  Narrow linewidth is a long-pursuing goal in precision measurement and
sensing. We propose a parity-time (PT )-symmetric feedback method to narrow the
linewidths of resonance systems. By using a quadrature measurement-feedback
loop, we transform a dissipative resonance system into a PT-symmetric system.
Unlike the conventional PT-symmetric systems which typically require two or
more modes, here the PT-symmetric feedback system contains only a single
resonance mode, which greatly extends the scope of applications. The method
enables remarkable linewidth narrowing and enhancement of measurement
sensitivity. We illustrate the concept in a thermal ensemble of atoms,
achieving a 48-fold narrowing of the magnetic resonance linewidth. By applying
the method in magnetometry, we realize a 22-times improvement of the
measurement sensitivity. This work opens the avenue for studying non-Hermitian
physics and high-precision measurements in resonance systems with feedback.
