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Point defect research in semiconductors has gained remarkable new momentum due to the identification of special point defects that can implement qubits and single photon emitters with unique characteristics. Indeed, these implementations are among the few alternatives for quantum technologies that may operate even at room temperature, and therefore discoveries and characterization of novel point defects may highly facilitate future solid state quantum technologies. First principles calculations play an important role in point defect research, since they provide a direct, extended insight into the formation of the defect states. In the last decades, considerable efforts have been made to calculate spin-dependent properties of point defects from first principles. The developed methods have already demonstrated their essential role in quantitative understanding of the physics and application of point defect qubits. Here, we review and discuss accuracy aspects of these novel ab initio methods and report on their most relevant applications for existing point defect qubits in semiconductors. We pay attention to the advantages and limitations of the methodological solutions and highlight additional developments that are expected in the near future. Moreover, we discuss the opportunity of a systematic search for potential point defect qubits, as well as the possible development of predictive spin dynamic simulations facilitated by ab initio calculations of spin-dependent quantities. ; Funding Agencies|Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Knut & Alice Wallenberg Foundation New States of Matter 2014-2019 (COTXS); Ministry of Education and Science of the Russian Federation [14.Y26.31.0005, K2-2017-080, 211]; National Research Development and Innovation Office of Hungary (NKFIH) within the Quantum Technology National Excellence Program [2017-1.2.1-NKP-2017-00001]; EU QuantERA project Q-Magine [127889]; EU H2020 ASTERIQS project; EU QuantERA project Nanospin [127902]

Study and design of magneto-optically active single point defects in semiconductors are rapidly growing fields due to their potential in quantum bit (qubit) and single photon emitter applications. Detailed understanding of the properties of candidate defects is essential for these applications, and requires the identification of the defects microscopic configuration and electronic structure. In multicomponent semiconductors point defects often exhibit several non-equivalent configurations of similar but different characteristics. The most relevant example of such point defect is the divacancy in silicon carbide, where some of the non-equivalent configurations implement room temperature qubits. Here, we identify four different configurations of the divacancy in 4H-SiC via the comparison of experimental measurements and results of first-principle calculations. In order to accomplish this challenging task, we carry out an exhaustive numerical accuracy investigation of zero-phonon line and hyperfine coupling parameter calculations. Based on these results, we discuss the possibility of systematic quantum bit search. ; Funding Agencies|Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish e-Science Centre (SeRC); Centre in Nano science and Nanotechnology (CeNano); Knut&Alice Wallenberg Foundation New States of Matter (COTXS); Swedish Research Council (VR) [2015-04391, 2016-04810, 2016-04068]; Linkoping University [LiU-2015-00017-60]; Swedish National Infrastructure for Computing Grants [SNIC 001/12-275, SNIC 2013/1-331, SNIC 2016/1-528]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005, K2-2017-080]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; National Research Development and Innovation Office of Hungary within the Quantum Technology National Excellence Program [2017-1.2.1-NKP-2017-00001]

Point defects in semiconductors are relevant for use in quantum technologies as room temperature qubits and single photon emitters. Among suggested defects for these applications are the negatively charged silicon vacancy and the neutral divacancy in SiC. The possible nonequivalent configurations of these defects have been identified in 4H-SiC, but for 6H-SiC, the work is still in progress. In this paper, we identify the different configurations of the silicon vacancy and the divacancy defects to each of the V1-V3 and the QL1-QL6 color centers in 6H-SiC, respectively. We accomplish this by comparing the results from ab initio calculations with experimental measurements for the zero-phonon line, hyperfine tensor, and zero-field splitting. Published under license by AIP Publishing. ; Funding Agencies|Swedish e-Science Centre (SeRC); Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Research Council (VR) [2016-04068, 2016-04810]; Centre in Nano Science and Nanotechnology (CeNano); Carl Trygger Stiftelse for Vetenskaplig Forskning [CTS 15: 339]; JSPS KAKENHI [A 17H01056, 18H03770]; Ministry of Science and High Education of the Russian Federation [K2-2019-001, 211]; MTA Premium Postdoctoral Research Program; National Research Development and Innovation Office of Hungary (NKFIH) within the Quantum Technology National Excellence Program [2017-1.2.1-NKP-2017-00001]; National Research Development and Innovation Office of Hungary (NKFIH) within NVKP Program [NVKP 16-1-20160043]; National Research Development and Innovation Office of Hungary (NKFIH) within Excellent Researcher Program [KKP129886]; EU QuantERA Nanospin (NKFIH) [127902]

Dynamic nuclear polarization (DNP) is an attractive method for initializing nuclear spins that are strongly coupled to optically active electron spins because it functions at room temperature and does not require strong magnetic fields. In this Letter, we theoretically demonstrate that DNP, with near-unity polarization efficiency, can be generally realized in weakly coupled electron spin-nuclear spin systems. Furthermore, we theoretically and experimentally show that the nuclear spin polarization can be reversed by magnetic field variations as small as 0.8 Gauss. This mechanism offers new avenues for DNP-based sensors and radio-frequency free control of nuclear qubits. ; Funding Agencies|Knut & Alice Wallenberg Foundation; Swedish Research Council (VR) [2015-04391]; Swedish Foundation for Strategic Research program SRL Grant [10-0026]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Swedish National Infrastructure for Computing [SNIC 2013/1-331, SNIC 2014/1-420]; "Lendulet program" of the Hungarian Academy of Sciences; NSF [EFMA-1641099]; UChicago NSF-MRSEC; AFOSR, AFOSR-MURI, ARO

Silicon-vacancy qubits in silicon carbide (SiC) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in 4H-SiC. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects. ; Funding Agencies|National Excellence Program of Quantum-Coherent Materials Project (Hungarian NKFIH Grant) [KKP129866]; European Union QuantERA Q-Magine Project [127889]; QuantERA Nanospin Project [127902]; European Union Horizon 2020 Quantum Technology Flagship project ASTERIQS [820394]; National Quantum Technology Program [2017-1.2.1NKP-2017-00001]; Swedish Research CouncilSwedish Research Council [VR 2016-04068]; Swedish Energy AgencySwedish Energy Agency [43611-1]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; European Union Horizon 2020 project QuanTELCO [862721]; Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [JSPS KAKENHI 17H01056, JSPS KAKENHI 18H03770]; U.S. Department of Energy, Office of ScienceUnited States Department of Energy (DOE) [DE-SC0019174]; Swedish National Infrastructure for Computing at the National Supercomputer Centre [SNIC 2019/3667, LiU-2015-00017-60]

Defect-based quantum systems in wide bandgap semiconductors are strong candidates for scalable quantum-information technologies. However, these systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. Here, we identify a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron X-ray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide as a near-stacking fault axial divacancy and show how this model explains these defects robustness against photoionization and room temperature stability. These results provide a materials-based solution to the optical instability of color centers in semiconductors, paving the way for the development of robust single-photon sources and spin qubits. ; Funding Agencies|MTA Premium Postdoctoral Research Program; Knut and Alice Wallenberg Foundation through WBSQD2 project [2018.0071]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish e-Science Centre (SeRC); Swedish Research CouncilSwedish Research Council [VR 2016-04068]; Carl-Trygger Stiftelse for Vetenskaplig Forskning [CTS 15:339]; Ministry of Education and Science of the Russian FederationMinistry of Education and Science, Russian Federation [K2-2019-001, 211]; Hungarian NKFIH grants of the National Excellence Program of Quantumcoherent materials project [KKP129866]; EU QuantERA Nanospin project [127902]; EU H2020 Quantum Technology Flagship project ASTERIQS [820394]; NVKP project [NVKP_16-1-2016-0043]; National Quantum Technology Program [2017-1.2.1-NKP-2017-00001]; U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering DivisionUnited States Department of Energy (DOE); DOE Office of ScienceUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User FacilityUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; DOE, Office of Basic Energy SciencesUnited States Department of Energy (DOE)