Researchers at Hewlett-Packard Laboratories are studying ways to connect defects in diamond to an on-chip optical network. A particular defect, the nitrogen-vacancy (NV) center, appears to be an excellent solid-state quantum bit (qubit) with electron spin coherence lifetimes exceeding 1 ms, and several groups are pursuing light-based schemes to connect multiple qubits. However, fabricating useful optical structures directly in diamond has so far proven difficult.

Instead of fabricating structures directly out of diamond, the HP group is pursuing hybrid cavity geometries in which diamond is coupled to the evanescent field of microcavities made from other materials. Initial experiments used diamond nanoparticles coupled to microdisk cavities made from SiO2, which support "whispering-gallery" modes that propagate in a circle around the disk edge. However, the utility of this approach was limited by the poor optical properties of NV centers found in nanoparticles.

More recently, the researchers have demonstrated microdisk cavities made from GaP, a material that has a high refractive index and is transparent at 637 nm, the zero-phonon emission wavelength for negatively charged NV centers. The GaP disks are placed onto a diamond surface and the sidewalls are extended by etching into diamond to improve the optical confinement. These structures have estimated mode volumes of a few cubic wavelengths, measured quality factors above 25,000 and demonstrated coupling to NV centers close to the diamond surface.

It is estimated that the spontaneous emission rate from the zero-phonon line could be enhanced by a factor of 17 for an optimally placed NV center that is spectrally on resonance with the cavity mode. This enhancement will be critical for creating photon-mediated electron entanglement for quantum computation and long distance quantum communication.

The researchers presented their work in the journal Nanotechnology.