  Recent advances in optical magnetometry have achieved record sensitivity at
both macro- and nano-scale. Combined with high bandwidth and non-cryogenic
operation, this has enabled many applications. By comparison, microscale
optical magnetometers have been constrained to sensitivities five
orders-of-magnitude worse than the state-of-the-art. Here, we report an ambient
optical micro-magnetometer operating for the first time in the picoTesla range,
a more than three order-of-magnitude advance on previous results. Unlike other
ultrasensitive optical magnetometers, the device operates at earth field,
achieves tens of MHz bandwidth, and is integrated and fiber coupled. Combined
with 60 micrometer spatial resolution and microWatt optical power requirements,
these unique capabilities open up a broad range of applications including
cryogen-free and microfluidic magnetic resonance imaging, and electromagnetic
interference-free investigation of spin physics in condensed matter systems
such as semiconductors and ultracold atom clouds
