Defects in diamond can have optical properties that are almost atomic-like, with well-defined energy levels that are identical for identical defects. This makes them ideal for quantum information applications that require single-photons emissions.

Most considered defects are nitrogen-vacancy (NV) or silicon-vacancy (SiV) centers. Despite remarkable properties, they still suffer from limitations. NV centers tend to have low optical emission, and their asymmetric structure makes them less stable. SiV centers are symmetric, but their energy levels can be significantly shifted in the presence of crystal strain.

Germanium is in the same periodic table group as silicon but is less studied as a defect center. Nahra et. al characterized germanium vacancy (GeV) defects in nanodiamonds and found notable advantages over NV and SiV centers for quantum information applications.

Using a variety of experimental techniques, the researchers characterized the lifetime, polarization, brightness, zero-phonon line, internal population dynamics, and emission properties of GeV defects in nanodiamonds. They found that GeV center in nanodiamonds have better optical properties than GeV-center in bulk diamond at room temperature -- including symmetry-induced stability, more well separated energy levels, and higher quantum efficiency. This makes them favorable candidates for solid state single photon source applications such as quantum key distribution.

In future work, author Christophe Couteau said they want to take advantage of nanodiamonds’ ability to couple to other photonic structures.

“If you can do that, you can hope to get all the light, all the single photons directed into a waveguide, or perhaps into an optical fiber directly,” he said. There is also characterization work to be done on GeV defects at low temperature, which is necessary for quantum computing.

Source: “Single germanium vacancy centers in nanodiamonds with bulk-like spectral stability,” by Christophe Couteau, Mackrine Nahra, Daniel Alshamaa, Regis Deturche, Valery Davydov, Ludmila F. Kulikova, and Viatcheslav Agafonov, AVS Quantum Science (2021). The article can be accessed at