One exciting application at the forefront of diamond research is in quantum science. Nonclassical (single-photon) light sources based on individual color centers in diamond, most notably the nitrogen-vacancy (NV) center have been used for secure communication based on quantum-cryptography protocols. Coupling between the NV center’s electronic spin and nearby nuclear spins can be used to form a large qubit register an essential ingredient for a quantum computer. Recently, techniques designed to manipulate the NV center have been applied to nanoscale magnetic-field sensing based on single spins. But practical implementations of these technologies require efficient excitation and extraction of single photons from NV centers using a simple optical system. This is a challenge because of the high refractive index of the diamond host, so that the majority of photons emitted from an embedded color center are not accessible even to sophisticated setups.
Diamond nanowire with an embedded NV center acts as an antenna that enables efficient incoupling of the pump power used to drive the NV center’s optical transition, as well as efficient outcoupling of emitted photons to an objective lens: see Figure 1(a). The diamond nanowires, ~2μm long and ~200nm in diameter, are fabricated from type Ib diamond (which contains randomly distributed NV centers) using electron-beam lithography and reactive ion etching see Figure 1(b).
The diamond nanowire has 10-100 times better performance while using less power than bulk material.
The diamond-nanowire antenna provides a natural and efficient interface for an individual color center and significantly increases the collection efficiency of emitted photons. However, we are working on further improvements. For example, coupling emission directly to an optical fiber could realize more compact systems with larger overall photon-extraction efficiencies. Photon flux can further be improved by increasing the photon-generation rate (Purcell effect) using plasmonic nanostructures or optical cavities fabricated directly in diamond. We believe that these developments will enable realization of large-scale quantum-information-processing systems as well as sensitive magnetometers based on nanostructured diamonds.