1. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
   
2. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA
   
3. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
   
4. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138, USA
   
5. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

Correspondence to: M. D. Lukin¹ Correspondence and requests for materials should be addressed to M.D.L. (Email: lukin@fas.harvard.edu).

Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences^{1, 2, 3, 4, 5}. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 T, and the corresponding field from a single proton is a few nanoteslas. A sensor able to detect such magnetic fields with nanometre spatial resolution would enable powerful applications, ranging from the detection of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules^{5, 6} to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory⁷. Here we experimentally demonstrate an approach to such nanoscale magnetic sensing, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature⁸. Using an ultra-pure diamond sample, we achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging. In addition, we demonstrate a sensitivity of 0.5 T Hz^-1/2 for a diamond nanocrystal with a diameter of 30 nm.