The construction of complex networks of qubits that preserve good quantum coherence in spite of their mutual interactions is an essential prerequisite for the realization of a hardware implementing quantum computation algorithms. Colour centres in diamond are solid-state systems that have been proposed for the realization of such qubits. They are attractive because of the ease of controllability of the spin state where the qubit is encoded, and because of the possibility to access the spin state optically. Such optical access has already allowed the realization of coherent optical–spin interfaces — the first concrete step towards networks of photonically coupled qubits.

In this respect, special interest has been given in recent years to silicon–vacancy defects in diamond nanophotonic devices. At variance with nitrogen–vacancy defects — widely investigated colour centres in diamond — the structural inversion symmetry is preserved in silicon–vacancy defects, in turn leading to reduced sensitivity to local electric field noise and improved properties as quantum emitters of indistinguishable photons.

Credit: APS

Now, Petr Siyushev et al. report on the control of the spin state via optical and microwave fields in another diamond colour centre that preserves inversion symmetry — the negatively charged germanium–vacancy defect (Phys. Rev. B 96, 081201(R); 2017 ). The researchers first determine the fluorescence properties of samples polished along different crystallographic planes, improving the detection conditions by fabricating solid immersion lenses (pictured). The measurements are performed at cryogenic temperatures and also under the effect of external magnetic fields ∼0.3 T. The detected optical transitions, of both spin-conserving and spin-flipping nature, are stable during few-hours-long measurements and map the characteristic diagram of energy levels in detail, evidencing an active spin-1/2 electronic degree of freedom.

Based on the observed energy levels, the researchers use two optical fields resonantly matching different electronic transitions between the Zeeman-split ground and excited states, resulting in dark spin superposition states — a condition known as coherent population trapping. Also, the irradiation of the system with properly tuned microwave radiation induces spin transitions and reshuffles the levels' populations within the ground state. With these experiments, the researchers demonstrate that the spin state of the considered germanium–vacancy defects can be controlled coherently by means of both optical and microwave fields. Both the methods reveal characteristic coherence times ∼20 ns for the ground state, and — in analogy with silicon–vacancy defects — the researchers identify orbital-relaxation processes mediated by phonons as the main limiting factor for this quantity.