Introduction

While atomic magnetometers can measure magnetic fields with exceptional sensitivity and without cryogenics, spin-altering collisions limit the sensitivity of sub-millimeter-scale sensors [1]. In order to probe magnetic fields with nanometer spatial resolution, magnetic measurements using superconducting quantum interference devices (SQUIDs) [2–4] as well as magnetic resonance force microscopes (MRFMs) [5–8] have been performed. However, the spatial resolution of the best SQUID sensors is still not better than a few hundred nanometers [9] and both sensors require cryogenic cooling to achieve high sensitivity, which limits the range of possible applications. A related challenge that cannot be met with existing technology is imaging weak magnetic fields over a wide field of view (millimeter scale and beyond) combined with sub-micron resolution and proximity to the signal source under ambient conditions.

Recently, a new technique has emerged for measuring magnetic fields at the nanometer scale, as well as for wide-field-of-view magnetic field imaging, based on optical detection of electron spin resonances of nitrogen-vacancy (NV) centers in diamond [10–12]. This system offers the possibility to detect magnetic fields with an unprecedented combination of spatial resolution and magnetic sensitivity [8, 12–15] in a wide range of temperatures (from 0 K to well above 300 K), opening up new frontiers in biological [10, 16, 17] and condensed-matter [10, 18, 19] research. Over the last few years, researchers have developed techniques for nanoscale magnetic imaging in bulk diamond [11, 12, 20] and in nanodiamonds [21–23] along with scanning probe techniques [10, 24].