a
practical quantum system.Then the state evolutions under $\mathcal{PT}$
symmetric Hamiltonians, which range from $\mathcal{PT}$ symmetric unbroken to
broken regions, have been experimentally observed with a single
nitrogen-vacancy (NV) center in diamond. Due to the universality of the
dilation method, our result opens a door for further exploiting and
understanding the physical properties of $\mathcal{PT}$ symmetric Hamiltonian
in quantum systems.
