Resonant Excitation Enhances V2 Ensemble Readout Contrast in 4H-SiC, Achieving 50x Improvement with 100times More Signal

The silicon vacancy defect known as V2 in 4H-SiC holds considerable promise for future technologies, owing to its unique optical and spin characteristics and the benefits offered by the silicon carbide material itself. Infiter Tathfif and Samuel G. Carter, from the Laboratory for Physical Sciences and the University of Maryland, have now significantly enhanced the ability to detect signals from these V2 defects. Their research demonstrates a dramatic improvement in readout contrast, achieving a value of 50, a nearly 100-fold increase over previous methods using non-resonant excitation. This breakthrough stems from a technique involving resonant optical excitation of V2 ensembles at cryogenic temperatures, paving the way for more sensitive and efficient applications of this material in areas such as quantum sensing and information processing.

However, the readout contrast, an important benchmark for quantum sensing, of V2 ensembles for optically-detected magnetic resonance (ODMR) is relatively low, usually less than 1% at room temperature. To overcome this challenge, researchers resonantly excite the V2 ensembles at cryogenic temperatures and compare the results with the off-resonant case. They report a maximum ODMR contrast of 50% with only 2 μW of resonant laser power, almost 100times improvement over off-resonant excitation. This high readout contrast is attributed to a subset of V2 centres that have one spin-selective optical transition resonant with the laser, and the ODMR contrast decreases with temperature, approaching the non-resonant contrast.

Silicon Carbide Hosts Promising Quantum Sensors

Silicon-vacancy (SiV) centers in silicon carbide (SiC) are emerging as promising quantum sensors, offering potential advantages over diamond-based sensors for magnetic field detection and imaging. These defects act as spin qubits, meaning their electron spin can be manipulated and read out using light and microwave techniques. Researchers are actively working to optimize the coherence and readout fidelity of these qubits, crucial steps towards building practical sensors. Significant progress has been made in using light to initialize and read out the spin state of SiV centers, demonstrating high sensitivity in magnetic field detection.

Researchers are also improving coherence times by using isotopically pure SiC, which reduces disruptive spin noise, and by carefully controlling strain within the SiC crystal to fine-tune the SiV’s energy levels. Furthermore, integrating SiV centers with photonic waveguides and nanophotonic structures is enabling the creation of compact and scalable quantum sensors. The potential for wide-field imaging with arrays of SiV centers is also being explored, and a key advantage of SiV centers is their potential for operation at room temperature, making them practical for real-world applications.

Resonant Excitation Boosts Silicon Carbide Sensing

This research demonstrates a significant improvement in the performance of silicon carbide (SiC) defects as potential quantum sensors. Scientists achieved a 100-fold enhancement in optical detection contrast by exciting these defects, known as V2 centers, with resonant laser light at very low temperatures, reaching a contrast of 50%. This improvement stems from selectively exciting a small group of defects with a specific optical transition matching the laser’s frequency, effectively increasing the signal strength. The team found that as temperature increases, the contrast diminishes, eventually matching that of non-resonant excitation at around 60 Kelvin, due to broadening of the optical transitions.

Despite the weaker light emission from this smaller, selectively excited group of defects, the system still achieves a sensitivity of 100 nanoteslas per root hertz, surpassing the performance of non-resonant excitation while requiring considerably less laser power. Researchers suggest that further gains are possible by narrowing the range of frequencies emitted by the defect ensemble or by employing more advanced laser technology. This resonant excitation method holds promise for wide-field magnetic imaging of quantum materials and devices, potentially utilizing specially structured SiC samples to enhance light coupling and spatial resolution.