An NV centre occurs when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. NV centres make outstanding solid-state quantum bits (or qubits) because they have electrons that are extremely well isolated from the surrounding lattice. Information can thus be stored in the spin of an NV by placing it in a certain electronic state, which can then be maintained for a long time, even at room temperature. This long “coherence” time is very useful for quantum computing.

Recently, researchers have turned their attention to coupling NV spins in diamond to mechanical resonators using locally applied magnetic fields since this would allow them to mechanically control the spins. Until now, however, coherently coupling a single NV spin and the quantized motion of a nanomechanical resonator has proved difficult because it is no easy task to control magnetic fields at the nanoscale.

A spin-nanomechanical hybrid device

“We have shown that NV centres in diamond interfaced with a suspended carbon nanotube carrying a dc current can be used to make a spin-nanomechanical hybrid device,” explains team member Peng-Bo Li of RIKEN in Japan and Xi’an Jiatong University in China. “This hybrid structure exploits the excellent mechanical and electrical characteristics of carbon nanotubes as well as the exceptional coherence properties of NV centres in diamond. We demonstrate that we can engineer the strong magnetomechanical interactions between a single NV spin and the vibrations of the suspended nanotube and then tune these interactions using applied microwave fields by changing the current flowing through the CNT.”

The researchers designed their hybrid quantum device by placing a single NV centre near a current-carrying CNT. “This set up corresponds to a quantum version of Oersted’s classical experiment, in which a current-carrying wire creates a magnetic field around it, which will produce a force acting on a magnetic needle,” Li tells nanotechweb.org. “We looked at two different designs. The first consists of a current-carrying CNT suspended above a diamond sample. Here individual NV centres (that we can observe using optical microscopy) are implanted 5-10 nm below the surface of the sample. The second feasible design is one in which a single NV defect is hosted in a diamond nanocrystal about 10 nm in size positioned near the nanotube.”

Towards the quantum regime

The good thing about the new hybrid system is that no additional components, such as external magnetic tips, are required to tune the coupling between the NV centre and the CNT, adds Li. The set up can also be scaled up to arrays of NV centres and so could be used to design larger hybrid quantum devices for information processing with NV spin qubits and novel nanoscale sensors.

The team, which includes researchers from the Vienna Center for Quantum Science and Technology in Austria and the University of Michigan, Ann Arbor in the US, is now busy looking into potential applications for its device in quantum sensors. “Researchers have known for a while that NV centres in diamond can be used as highly sensitive room temperature nanoscale probes of magnetic and electric fields, as well as temperature, but we may be able to use our hybrid device to sense current transport properties of materials in the quantum regime,” says Li.

The work is detailed in Physical Review Letters DOI:http://dx.doi.org/10.1103/PhysRevLett.117.015502.