  The nonlinear magneto-optical effect has significantly impacted modern
society with prolific applications ranging from precision mapping of the
Earth's magnetic field to bio-magnetic sensing. Pioneering works on collisional
spin-exchange effects have led to ultra-high magnetic field detection
sensitivities at the level of $fT/\sqrt{Hz}$ using a single linearly-polarized
probe light field. Here we demonstrate a nonlinear Zeeman-coherence parametric
wave-mixing optical-atomic magnetometer using room temperature rubidium vapor
that results in more than a three-order-of-magnitude optical signal-to-noise
ratio (SNR) enhancement for extremely weak magnetic field sensing. This
unprecedented enhancement was achieved with nearly a two-order-of-magnitude
reduction in laser power while preserving the sensitivity of the widely-used
single-probe beam optical-atomic magnetometry method. This new method opens a
myriad of applications ranging from bio-magnetic imaging to precision
measurement of the magnetic properties of subatomic particles.
