  Quantum mechanics imposes a limit on the precision of a continuous position
measurement of a harmonic oscillator, as a result of quantum backaction arising
from quantum fluctuations in the measurement field. A variety of techniques to
surpass this standard quantum limit have been proposed, such as variational
measurements, stroboscopic quantum non-demolition and two tone
backaction-evading (BAE) measurements. The latter proceed by monitoring only
one of the two non-commuting quadratures of the motion. This technique,
originally proposed in the context of gravitational wave detection, has not
been implemented using optical interferometers to date. Here we demonstrate
continuous two-tone backaction-evading measurement in the optical domain of a
localized GHz frequency mechanical mode of a photonic crystal nanobeam
cryogenically and optomechanically cooled in a $^3$He buffer gas cryostat close
to the ground state. Employing quantum-limited optical heterodyne detection, we
explicitly show the transition from conventional to backaction-evading
measurement. We observe up to 0.67 dB (14%) reduction of total measurement
noise, thereby demonstrating the viability of BAE measurements for optical
ultrasensitive measurements of motion and force in nanomechanical resonators.
