Nature Commun. 5, 3328 (2014)

The most studied defect in diamond is the nitrogen–vacancy (NV) centre, which consists of a nitrogen impurity atom located in the vicinity of a carbon vacancy. The spin of an electron associated with an NV centre has a long coherence time and can be manipulated by photons. These two properties make NV centres good candidates for quantum communication devices.

One problem with NV centres is that their optical emission is intrinsically weak. A promising alternative is the silicon–vacancy (SiV) centre in diamond, which has much stronger emission. So far, however, it has been difficult to detect an optical emission from SiV centres associated with a specific state of the electron spin. This is principally due to spin mixing in the ground state of the electron. Now, Mete Atatüre and colleagues at the University of Cambridge and institutes in Germany, Switzerland and Austria have succeeded in tagging optical emission lines from a negatively charged SiV centre with the electron spin states.

Measurements of the fluorescence of the SiV− centres revealed two peaks associated with the ground state of the electron and two peaks associated with an excited state. These peaks were split into quadruplets when a magnetic field was applied, as would be expected for a spin-1/2 electron. When a laser excites electrons of a specific spin configuration to the highest energy state, the optical emission shows a relaxation to a lower excited state before the electron decays to the ground state. The decay to the ground state does not conserve the spin perfectly due to misalignment of the magnetic field with the natural magnetization of the SiV− centres. However, this could be improved by changing the growth conditions of the samples.