In the field of semiconducting materials, defects have proven to be a valuable asset for quantum sensors. These defects, which are essentially jostled arrangements of atoms, can sometimes contain electrons with a spin that can store and process information. This spin degree of freedom has the potential to be harnessed for a wide range of purposes, including sensing magnetic fields and building a quantum network.

Discovering Spin in Gallium Nitride

A team of researchers led by Greg Fuchs, Ph.D., a professor of applied and engineering physics at Cornell Engineering, embarked on a quest to find such spin in the popular semiconductor known as gallium nitride. Surprisingly, they discovered spin in two distinct species of defects within the material, one of which can be manipulated for future quantum applications. The group’s findings were published in a paper titled “Room Temperature Optically Detected Magnetic Resonance of Single Spins in GaN” in the journal Nature Materials, with Jialun Luo, a doctoral student, as the lead author.

The Significance of Defects in Semiconductors

Defects are commonly referred to as color centers and are responsible for giving gems their vibrant hues. For example, nitrogen-vacancy centers are defects that contribute to the pink color of diamonds. Despite the widely known presence of defects in various materials, there are still many color centers that remain unidentified. Gallium nitride, in particular, is a mature semiconductor that has been extensively used in wide-bandgap high-frequency electronics. However, its potential as a material for quantum defects has not been thoroughly explored.

Collaborative Research Efforts

To investigate the spin degree of freedom in gallium nitride, Fuchs and Luo collaborated with Farhan Rana, a professor of engineering, and Yifei Geng, a doctoral student, with whom they had previously worked on exploring the material. The research team employed confocal microscopy to identify defects using fluorescent probes and conducted a series of experiments at room temperature. These experiments included measuring the changes in a defect’s fluorescence rate relative to the magnetic field and inducing spin resonant transmissions by applying a small magnetic field.

Unveiling Multiple Spin Spectra

The preliminary data from the experiments indicated the presence of intriguing spin structures, although initially, the researchers were unable to drive the spin resonance. However, by determining the defect symmetry axes and applying a magnetic field in the correct direction, they eventually unraveled the spin spectra of gallium nitride. The material exhibited two types of defects, each with its own distinct spin spectrum. In one type, the spin was coupled to a metastable excited state, while in the other type, it was coupled to the ground state.

Of particular interest was the fact that the researchers were able to observe significant changes in fluorescence, up to 30%, when driving the spin transition in the defects coupled to the ground state. This level of contrast and change in fluorescence is relatively rare for a quantum spin at room temperature. The ability to achieve larger changes in fluorescence is crucial for efficient and quick measurements in technological applications.

Furthermore, the research team conducted a quantum control experiment, which demonstrated the manipulation of the ground-state spin in gallium nitride. The ground-state spin also exhibited quantum coherence, which is essential for quantum bits, or qubits, to retain their encoded information. This observation opens up new possibilities for utilizing gallium nitride defects as stable and coherent quantum systems.

Future Directions and Challenges

While the findings are significant, there is still a wealth of fundamental work to be done in this field. Many questions remain unanswered, and further research is needed to fully understand the potential of defects in gallium nitride for quantum applications. Nevertheless, these discoveries represent a significant step forward in the exploration of spin systems in semiconducting materials, potentially paving the way for the development of advanced quantum technologies.

The study conducted by Fuchs and his team sheds light on the previously unexplored potential of gallium nitride as a material for quantum defects. The existence of multiple spin spectra and the ability to control and manipulate the ground-state spin demonstrate promising avenues for the development of quantum sensors and quantum information processing. As the researchers continue to delve deeper into this field, it is hoped that their findings will contribute to the advancement of quantum technologies and our understanding of the fascinating properties of semiconducting materials.

Physics

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