Optical properties of GaN defects. A, PL image of an isolated defect (n°2), indicated by an arrow, and its surroundings. Scale bar, 2 μm. b, Optical spectrum of defect no. 2. The inset shows a scanning electron microscope image of a solid immersion lens sculpted around the defect. Scale bar, 4 μm. vsSecond-order photon autocorrelation g(2)(τ) of defect no. 2, where τ it’s the delay. Autocorrelation without delay g(2)(0) = 0.3 < 0.5, which is consistent with a single photon emitter. dPL dependent on the measured magnetic field with the magnetic field approximately aligned with the vs axis of the GaN crystal showing two groups of behavior, as discussed in the text. eMinimum level diagram consistent with a S ≥ 1 spin in the ground state (g) and in the excited state (e). The non-radiative intersystem crossing rate (ISC) yISC in a metastable state (M) depends on the spin. FMinimum level diagram consistent with a S≥ 1 metastable state. The rate of non-radiative intersystem crossing yISC,g from a metastable state depends on spin and radiative relaxation rate yFor example is independent of spin. Credit: Natural materials (2024). DOI: 10.1038/s41563-024-01803-5
In diamonds (and other semiconductor materials), defects are the quantum sensor’s best friend. Indeed, the defects, essentially an arrangement of jostled atoms, sometimes contain electrons with angular momentum, or spin, which can store and process information. This “spin degree of freedom” can be exploited for various purposes, such as detecting magnetic fields or creating a quantum network.
Researchers led by Greg Fuchs, Ph.D. ’07, professor of applied physics and engineering at Cornell Engineering, went looking for such a spin in the popular semiconductor gallium nitride and found it, surprisingly, in two distinct species of defects, one of which can be manipulated for future quantum applications.
The group’s paper, “Room Temperature Optically Detected Magnetic Resonance of Single Spins in GaN,” was published in Natural materials. The lead author is doctoral student Jialun Luo.
Flaws are what give gemstones their color, and for this reason they are also called color centers. Pink diamonds, for example, get their hue from defects called nitrogen vacancy centers. However, many color centers have yet to be identified, even in commonly used materials.
“Gallium nitride, unlike diamond, is a mature semiconductor. It was developed for wide-bandgap high-frequency electronics, and that required a very intense effort over many, many years,” Fuchs said . “You can buy a slice of it; it’s probably in your computer charger or in your electric car. But in terms of a material for quantum defects, it hasn’t been explored much.”
To research the spin degree 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, then conducted a multitude of experiments, such as measuring how a defect’s fluorescence rate changes as a function of magnetic field and using of a small magnetic field to drive the resonant spin transmissions of the defect. 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 needed to know the symmetry axes of the defects and apply a magnetic field in the right direction to probe the resonances; the results gave us further questions waiting to be answered.”
The experiments showed that the material exhibited two types of defects with distinct spin spectra. In one of them, the spin was coupled to a metastable excited state; in the other, it was coupled to the ground state.
In the latter case, the researchers were able to observe fluorescence changes of up to 30% when driving the spin transition, a large and relatively rare contrast change for quantum spin at room temperature.
“Usually fluorescence and spin are very weakly related, 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, it’s not great because you want a big change so you can measure it quickly and efficiently.”
The researchers then carried out 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 retain their information.
“There’s something quite exciting about this observation,” Fuchs said. “There’s still a lot of fundamental work to be done, and there are a lot more questions than answers. But the fundamental discovery of spin in this color center, the fact that it has a strong spin contrast going up to at 30%, which it exists in a mature semiconductor material, which opens up all kinds of interesting possibilities that we are now eager to explore.
More information:
Jialun Luo et al, Room temperature optically detected magnetic resonance of single spins in GaN, Natural materials(2024). DOI: 10.1038/s41563-024-01803-5
Provided by Cornell University
Quote: How semiconductor defects could boost quantum technology (February 12, 2024) retrieved February 12, 2024 from
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