In diamonds (and other semiconductor materials), defects are a quantum sensor’s best friend.
That’s because defects, essentially a jumbled arrangement of atoms, sometimes contain electrons with an angular momentum, or spin, that can store and process information. This “rotational degree of freedom” can be exploited for a variety of purposes, such as sensing magnetic fields or creating a quantum network.
Researchers led by Greg Fuchs, Ph.D. ’07, professor of applied and engineering physics at Cornell Engineering, looked for such spin in the popular semiconductor gallium nitride and found it, surprisingly, in two distinct types of defects, one of which can be manipulated for quantum applications of next.
The group’s paper, “Room temperature optically detected magnetic resonance of single spins in GaN,” was published Feb. 12 in Nature Materials. Lead author is PhD student Jialun Luo.
Flaws are what give gemstones their color and are therefore also known as color centers. Pink diamonds, for example, get their color from defects called free nitrogen centers. However, there are many color centers that have yet to be identified, even in commonly used materials.
“Gallium nitride, unlike diamond, is a mature semiconductor. It was developed for broadband high-frequency electronics, and that was a very intensive effort over many, many years,” Fuchs said. “You can go and buy a mass of it, it’s in your computer charger, maybe, or in your electric car. But in terms of a quantum defect material, it hasn’t been explored much.”
To search for the rotational degrees of freedom in gallium nitride, Fuchs and Luo teamed up with Farhan Rana, the Joseph P. Ripley Professor of Engineering, and doctoral student Yifei Geng, with whom they had previously explored the material.
The group used confocal microscopy to identify defects via fluorescent probes and then performed a series of experiments, such as measuring how a defect’s fluorescence intensity changes as a function of magnetic field and using a small magnetic field to direct transmissions. defect spin resonances. all at room temperature.
“At first, preliminary data showed signs of interesting spin structures, but we couldn’t drive the spin resonance,” Luo said. “It turns out that we had to know the symmetry axes of the fault and apply a magnetic field along the right direction to probe the resonances; the results brought us more questions, waiting to be worked out.”
Experiments showed that the material had two types of defects with a distinct rotation spectrum. In one, the spin was coupled to a metastable excited state; in the other, it was related to the baseline condition.
In the latter case, the researchers were able to see fluorescence changes of up to 30% when they drove the spin transition—a large change in contrast, and relatively rare, for a quantum spin at room temperature.
“Usually fluorescence and spin are very loosely coupled together, so when you change the spin projection, the fluorescence can change by 0.1%, or something very, very small,” Fuchs said. “From a technology standpoint, that’s not great, because you want a big change so you can measure it quickly and efficiently.”
The researchers then performed a quantum control experiment. They found that they could manipulate the spin of the ground state and that it had quantum coherence – a quality that allows quantum bits, or qubits, to store their information.
“That’s something that’s very exciting about this observation,” Fuchs said. “There is still a lot of ground work to be done and there are many more questions than answers. But the fundamental discovery of spin in this color center, the fact that there’s a strong spin contrast of up to 30%, that exists in a mature semiconductor material—that opens up all kinds of interesting possibilities that we’re now excited to explore. “
The research was supported by the Cornell Center for Materials Research (CCMR), with funding from the Science and Engineering Center for Materials Research program of the National Science Foundation; the Cornell Engineering Sprout program; and NSF’s Quantum Sensing Challenges for transformative advances in the quantum systems program.
The researchers used the Cornell NanoScale device, also supported by the NSF.