  Exceptional points (EPs) are special spectral degeneracies of non-Hermitian
Hamiltonians governing the dynamics of open systems. At the EP two or more
eigenvalues and the corresponding eigenstates coalesce. Recently, it has been
proposed that EPs can enhance the sensitivity of optical gyroscopes. Here we
report measurement of rotation sensitivity boost by over 4X resulting from
operation of a chip-based stimulated Brillouin gyroscope near an exceptional
point. A second-order EP is identified in the gyroscope and originates from the
dissipative coupling between the clockwise and counterclockwise lasing modes.
The modes experience opposing Sagnac shifts under application of a rotation,
but near the exceptional point new modal admixtures dramatically increase the
Sagnac shift. Modeling confirms the measured enhancement. Besides the ability
to operate an optical gyroscope with enhanced sensitivity, this result provides
a new platform for study of non-Hermitian physics and nonlinear optics with
precise control.
