Quantum computing could soon become a mainstream reality, with the news that nitrogen-vacancy centres in diamonds could be used to construct vital components for quantum computers.

Before now it has been impossible to read optically-written information from quantum computers electronically. Now, using a graphene layer, a team of scientists headed by Professor Alexander Holleitner of the Technische Universität München (TUM) has implemented a read unit that seems to work.

Ideally, diamonds consist of pure carbon. But natural diamonds always contain defects. The most researched defects are nitrogen-vacancy centres comprising a nitrogen atom and a vacancy. These might serve as highly sensitive sensors or as register components for quantum computers. However, until now it has not been possible to extract the optically stored information electronically.

A team headed by Prof Holleitner, physicist at the TU München and Frank Koppens, physics professor at the Institut de Ciencies Fotoniques near Barcelona, have now devised a methodology for reading the stored information. The technique builds on a direct transfer of energy from nitrogen-vacancy centres in nanodiamonds to a directly neighbouring graphene layer.

When laser light shines on a nanodiamond, a light photon raises an electron from its ground state to an excited state in the nitrogen-vacancy centre.

“The system of the excited electron and the vacated ground state can be viewed as a dipole,” says Prof Holleitner. “This dipole, in turn, induces another dipole comprising an electron and a vacancy in the neighbouring graphene layer.”

In contrast to the approximately 100 nanometre large diamonds, in which individual nitrogen-vacancy centres are insulated from each other, the graphene layer is electrically conducting. Two gold electrodes detect the induced charge, making it electronically measureable.

Essential for this experimental setup is that the measurement is made extremely quickly, because the generated electron-vacancy pairs disappear after only a few billionths of a second. However, the technology developed in Holleitner’s laboratory allows measurements in the picosecond domain (trillionths of a second). The scientists can thus observe these processes very closely.

“In principle our technology should also work with dye molecules,” says doctoral candidate Andreas Brenneis, who carried out the measurements in collaboration with Louis Gaudreau. “A diamond has some 500 point defects, but the methodology is so sensitive that we should be able to even measure individual dye molecules.”

As a result of the extremely fast switching speeds of the nanocircuits developed by the researchers, sensors built using this technology could be used not only to measure extremely fast processes. Integrated into future quantum computers they would allow clock speeds ranging into the terahertz domain.

The research was funded by the German Research Foundation (Cluster of Excellence Nanosystems Initiative Munich, NIM, and SFB 631), the European Research Council (ERC Grants NanoREAL, CarbonLight), the Fundacio Cellex Barcelona and the Marie Curie International Fellowship COFUND, as well as the ICFOnest Program and the Centre for NanoScience (CeNS) München. Physicists from TU München, Universität Augsburg, the Walther-Meißner-Institut
of the Bavarian Academy of Sciences and the ICFO Institut de Ciencies Fotoniques in Castelldefels near Barcelona participated in the project.

Pictured: Vision of a future quantum computer with chips made of diamond and graphene. Image: Christoph Hohmann/NIM