Stabilization of point-defect spin qubits by quantum wells

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.

). This effect is not relevant in the context. b After light chemical etching on one side, the corresponding left branch of PL6 ODMR line disappears.
Note that the corresponding resonant frequencies are shifted with respect to a because of the smaller thickness of the sample.
In order to facilitate the fabrication of PL6 qubits, we carry out optically detected magnetic resonance (ODMR) experiments to locate PL6 centers in 4H-SiC. We apply inhomogeneous magnetic field along the c-axis of an as-grown 4H-SiC sample and record the ODMR signal of bulk  Table 1 and Supplementary Table 2 Table 3 and Supplementary Table 4   which is the focus in this section. We also note that the excited states of the high symmetry c-axis-oriented divacancies are dynamic Jahn-Teller (DJT) unstable. We approximated the DJT excited state energies by the static Jahn-Teller excited state energies in these cases. As the energy of the Jahn-Teller distortion is ≈30 meV, neglect of electron phonon coupling presumable causes uncertainties in the ZPL energy differences in the order of 10 meV.
The calculated and experimental ZPL energies of the axial divacancies are provided in Supplementary Table 5. Considering the small splitting of the ZPL lines and the estimated error bar of the theoretical results, only limited statements can be made. Similar classes of the ZPL energies can be observed as for the hyperfine and the zero-field-splitting parameters seen in the main text, however, the splitting between the PL1 and PL2 configurations are highly overestimated. Most importantly, the ZPL energy of k 2 k 2 -ssf configuration is well separated from the ZPL energy of other axial configurations and possesses the largest ZPL energy. These observations further support the identification of PL6 qubit as k 2 k 2 -ssf configuration.
The ZPL energies of the basal plane oriented configurations closely follow each other and several configurations have been found closer to each other than 10 meV, which is our numerical accuracy in these calculations. Consequently, no conclusive statement can be made from the ZPL results of the basal plane configurations. reasonable accuracy even within this approximation.
The single particle absorption spectrum can be seen in Supplementary Figure 2a and b for the neutral and negative charge state of k 2 k 2 -ssf divacancy configuration, respectively. In the neutral case, the lowest energy non-intra-defect transition happens between the VBM and e C defect state.
This result supports the negative charge state to be identified as the dark state of divacancy defect.
Comparing the single particle absorption spectra with higher level theory absorbtion spectra reported in Supplementary Reference [ 5 ], one can see that this statement is further confirmed. On the other hand, the transition energy of e C → e Si intra-defect transition falls below the VBM → e C transition in Supplementary Reference [ 5 ], which is in contrast to the single particle spectrum depicted in Supplementary Figure 2. The differences can be attributed to the electron-hole interaction that might be enhanced for localized defect states. Note that similar effect can be expected in the case of negative charge state, i.e. the e C → e Si transition may have lower energy than the e C → CBM and e C → ssf transitions. Note, on the other hand, that intra-defect transitions cannot change the charge state of the defect thus the defect remains in its dark negative charge state. Therefore, the actual position of e C → e Si transition does not influence our statements made for non-intra-defect transitions.
As can be seen in Supplementary Figure 2b, the presence of the divacancy defect states do not alter the appearance of the single stacking fault states below the CBM. Most importantly, there is a finite transition strength for e C → ssf transitions that may thus allow for bright state re-pumping processes.
Due to the overlap with the strong intra-defect a 1 → e C and e C → e Si transitions, relative transition strength of free-to-bound and bound-to-free like transitions cannot be reliably determined from the absorption spectra. Therefore, in the following section we provide transition dipole moments for pairs of Kohn-Sham defect states.