Hybrid Quantum Registers Based on Group IV Defects in Diamond

Nowadays, quantum information processing (QIP) gets more and more important regarding the increasing demand for secure communication. For QIP a quantum platform is required which is able to perform computations and transfer entanglement over long distances.<br/>Among others, negatively charged group IV defects in diamond gained increasing interest as promising candidates due to their efficient spin photon interface and the long coherence times they can provide. The best known example is the SiV [1], for which it was furthermore demonstrated that neighboring nuclear spins can be accessed and manipulated [2]. Exploiting the dipolar coupling individual ¹³C can be addressed, forming a quantum register. However, at T&gt;500mK in the ground state phonon mediated relaxation has a severe detrimental effect on the electron spin coherence. This makes the operation in expensive dilution refrigerators or dedicated control over phononic coupling via strain or geometry necessary.<br/>Point defects with a substitutional group IV atom of a higher atomic number could serve as an alternative as they provide higher ground state splitting, requiring less low temperatures to get rid of this limiting effect. Recently, the focus especially has shifted to SnV as it was shown to have long coherence times already at liquid helium temperatures [3]. However, it is still an outstanding challenge to produce decent samples with good optical properties, as the heavy atom can cause strain or even damage to the diamond crystal during implantation. Thus, these samples are often lacking spectral or charge state stability and showing a broad inhomogeneous distribution. <br/>Here we report on recent progress of GeV operated in the mK regime. The smaller linewidths of the GeV [4] allow for separation of the spin selective transitions at lower magnetic fields compared to the SiV. This results also in smaller microwave frequencies required for the coherent control. As the transmission gets better at lower frequencies, this leads to less losses, which enabled us to drive the spin with up to 5MHz. Moreover, we were able to measure a coherence time of 160µs.<br/>As the Ge atom is smaller, it can be incorporated in the diamond lattice more easily leading to a higher number of indistinguishable single photon emitters. Furthermore, compared to the SiV the GeV has a higher quantum yield resulting in a higher fluorescence. These features can be beneficial considering entanglement distribution schemes.<br/>Moreover, by transferring the techniques of addressing surrounding nuclear spins developed for the SiV, this could provide a nice hybrid quantum register, which can be considered as a platform for quantum information processing.<br/> <br/>[1] Sukachev, Denis D., et al. "Silicon-vacancy spin qubit in diamond: a quantum memory exceeding 10 ms with single-shot state readout." <i>Physical review letters</i> 119.22 (2017): 223602.<br/>[2] Metsch, Mathias H., et al. "Initialization and readout of nuclear spins via a negatively charged silicon-vacancy center in diamond." <i>Physical review letters</i> 122.19 (2019): 190503.<br/>[3] Görlitz, Johannes, et al. "Coherence of a charge stabilised tin-vacancy spin in diamond." <i>arXiv preprint arXiv:2110.05451</i> (2021).<br/>[4] Siyushev, Petr, et al. "Optical and microwave control of germanium-vacancy center spins in diamond." <i>Physical Review B</i> 96.8 (2017): 081201.