Improved ensemble spin coherence of silicon vacancies using isotopically purified 4H-SiC*

The silicon vacancy (V_Si) in SiC is a promising defect for quantum information science and technology. In particular, the V2 V_Si is one of the few defects with long-lived spin memory at room temperature and in a host material having a low abundance of nuclear spins. The 4.7% abundance of ²⁹Si and 1.1% abundance of ¹³C, however, do have a significant effect on the spin coherence of V_Si, resulting in hyperfine-induced side peaks in the spin transitions and strong echo modulation effects that limit the spin echo decay time for low magnetic fields (< ~10 mT) to less than 10 μs. Here, we report on ensemble spin coherence measurements of the V2 V_Si for isotopically purified 4H-SiC epilayers, with much lower concentrations of ²⁹Si and ¹³C. With room temperature optically detected magnetic resonance, we show very sharp ensemble transition linewidths down to 0.25 MHz and a T₂^* of up to 5 μs. Spin echo measurements show no sign of echo modulation from nuclear spins, giving an echo decay time of about 100 μs at a low magnetic field of 0.3 mT. We have also measured the effects of defect density and magnetic field on spin coherence.