Title: Laser-written micro-channel atomic magnetometer

Abstract: We demonstrate a sensitive optically-pumped magnetometer using rubidium vapor and 0.75 amg of nitrogen buffer gas in a sub-mm-width sensing channel excavated by femtosecond laser writing followed by chemical etching. The channel is buried less than 1 mm below the surface of its fused silica host material, which also includes reservoir chambers and micro-strainer connections, to preserve a clean optical environment. Using a zero-field-resonance magnetometry strategy and a sensing volume of 2.25 mm$^3$, we demonstrate a sensitivity of $\approx$ 1 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at $10$ Hz. The device can be integrated with photonic structures and microfluidic channels with 3D versatility. Its sensitivity, bandwidth and stand-off distance will enable detection of localized fields from magnetic nanoparticles and \mul NMR samples.