Abstract Quantum computing technology has made great progress in recent years. Among many candidate systems for quantum computing, the nitrogen-vacancy luminescence center in diamond is the most special one, because with this system, high fidelity quantum logic gate can It is realized under normal temperature and pressure. People found that King Kong...
Quantum computing technology has made great progress in recent years. Among many candidate systems for quantum computing, the nitrogen-vacancy luminescence center in diamond is the most special one, because with this system, high fidelity quantum logic gates can It is realized under normal temperature and pressure. It has been found that the electron spin of a negatively charged nitrogen-vacancy luminescence center (color center) in diamond can be polarized with a 532 nm laser. The color center electron spin quantum number is 1. Under zero magnetic field, the energy level split between m=0 and m=\pm 1 is 2.87 GHz, and m=\pm 1 two levels are degenerate. Although the color spectrum emitted by the color center is relatively wide, it is distributed from 637 nm to 800 nm, but its zero phonon line is very narrow, around 637 nm. The intensity of the fluorescence spectrum is related to the electron spin state of the color center. Therefore, we can use a filter to filter out the 532 nm laser and detect the fluorescence emitted by the remaining color centers to determine the state of the color core electron spin. To some extent, the diamond color center can be seen as an ion trap system trapped in the diamond lattice.
How to understand the coherence characteristics of diamond color center? First, we noticed that diamond is one of the hardest materials on the planet, and its Debye temperature is 1800. Therefore, the phonon spectral density in diamond at room temperature is relatively low. This is reflected in the spectrum of the color center and it can be seen that the red sideband of the line has a very broad distribution, while the spectrum on the blue sideband is essentially zero. The proportion of fluorescence falling on the zero phonon line is about one percent. It is precisely because of the imbalance of the red and blue sidebands that we can use the 532 nm laser to polarize the electron spin of the color center. Almost ninety-nine percent of the carbon atoms in diamond are isotopic carbons 12, and there is no hyperfine structural coupling between them and the color centers. Only the remaining one percent of the carbon 13 will be coupled, and the randomly distributed nuclear magnetic field will affect the coherence properties of the color center. If we use the isotope purification technique to obtain a carbon 12 with a purity of more than 99.7% diamond, the coherence time of the color center electron spin at room temperature can exceed one millisecond.
If there are several carbon 13-nuclear spins near the color center, then we can control the electron and nuclear spins with the added RF signal and microwave signal to realize the general quantum logic gate. So far, the highest fidelity logic gate has exceeded 99.9%. We know that to achieve error-correctable quantum computing, we must have at least 5 qubits. So we need at least 4 nuclear spins coupled with the same color center electron spin. This is a great challenge for the experiment. We can also use ion implantation to precisely control the position of the color center to achieve effective coupling between two adjacent color centers. However, at this time, the pitch of the color centers is on the order of ten nanometers, and it is difficult to distinguish different color centers by laser. To more efficiently couple the diamond color center while ensuring that the color center can be read independently, a new approach is needed.

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