Different approaches have improved the sensitivity of either electron or nuclear magnetic resonance to the single spin level. For optical detection it has essentially become routine to observe a single electron spin or nuclear spin.
The world's smallest magnetic field sensor - the NV center
Every NV center is a magnetic field sensor. It has a spin and the energy of its spin sublevels changes when it is immersed into a magnetic field. This shift can be easily read out by performing optical and microwave spectroscopy. In a spectrum, the shift of the spin resonance lines can be used to infer the magnetic field (see graphics on the right).
Advanced measurement protocols (e.g. Ramsey interferometry or Hahn echo) exploit the quantum nature of the spin and allow for a highly precise readout.
The unique property of such an NV spin sensor is its size: it is an atomic scale object and should therefore allow to measure magnetic fields with sub-nanometer spatial resolution. In a complementary approach, high precision measurements can be performed by integrating over many such sensors, e.g. by performing spectroscopy on diamonds densely filled with NV centers.
We explore this new type of sensor in a variety of applications, ranging from the detection of single spins over magnetic field imaging to bulk magnetometry with mm-sized crystals.
Nanoscale magnetometry - towards magnetic resonance imaging of single atoms
A single NV center can sense magnetic fields with sub-nanometer spatial resolution. Such a sensor, attached to the tip of a scanning probe microscope, could provide a completely new view onto the nanoworld. The most interesting magnetic fields at the nanoscale probably will be the fields of single electron and nuclear spins. Since they rise with r^3, they should become measurable quantities for a sensor which approaches them closer than some tens of nanometers. A sensor capable of detecting these fields might enable structure determination of single molecules and become an invaluable tool for solid state physicists, chemists and biologists.
Microscale magnetometry - Imaging magnetic fields of micron-sized objects
A thin disk of diamond, densely filled with NV centers, allows to image magnetic fields. Technically, this is done by performing spectroscopy on the entire disk and observing its flourescence with a camera. The resulting fluorescence image allows to reconstruct two-dimensional vector maps of the fields of micron-scale objects (see graphics on the right for an example). With such a device, it might become possible to observe the magnetic fields of single nerve cells or to perform nuclear magnetic resonance imaging with microscopic resolution.
Macroscale magnetometry - the most sensitive sensors we can build
We investigate mm-sized diamonds, filled with the highest possible density of NV centers. Averaging over such a large number of sensors allows to achieve an extremely high sensitivity. Theoretical estimates predict these devices to reach the level of fT/sqrt(Hz). This would be comparable to SQUIDs, the best magnetic field sensors known today, without the need for cryogenic operation.
Such a sensor might enable the realization of compact NMR spectrometers or the imaging of small, but macroscopic currents in biological tissue, e.g. the brain or the heart.
(Picture on the left: One of our sensor diamonds in the spectroscopy setup; the NV centers glow red under green laser illumination.)