How semiconductor defects can drive quantum technology

How semiconductor defects can drive quantum technology

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Optical properties of GaN defects. or, 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 hard immersion lens etched around the defect. Scale bar, 4 μm. cSecond-order photon autocorrelation g(2)(t) of defect no. 2, where t it’s the delay. Autocorrelation with zero lag g(2)(0) = 0.3 < 0.5, which is consistent with a one-photon emitter. dMagnetic field-dependent PL measured with the magnetic field approximated by c axis of the GaN crystal showing two sets of behaviors, as discussed in the text. eThe minimum level diagram that is consistent with a S ≥ 1 spin in ground state (g) and excited state (e). Non-radiative intersystem crossing (ISC) rate. cISC in a metastable state (M) is spin dependent. pThe minimum level diagram that is consistent with a S≥ 1 metastable state. Non-radiative intersystem crossing rate cISC, g from a metastable state is spin dependent and the rate of radiative relaxation ce.g is independent of rotation. Credit: Materials of Nature (2024). DOI: 10.1038/s41563-024-01803-5

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Optical properties of GaN defects. or, 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 hard immersion lens etched around the defect. Scale bar, 4 μm. cSecond-order photon autocorrelation g(2)(t ) of defect no. 2, where t it’s the delay. Autocorrelation with zero lag g(2)(0) = 0.3 < 0.5, which is consistent with a one-photon emitter. dMagnetic field-dependent PL measured with the magnetic field approximated by caxis of the GaN crystal showing two sets of behaviors, as discussed in the text. eThe minimum level diagram that is consistent with a S ≥ 1 spin in ground state (g) and excited state (e). Non-radiative intersystem crossing (ISC) rate. cISC in a metastable state (M) is spin dependent. pThe minimum level diagram that is consistent with a S≥ 1 metastable state. Non-radiative intersystem crossing rate cISC, g from a metastable state is spin dependent and the rate of radiative relaxation ce.g is independent of rotation. Credit: Materials of Nature(2024). DOI: 10.1038/s41563-024-01803-5

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 in Materials of Nature. Lead author is PhD student Jialun Luo.

Flaws are what give gemstones their color and therefore, they are also known as color centers. Pink diamonds, for example, get their color from defects called nitrogen vacancies. 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’s been developed for wideband high-frequency electronics, and that’s been a very intensive effort for many, many years,” Fuchs said. “You can go and buy a wafer 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 the fluorescence rate of a defect changes as a function of magnetic field and using a small magnetic field to drive resonant transmissions. of fault rotation. 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 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 distinct rotation spectra. 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.

“Typically, 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 fundamental work to be done, and there are many more questions than there are answers. But the basic finding of spin in this color center, the fact that there is a strong spin contrast of up to 30%, that it exists in a semiconductor material ripe – which opens up all sorts of interesting possibilities that we’re now excited to explore.”

More information:
Jialun Luo et al, Room Temperature Revealed Magnetic Optical Resonance of Single Spins in GaN, Materials of Nature(2024). DOI: 10.1038/s41563-024-01803-5

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