Diamond Spins Have Come into Focus with New Measurement Method

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Recently, a new technique was developed to read out single negatively charged Nitrogen-Vacancy centers’ spin states. The method could make bulky read-out systems of today obsolete as well as allow new uses of electronic devices’ N-V centers. It could be used to read out N-V centers that are extremely close together as well, which could be of use to develop quantum-information technologies.

One Nitrogen-Vacancy in a diamond lattice occurs when two nearby carbon atoms are replaced by both a nitrogen atom and vacant lattice site. Together, the atom as well as the vacancy can act as a negatively charged entity having an intrinsic spin. As N-V centers are isolated from the surroundings of them, their quantum behavior is not washed out right away by thermal fluctuations. Therefore, they can also be used to make an array of quantum technologies, operating at room temperature.

A green photon that hits an N-V center can promote the electron to an excited state. When the electron decays back to its ground state, it may give off a red photon. The Nitrogen-Vacancy center has 3 spin sublevels, and their excited states come with different odds of giving off a photon as they decay. Researchers can read out its spin state by repeatedly exciting an N-V center and collecting the photons emitted, which is very helpful for quantum computation. Furthermore, since external variables like the magnetic field, temperature, force and pressure, electric field, etc., can influence the spin state, an N-V center can be used as an atomic-scale sensor.

Bulky Detection

Even though Nitrogen-Vacancy centers are small, the equipment necessitated to collect the photons is complicated and bulky. This has prevented their integration into devices that are the size of a chip. It also poses an issue for using N-V centers in quantum computing at room temperature. Entangling a couple of N-V centers necessitates them to be around 30 nm apart, smaller than the limit of diffraction for the red light. Therefore, expensive and difficult microscopy techniques are required to individually detect the spin states.

In addition, the excited state’s finite lifetime slows down experiments. Hasselt University’s Milos Nesladek explained, “To get information about the NV centre’s spin state, you have to repeat the measurement many times. You can put in only a certain amount of laser power before the optical signature saturates.”

Nesladek and his colleagues have made an alternative technique to detect the spin of an N-V center that makes use of green laser light as well. Although, the same physics causing their excited states with different spins in order to have different fluorescence upon decay causes Nitrogen-Vacancy centers to also have different odds of absorbing another photon from the very same laser. This then eliminates the surplus electron from the N-V center into the diamond’s conduction band.

If an expert applies a voltage, then the electron can freely move through the natural diamond, and be detected in the process. Measuring the photocurrent generated when the light hits an N-V center, therefore, lets researchers infer the spin state of it. This process was unveiled first by Milos Nesladek and his colleagues four years ago for N-V center ensembles. Their new work expands this to individual N-V center spins’ detection.

Stronger Signal

The researchers also demonstrated a greater Signal-to-Noise Ratio (SNR) than what is possible under the very same conditions with optical detection. What’s more, the researchers found, as the photocurrent was generated when electrons were promoted, not decayed, from the state, it kept increasing when the laser power was turned up. According to Ulm University’s Petr Siyushev, who was one of the research team members, “You don’t need to implement complicated optical detection: you can just integrate everything into a tiny diamond chip which will be compatible with all current electronic technology.”

Another researcher, Harvard University’s Ronald Walsworth, described the work as an extremely “important technical step.” Walsworth cautions that the required optics to target the specific N-V centers using green light are rather complex, and said electrical read-out presents difficulties of its own. “You need to fabricate electrodes in specific places on the diamond. Once you do that you can’t easily image many NVs over a wide field of view,” Walsworth said. Nevertheless, he feels the method holds real promise for the applications like the use of N-V centers in cryogenics.

“Getting good optical detection of red photons coming all the way out of a cryostat is a real challenge. With electrical detection that would become very straightforward,” Walsworth further said. The research on the Nitrogen-Vacancy centers’ spin state is described in the journal named “Science”. The N-V center is one of the many point defects in the natural diamond.

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