March 1, 2024

How defects in semiconductors could boost quantum technology

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Optical properties of GaN defects. The, PL image of an isolated defect (nº 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 carved around the defect. Scale bar, 4 μm. wSecond-order photon autocorrelation g(two)(τ) of defect no. 2, where τ is the delay. The zero-lag autocorrelation g(two)(0) = 0.3 < 0.5, which is consistent with a single photon emitter. dMagnetic field dependent PL measured with the magnetic field approximately aligned to the w axis of the GaN crystal showing two groups of behavior, as discussed in the text. It isMinimum level diagram that is consistent with a s ≥ 1 spin in the ground state (g) and in the excited state (e). The nonradiative intersystem crossing rate (ISC) γISC in a metastable state (M) is spin dependent. fMinimum level diagram that is consistent with a s≥ 1 metastable state. The nonradiative intersystem crossing rate γISC,g of a metastable state is dependent on the spin and rate of radiative relaxation γfor example is independent of spin. Credit: Materials from Nature (2024). DOI: 10.1038/s41563-024-01803-5

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Optical properties of GaN defects. The, PL image of an isolated defect (nº 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 carved around the defect. Scale bar, 4 μm. wSecond-order photon autocorrelation g(two)(τ ) of defect no. 2, where τ is the delay. The zero-lag autocorrelation g(two)(0) = 0.3 < 0.5, which is consistent with a single photon emitter. dMagnetic field dependent PL measured with the magnetic field approximately aligned to the waxis of the GaN crystal showing two groups of behavior, as discussed in the text. It isMinimum level diagram that is consistent with a s ≥ 1 spin in the ground state (g) and in the excited state (e). The nonradiative intersystem crossing rate (ISC) γISC in a metastable state (M) is spin dependent. fMinimum level diagram that is consistent with a s≥ 1 metastable state. The nonradiative intersystem crossing rate γISC,g of a metastable state is dependent on the spin and rate of radiative relaxation γfor example is independent of spin. Credit: Materials from Nature(2024). DOI: 10.1038/s41563-024-01803-5

In diamonds (and other semiconductor materials), defects are a quantum sensor’s best friend. This is because defects, essentially a jostling arrangement of atoms, sometimes contain electrons with angular momentum, or spin, that can store and process information. This “rotational degree of freedom” can be harnessed for a number of purposes, such as detecting 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 a spin in the popular semiconductor gallium nitride and found it, surprisingly, in two distinct defect species, 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 Materials from Nature. The lead author is doctoral student Jialun Luo.

These are the defects that give color to gemstones and, therefore, are also known as color centers. Pink diamonds, for example, get their hue from defects called nitrogen vacancy centers. However, there are many color centers that have not yet been identified, even in commonly used materials.

“Gallium nitride, unlike diamond, is a mature semiconductor. It was developed for wide-bandgap high-frequency electronics, and this has been a very intense effort over many, many years,” Fuchs said. “You can buy a wafer of it; it’s probably in your computer charger, or in your electric car. But in terms of material for quantum defects, it hasn’t been explored very much.”

To look for the degree of spin 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 the defects using fluorescent probes and then conducted a series of experiments, such as measuring how a defect’s fluorescence rate changes as a function of the magnetic field and using a small magnetic field to drive the resonant transmissions of spin of the defect, all at room temperature.

“At first, preliminary data showed signs of interesting spin structures, but we were unable to drive spin resonance,” Luo said. “It turns out we needed to know the defect’s axes of symmetry and apply a magnetic field in the correct direction to probe the resonances; the results brought us more questions waiting to be resolved.”

The experiments showed that the material presented 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 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.

“Normally, fluorescence and spin are very weakly linked, so when you change the spin projection, the fluorescence can change by 0.1% or something very, very small,” Fuchs said. “From a technology perspective, 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 discovered that they could manipulate the ground state’s spin and that it had quantum coherence – a quality that allows quantum bits, or qubits, to retain their information.

“That’s something very exciting about this observation,” Fuchs said. “There is still a lot of fundamental work to do and there are many more questions than answers. But the basic discovery of the spin in this color center, the fact that it has a strong spin contrast of up to 30%, is that it exists in a mature semiconductor material – which opens up all kinds of interesting possibilities that we’re now excited to explore.”

More information:
Jialun Luo et al, Room temperature optically detected magnetic resonance of single spins in GaN, Materials from Nature(2024). DOI: 10.1038/s41563-024-01803-5

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