  We demonstrate a portable all-optical intrinsic scalar magnetic gradiometer
composed of miniaturized cesium vapor cells and vertical-cavity
surface-emitting lasers (VCSELs). Two cells, with an inner dimension of 5 mm x
5 mm x 5 mm and separated by a baseline of 5 cm, are driven by one VCSEL and
the resulting Larmor precessions are probed by a second VCSEL through optical
rotation. The off-resonant linearly polarized probe light interrogates two
cells at the same time and the output of the intrinsic gradiometer is
proportional to the magnetic field gradient measured over the given baseline.
This intrinsic gradiometer scheme has the advantage of avoiding added noise
from combining two scalar magnetometers. We achieve better than 18 fT/cm/rt-Hz
sensitivity in the gradient measurement. Ultra-sensitive short-baseline
magnetic gradiometers can potentially play an important role in many practical
applications, such as nondestructive evaluation and unexploded ordnance (UXO)
detection. Another application of the gradiometer is for magnetocardiography
(MCG) in an unshielded environment. Real-time MCG signals can be extracted from
the raw gradiometer readings. The demonstrated gradiometer greatly simplifies
the MCG setup and may lead to ubiquitous MCG measurement in the future.
