Optical spectroscopy of shallow silicon vacancy centers in diamond nanostructures

Color centers in diamond represent the basic building blocks for various quantum technology applications, including sensing [1–5], quantum information processing [6], and quantum communication [7–9]. In quantum sensing, the negatively charged silicon vacancy (SiV−) center is a particularly promising candidate for all-optical single-spin quantum sensing at sub-kelvin temperatures and tesla-range magnetic fields due to its outstanding optical and spin qualities in this regime [10, 11]. The SiV− boasts low optical lifetimes (~ 1.7ns), a high Debye-Waller factor (~ 0.7) and optical coherence protected by its crystallographic inversion symmetry [12, 13], as well as an all-optically addressable spin with long coherence times [10]. What is more, the study of complex physical systems such as skyrmions [14] and other strongly correlated electron systems below 1K and high magnetic fields would benefit immensely from the high spatial resolution and sensitivity offered by single-spin scanning probe magnetometry, such as demonstrated with near-surface nitrogen vacancy (NV) centers in diamond from 4-300K [1, 15] incorporated in this geometry.
At ultra-low temperatures (< 1K), however, charge-state instabilities of near-surface NV centers obstruct their use for such applications. Therefore, the SiV− center presents an excellent candidate in this particular regime, although the combination of an emitter close (< 100 nm) to the surface together with high optical coherence in nanostructures is still difficult to attain, a vital prerequisite for all-optical SiV scanning probe magnetometry.
In this thesis, we address this issue by demonstrating lifetime-limited optical coherence of single shallow (< 50nm) SiV− centers incorporated into diamond parabolic reflectors [16], suitable for scanning probe operation, paving the way towards all-optical ultra-low temperature scanning probe magnetometry at high magnetic fields. Further, the unlocked spectral stability of the SiV− enables a detailed study of its orbital structure using photon correlation measurements under resonant optical driving, shedding light on its photon bunching dynamics.
Finally, we turn to yet another promising defect center, the neutrally charged silicon vacancy (SiV0) center in diamond. While it offers a unique combination of excellent optical properties and long spin coherence times even above 1K [17], its deterministic stabilization in the diamond host is challenging. Here, we present a method for charge-state control of the SiV center, where SiV0 is produced by surface transfer doping enabled by hydrogen termination and subsequent recovery of SiV− using a continuous optical charge-state conversion technique. This enables accessible charge-state control and exciting prospects for SiV-based quantum technologies by incorporating SiV0 into open microcavities [18]. Our results constitute a key step towards the use of near-surface SiV− centers for sensing in extreme conditions and offer powerful approaches for charge-state control of diamond color centers for quantum technology applications.