Microfabricated spin exchange relaxation free atomic magnetometer

Methods first developed at NIST for MEMS-based atomic clocks have been applied to magnetic field sensors. The sensors are built around microfabricated alkali-atom vapor cells integrated with micro-optics and a VCSEL light source. Exceptional magnetic field sensitivities can be achieved in a small volume vapor cell, especially when operated in the spin-exchange relaxation free (SERF) regime. In this technique, magnetic resonance broadening due to spin-exchange collisions is suppressed under conditions of high alkali density and low magnetic fields. We have demonstrated sensitivities better than 100 fT/Hz$^{1/2}$ with a millimeter scale SERF sensor.\footnote{V. Shah, S. Knappe, P.D.D. Schwindt, and J. Kitching, Nature Photonics, {\bf 1}, 649 (2007).} Adding flux concentrators\footnote{W.C. Griffith, R. Jimenez-Martinez, V. Shah, S. Knappe, and J. Kitching, Appl. Phys. Lett., {\bf 94}, 023502 (2009).} around the vapor cell further improves the sensitivity to 10 fT/Hz$^{1/2}$, potentially providing a low power, noncryogenic alternative to SQUID sensors.