Online Supplementary Information : Nanoscale magnetic sensing with an individual electronic spin in diamond

Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 mT, 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 to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. 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 temperature. 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 mT Hz for a diamond nanocrystal with a diameter of 30 nm. Sensitive solid-state magnetometers typically use phenomena such as superconducting quantum interference in SQUIDs or the Hall effect in semiconductors. Intriguing avenues such as magnetic resonance force microscopy are also currently being explored. Our approach to magnetic sensing uses the coherent manipulation of a single quantum system, an electronic spin qubit. As illustrated in Fig. 1, the electronic spin of an individual nitrogen-vacancy impurity in diamond can be polarized by optical pumping and measured through state-selective fluorescence. Conventional electron spin resonance (ESR) techniques are used to coherently manipulate its orientation. To achieve magnetic sensing, we monitor the electronic spin precession, which depends on external magnetic fields through the Zeeman effect. This method is directly analogous to precision measurement techniques in atomic and molecular systems, which are widely used to implement ultra-stable atomic clocks and sensitive magnetometers. The principal challenge for achieving high sensitivity using solidstate spins is their strong coupling to the local environment, which limits the free precession time and thus the magnetometer’s sensitivity. Recently, there has been great progress in understanding the local environment of nitrogen-vacancy spin qubits, including C nuclear spins and electronic spin impurities. Here we use coherent control over a coupled electron–nuclear system, similar to techniques used in magnetic resonance, to decouple the magnetometer spin from its environment. As illustrated in Fig. 1d, a spin-echo sequence refocuses the unwanted evolution of the magnetometer spin due to environmental fields fluctuating randomly on timescales much longer than the length of the sequence. However, oscillating external magnetic fields matching the echo period will affect the spin dynamics constructively, allowing sensitive detection of its amplitude. The ideal preparation, manipulation and detection of an electronic spin would yield a so-called quantum-projection-noise-limited minimum detectable magnetic field