How Ulm University Detects NV Center Spin via Photocurrent

Researchers at Ulm University have achieved photoelectrical readout of the spin state of nitrogen-vacancy (NV) centers implanted just 10 nanometers beneath the surface of diamond, a surprising feat given the challenges of detecting such shallowly positioned defects. The team successfully demonstrated coherent control of these NV centers even after diamond overgrowth, establishing a robust method for spin state detection. This photoelectrical readout offers an advantage over conventional fluorescence techniques due to a higher charge carrier rate, potentially accelerating progress in quantum sensing and computing with color centers in diamond. The results, the paper reports, “establish photoelectric readout as a viable route to chip-integrated, electrically detected nanoscale sensing and to spin registers based on engineered shallow NV centers.”

Shallow NV Center Creation via Implantation & Overgrowth

This breakthrough addresses a critical challenge in advancing nanoscale sensing and quantum computing, as many applications demand artificially created, precisely positioned shallow NV centers. The team’s success hinges on a novel approach: burying the implanted centers with a layer of overgrowth diamond, a technique that allows for shallow implantation and enhances signal detection. Conventional methods for creating NV centers often result in defects and charge traps near the diamond surface, hindering clear photoelectrical readout. The Ulm University researchers tackled this issue by first implanting nitrogen ions, then covering the implanted region with an ultrapure diamond layer. Subsequent annealing at 1000°C in vacuum further refined the structure, resulting in NV centers approximately 10 nanometers deep.

This overgrowth strategy improves the photoelectric readout signal by suppressing background photocurrent, a common source of interference in shallow-defect detection. Experiments demonstrate that the overgrowth process doesn’t compromise the NV center’s coherence, a crucial property for quantum applications. They found that the Ramsey dephasing time, a measure of coherence, remained consistent with measurements obtained using traditional fluorescence readout, suggesting that the protective overgrowth layer shields the NV center from environmental noise without disrupting its quantum properties.

Photoelectrical Readout Compared to Fluorescence Detection

Following advancements in manipulating individual nitrogen-vacancy (NV) centers in diamond, researchers are increasingly focused on refining methods for reading their quantum states. While conventional optical fluorescence detection remains prevalent, photoelectrical readout, measuring the spin state via changes in electrical current, is gaining traction due to its potential for miniaturization and integration into solid-state devices. Recent work from Ulm University and collaborators demonstrates a significant leap forward: the successful photoelectrical detection and coherent control of shallowly implanted NV centers, a feat previously limited to deeper or ensemble defects. This is noteworthy because shallowly implanted centers present a greater challenge for photoelectrical detection, owing to increased susceptibility to surface-related charge instability and implantation-induced damage.

A key advantage of photoelectrical readout lies in its inherent speed. The process relies on detecting charge carriers, electrons and holes, generated by the NV center’s spin state, which is faster than collecting the comparatively fewer photons emitted in fluorescence. This higher charge carrier rate promises to accelerate progress in both quantum sensing and quantum computing.

This shallow implantation depth is crucial for applications requiring nanoscale sensing of surface phenomena. Nitrogen implantation, while enabling shallow NV formation, often leaves behind defects and residual nitrogen that can interfere with signal clarity. These defects act as charge traps, reducing the contrast of the photoelectrical signal. To mitigate this, the researchers employed a diamond overgrowth technique, effectively burying the implanted NV centers beneath an ultrapure diamond layer. This process, detailed in their work, not only protects the centers but also modifies the surrounding defect environment, improving the overall signal. The team reports that “Overgrowth improves photoelectric readout by suppressing the background photocurrent,” highlighting a key benefit of their approach.

Impact of Defect Environment on PE Signal Contrast

The expectation that deeper NV centers in diamond are easier to read via photoelectric readout has been challenged by recent work at Ulm University, demonstrating robust spin detection of artificially implanted centers just 10 nanometers below the surface, a depth previously considered problematic for this technique. This achievement addresses a significant hurdle in developing nanoscale sensors and quantum computing architectures reliant on precisely positioned NV centers. The difficulty lies in the creation of a problematic defect environment during shallow implantation. Nitrogen implantation, while enabling precise placement, also leaves behind residual nitrogen and vacancy-related complexes. These defects act as charge traps, potentially diminishing the photoelectric readout contrast and signal-to-noise ratio.

The team specifically notes that a deep donor defect, with an ionization threshold corresponding to wavelengths commonly used to address NV centers, can generate parasitic photocurrent, obscuring the desired spin-dependent signal. To circumvent this issue, the team utilized diamond overgrowth, growing an ultrapure diamond layer atop the implanted NV centers.

The team’s approach tackles a significant hurdle: reliably reading the spin of artificially implanted NV centers close to the diamond surface, where defect environments and implantation damage typically degrade signal quality. The innovation centers on a diamond overgrowth technique. After implanting nitrogen ions, the researchers capped the NV centers with an ultrapure diamond layer, effectively burying them while maintaining a shallow depth suitable for near-surface sensing.