Picosecond readout of diamond nitrogen-vacancy centres coupled to graphene for future Quantum Computers with Terahertz clockspeeds

Nitrogen-vacancy centers in diamonds could be used to construct vital components for quantum computers. But hitherto it has been impossible to read optically written information from such systems electronically. Using a graphene layer, a team of scientists headed by Professor Alexander Holleitner of the Technische Universität München (TUM) has now implemented just such a read unit.

Natural diamonds always contain defects. The most researched defects are nitrogen-vacancy centers 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.

TUM researchers have now devised just such a methodology for reading the stored information in diamond nitrogen vacancy centers. The technique builds on a direct transfer of energy from nitrogen-vacancy centers in nanodiamonds to a directly neighboring graphene layer.

Non-radiative energy transfer

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

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

Laboratory set-up measuring the interaction between graphene and nano-diamonds with implanted nitrogen-vacancy centers.
Credit: Astrid Eckert / TUM

Nature Nanotechnology – Ultrafast electronic readout of diamond nitrogen-vacancy centres coupled to graphene

Picosecond electronic detection

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 Holleitners 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 results may pave the way for incorporation of nitrogen-vacancy centers into ultrafast electronic circuits for quantum computers and for electronic read-out of non-radiative transfer processes.

Non-radiative readout scheme.

Previously Harvard researchers had made room temperature quantum computer memory from diamond nitrogen vacancy

Science journal – Room-Temperature Quantum Bit Memory Exceeding One Second

Stable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. We demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature. The qubit consists of a single 13C nuclear spin in the vicinity of a nitrogen-vacancy color center within an isotopically purified diamond crystal. The long qubit memory time was achieved via a technique involving dissipative decoupling of the single nuclear spin from its local environment. The versatility, robustness, and potential scalability of this system may allow for new applications in quantum information science.

Self Assembled nanodiamond qubits

German researchers devised a technique of creating self-assembled nanodiamond quantum bits (qubits) that could form the basis of quantum computers and storage devices that, unlike other quantum tech could operate at room temperature.

They used DNA to build a scaffold that allows six nanodiamonds to self-assemble in a ring, potentially creating a six-qubit room-temperature quantum computer.

Arxiv – Self-assembling hybrid diamond-biological quantum devices

The realization of scalable arrangements of nitrogen vacancy (NV) centers in diamond remains a key challenge on the way towards efficient quantum information processing, quantum simulation and quantum sensing applications. Although technologies based on implanting NV-center in bulk diamond crystals or hybrid device approaches have been developed, they are limited in the achievable spatial resolution and by the intricate technological complexities involved in achieving scalability. We propose and demonstrate a novel approach for creating an arrangement of NV-centers, based on the self-assembling capabilities of biological systems and its beneficial nanometer spatial resolution. Here, a self-assembled protein structure serves as a structural scaffold for surface functionalized nanodiamonds, in this way allowing for the controlled creation of NV-structures on the nanoscale and providing a new avenue towards bridging the bio-nano interface. One-, two- as well as three-dimensional structures are within the scope of biological structural assembling techniques. We realized experimentally the formation of regular structures by interconnecting nanodiamonds using biological protein scaffolds. Based on the achievable NV-center distances of 11nm, we evaluate the expected dipolar coupling interaction with neighboring NV-center as well as the expected decoherence time. Moreover, by exploiting these couplings, we provide a detailed theoretical analysis on the viability of multiqubit quantum operations, suggest the possibility of individual addressing based on the random distribution of the NV intrinsic symmetry axes and address the challenges posed by decoherence and imperfect couplings. We then demonstrate in the last part that our scheme allows for the high-fidelity creation of entanglement, cluster states and quantum simulation applications.

Abstract for original readout work

Non-radiative transfer processes are often regarded as loss channels for an optical emitter because they are inherently difficult to access experimentally. Recently, it has been shown that emitters, such as fluorophores and nitrogen-vacancy centres in diamond, can exhibit a strong non-radiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here we demonstrate that the transferred excitations can be read out by detecting corresponding currents with a picosecond time resolution. We detect electronically the spin of nitrogen-vacancy centres in diamond and control the non-radiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen-vacancy centres into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.

8 pages of Supplemental information

SOURCES – Technische Universität München (TUM), Nature Nanotechnology, Journal Science

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