Long, thin slivers of diamond like the strings of a guitar are extremely sensitive to minute changes in mass, force, charge, or displacement. As the hardest of materials, diamond has the highest mechanical spring constant, making such resonator detectors suitable for studying the tiniest quantum systems. But to apply diamond nano-mechanical systems to quantum information technology, resonators have to be coupled to light or superconducting circuits.

The solution could lie in superconducting nano-mechanical resonators made from boron-doped diamond (BDD) devised by a team of researchers from the Institut NEEL – CNRS in Grenoble, France, the Fraunhofer-Institut für Angewandte Festkörperphysik in Germany, and the University of Cardiff in the UK [Bautze, T., et al., Carbon 72 (2014) 100-105 DOI: 10.1016/j.carbon.2014.01.060]. The tiny resonators, approximately 25-30 µm long and 350-480 nm wide, are made superconducting by the addition of boron as a dopant during fabrication.

The devices are simple to produce, say the researchers. Small diamond particles are used to seed growth on a Si/SiO2 wafer. Microwave plasma chemical vapour deposition (CVD) then enables controlled growth of a diamond film and the inclusion of dopants, such as boron, to convey the desired characteristics. The final nano-mechanical structures are simply defined using standard electron beam lithography.

“We have shown that the remarkable properties [of diamond] are maintained even in devices on the nanometer scale, that their quality factor is competitive with state-of-the-art resonators and that their fabrication is simple,” Christopher Bäuerle and Tobias Bautze of Institut NEEL – CNRS told Materials Today.

The BDD oscillators can be driven and read out in the superconducting state, showing excellent performance defined in terms of quality factors, which can reach up to 40,000 at a resonance frequency of 10 MHz. Furthermore, explain the researchers, the superconducting state in the BDD devices persists even in high magnetic fields of several Teslas.

“[The] very simple fabrication process should allow for the integration of such resonators in future quantum devices,” say Bäuerle and Bautze. “Also for industrial applications, diamond can be used for ultra-fast switches and actuators. Here the fact that BDD is conductive is of importance.”

The resonators could find application in advanced technologies like superconducting quantum interference devices (SQUIDs), which detect extremely subtle magnetic fields, or very low-loss microwave cavity resonators. The simple fabrication of the BDD nano-mechanical resonators and their ability to be coupled to superconducting circuits, along with their state-of-the-art performance, make these devices a viable proposition in the emerging area of quantum opto-mechanics.

“Our work demonstrates the proof of principle for superconducting diamond electronics. The next step is to integrate the different circuit elements towards this goal,” say Bäuerle and Bautze.

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