Researchers Develop an Aligned Ensemble of Quantum Sensors

Envision a sensor capable of detecting changes in the proton concentration of a single protein, inside a single cell, due to its extreme sensitivity. This level of understanding would expose elusive quantum-scale dynamics of that protein’s function, possibly even in real time, but will need a sensor with controllable features at a similar scale.

A new fabrication technique is now allowing quantum sensing abilities to approach this scale of precision. In this week’s Applied Physics Letters, from AIP Publishing, Researchers in Japan have reported on their development of an aligned ensemble of quantum sensors known as nitrogen vacancy (NV) centers, only nanometers from its substrate’s surface.

These results have been confirmed by nanoscale nuclear magnetic resonance (NMR) measurements and thus reveal a clear path towards atomic level design of quantum sensors with surface areas that are bigger than those usually achievable. This is considered to be the first demonstration of this nanoscale NMR measurement with high-density NV centers that have been perfectly aligned near the surface, indicating a major progress for quantum magnetometry research.

The way to combine both high counts and high contrast is to have the alignment, because when you have the alignment you basically have the benefit of the single NVs combined with the high counts obtained from the ensemble NV centers. So that’s what we basically did, really close to the surface -- within 10 nanometers -- and we demonstrated that with a SIMS [Secondary Ion Mass Spectrometry] measurement, as well as measuring nano NMR, which shows you the approximation of the distance of NVs from the surface.

Hitoshi Ishiwata, the Tokyo Institute of Technology and Lead Author of the paper

Already existing as a common tool in the world of quantum sensing, NV centers are known to be particular types of impurities found in the crystal structure of diamond. The NV center comprises of a nitrogen atom adjacent to a missing (vacant) atom in the crystal’s lattice for one unit of diamond’s otherwise purely carbon configuration. This defect can take place in one of four possible locations within the unit crystal and each delivers a single-photon signal whose spectral signature is nuclear spin-dependent.

The new technique employs a combination of directional polishing and chemical vapor deposition (CVD) in order to control how the NVs develop in the lattice. Ensembles of NVs all with the same orientation were achieved by Ishiwata and his colleagues, for their diamond substrate comprising of a commonly aligned surface, where the lattice is oriented alongside the same crystollagraphic plane (referred to as 111 in this case). For a substrate measuring about 10 μm across, only less than the width of a strand of human hair, their method is capable of developing somewhere around 10,000 such centers within 10 nm from surface.

The Researchers were able to perform nanoscale NMR detection of the fluorine in oil making contact with the substrate since the NVs are in the same respective locations of their crystal units and extremely close to the surface. The reliability of their fabrication method has literally far reaching applications for wide field measurements, guaranteeing the high contrast detection over comparatively bigger sample areas.

The other benefit of high density NV centers with alignment is to perform wide field imaging with high sensitivity. Before it was impossible to have high sensitivity for wide field imaging due to the difficulty of obtaining alignment of NV centers with high density. With our technique, high contrast wide field imaging with high signal to noise ratio is now possible, which leads to high sensitivity wide field imaging.

Hitoshi Ishiwata, the Tokyo Institute of Technology and Lead Author of the paper

Besides continuing to discover ways to further enhance the method, the Researchers are also keen on exploring applications of these ensembles in time resolved sensing, employing pulsed lasers in order to provide real-time proton information of dynamic samples. Ishiwata himself was specifically enthusiastic about the options for understanding biological cells like never before.

A future application of this material is the observation of individual cell membranes because our material is suited for observing nanoscale NMR on the volume scale of 17 cubic nanometers, which is comparable to the thickness of cell membranes (~5 nanometers). So we could use this material and measurement technique for locally probing nanoscale activity of proteins that exists in cell membrane with high sensitivity.

Hitoshi Ishiwata, the Tokyo Institute of Technology and Lead Author of the paper