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 1-3 . Recently, it has been proposed that EPs can enhance the sensitivity of optical gyroscopes 4,5 . Here we report measurement of rotation sensitivity boost by over 4× 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.High-Q optical microresonators have received considerable attention as sensors across a wide range of applications including biomolecule 6-8 and nanoparticle detection 9 , temperature measurement 10 , and rotation measurement [11][12][13][14][15] . In recent years, a new approach to enhance the sensitivity of microresonator sensors using the physics of exceptional points is being studied 4,5,16-20 . Traditionally, for precise sensing, a perturbation to an optical microcavity (or to its reference frame as in the case of a gyroscope) introduces either a linewidth change, a frequency shift, or a frequency splitting of a resonance that monotonically changes with the strength of the perturbation. However, operation of these systems near an exceptional point changes this situation by introduction of a square-root dependence into the transduction that can boost the sensor's ability to transduce perturbations 16 .In this work, we experimentally and theoretically demonstrate the existence of EPs in a microresonatorbased laser gyroscope, and then measure the enhanced rotation-rate transduction sensitivity near the EP. The laser gyroscope is described elsewhere 11 and uses counterpropagating Brillouin lasers in a high-quality-factor (Q ≈ 10 8 ) silica wedge resonator 21 . As shown in Fig. 1a pump light at frequencies ω pj (j = 1, 2) determined by radio-frequency modulation of a single laser (∼1552.5 nm) is coupled into the resonator from both ends of a fiber taper 22,23 . One of the pump frequencies is Pound-Drever-Hall locked to a resonator mode by feedback control to the laser. The second pump frequency is then varied to affect pump detuning change as described below. The two pump powers are stabilized via power feedback. Brillouin scattering causes a pump photon with frequency ω pj to scatter from a co-propagating acoustic phonon with frequency Ω phonon into a backwardpropagating Stokes photon with frequency ω sj . In the context of a resonator (and as illustrated in Fig. 1b), the associated phase matching con...