Dynamical polarization of nuclear spin ensembles is of central importance for magnetic resonance studies, precision sensing and for applications in quantum information theory.
Visualizing the tree-rings of magnetic fields
Optimal-control spectroscopy on a scanning qubit reveals multiple contour lines of the magnetic field in a single scan.
Microscopes able to image magnetic fields are an increasingly important tool, not only for material physics but also for industry where they are used to check the magnetic field of hard disk write heads.
The NV center in diamond – an atomic-size color defect – is a promising candidate for the next generation of sensor heads for magnetic field microscopes. It can sense magnetic fields with high (nT) sensitivity and, more importantly, sub-nm resolution. This latter property is an order of magnitude better than existing techniques such as magnetic force microscopy.
Imaging of magnetic fields with NV centers has been demonstrated before, but has so far required sophisticated and error-prone control techniques (e.g. lock-in detection). We now managed to radically simplify this technique with the help of a computer-optimized spectroscopy protocol based on a technique known as optimal control. Our protocol switches off fluorescence of a scanning color center whenever the magnetic field reaches one of several magic values. Scanning through a spatially varying magnetic field, fluorescence of the center simultaneously images multiple contour lines of the magnetic field – lines where the field takes on one of the magic values. This pattern is visually and conceptually analogous to tree lines in biology, which – mathematically phrased – reveal all parts of a piece of wood that were grown by the tree at a particular instant in time.
Technically, this contour line pattern can be used to quantitatively reconstruct a map of the full magnetic field, just as tree-rings can reveal the growth history of a tree.
We demonstrated our technique by imaging a MFM-tip as an example of a magnetic nanostructure, gaining previously unknown insights into the magnetic field structure of the MFM-tip, which was then used to check different models of the MFM-tip's magnetic field for validity.
High-Dynamic-Range Imaging of Nanoscale Magnetic Fields Using Optimal Control of a Single Qubit
Phys. Rev. Lett. 111, 170801 (2013)
Thomas Häberle, Dominik Schmid-Lorch, Khaled Karrai, Friedemann Reinhard, and Jörg Wrachtrup