The quest for easily fabricated, addressed, and manipulated quantum architectures is driving significant research. Diamond and SiC are promising platforms for quantum technologies, offering potential for applications in quantum teleportation, networks, and memories. Current solid-state qubit implementations, however, are limited by short-range spin-spin interactions or require probabilistic photon detection in macroscopic networks, both of which possess drawbacks. This research aims to overcome these limitations by utilizing a system with coherent, long-range interactions (>10 nm) and addressable optical transitions. Previous research into systems like giant Rydberg excitons in cuprous oxide and donor-acceptor pairs (DAPs) in various semiconductors showed promise but faced challenges. This work explores DAPs in wide-bandgap semiconductors as a novel quantum platform where coherent, long-range interactions are mediated by optically switchable dipole-dipole coupling, analogous to Rydberg interactions in atomic systems. This approach offers the advantage of scalability through mature nanofabrication techniques and readily available control infrastructures like cavities and waveguides. Furthermore, DAPs in solid-state systems boast significantly longer lifetimes compared to molecular DAP systems, facilitating the realization of long-lived, coherently controlled states for quantum applications. The wide variety of available DAPs in materials commonly used in quantum information science enhances the platform's versatility and generalizability.

The paper reviews existing research on quantum architectures, highlighting the limitations of current solid-state qubit implementations. It discusses alternative approaches such as giant Rydberg excitons in cuprous oxide and donor-acceptor pairs (DAPs) in various semiconductors. The literature review notes that while DAP systems have been studied extensively, their applications in quantum information science are relatively recent. It contrasts the advantages of solid-state DAPs over molecular systems, emphasizing the potential for scalability and mature control techniques afforded by solid-state systems. The review points out the importance of the static electric dipole moments in DAPs for optically controlled long-range interactions, similar to Rydberg atoms, and the potential for engineering these dipoles via nanofabrication techniques. Finally, the review acknowledges the existing discrepancies in literature regarding DAP radiative recombination lifetimes, spanning from picoseconds to milliseconds, depending on factors such as host material, temperature, and dopant concentration.

The researchers employed spin-polarized density functional theory (DFT) calculations using the Quantum Espresso package and the WEST code to investigate the electronic structure of DAPs in diamond and 3C-SiC. Norm-conserving pseudopotentials and an energy cutoff of 90 Ry were used. Charge transition levels (CTLs) for various substitutional defects (B, N, P in diamond; B, N, Al in SiC) were computed using both the PBE and HSE functionals, accounting for finite-size effects. The electric dipole moments of selected DAPs were calculated using two methods: a simple approximation (eRm) and a more sophisticated approach involving the difference in polarization between ground and excited states, employing maximally localized Wannier functions (MLWFs). Zero-phonon lines (ZPLs) were evaluated using constrained DFT and compared with a simple DAP model to assess the accuracy of the model. The sensitivity of ZPLs to applied electric fields was also investigated, examining the Stark shift using both DFT and the MLWF method. To assess the resolvability of ZPLs from DAPs at various distances, photoluminescence (PL) spectra were calculated using the effective one-dimensional configurational coordinate (CC) approximation, incorporating Huang-Rhys (HR) factors to quantify electron-phonon coupling. Finally, radiative lifetimes were estimated using an expression involving the optical transition dipole moment and energy difference between states. Experimental measurements of photoluminescence spectra were conducted on diamond samples containing nitrogen and boron, with electrostatic gates fabricated on selected samples to assess the electric dipole moments under applied electric fields.

The calculations revealed that several DAPs in diamond and SiC possess unusually large electric dipole moments (>25 Debye), significantly larger than that of the nitrogen-vacancy center (~0.8 Debye). These large dipole moments enable strong interactions at length scales exceeding 10 nm. The ZPLs were found to be highly sensitive to applied electric fields, demonstrating significant tunability (a few THz). A simple DAP model accurately reproduced the ZPL energy dependence on donor-acceptor distance, although deviations were noted for deeper defects. The analysis of photoluminescence spectra indicated that shallower DAPs (e.g., Al-N pairs in SiC) exhibit weaker electron-phonon coupling, resulting in sharper, more resolvable ZPLs compared to deeper DAPs (e.g., B-N pairs in diamond). Radiative lifetimes were predicted to be in the nanosecond range for some DAPs in diamond and in the microsecond range for DAPs in SiC. Experimental measurements on diamond samples confirmed the computational findings that B-N pairs in diamond have a weak and unresolvable ZPL emission due to their strong phonon coupling. The discrepancy between computed results and experimental findings for B-N pairs in diamond highlights the crucial role of electron-phonon coupling. The study finds that Al-N pairs in 3C-SiC exhibit the most promising combination of properties for the proposed quantum platform: large electric dipole moments enabling long-range interactions, resolvable ZPLs due to weak electron-phonon coupling, and sufficiently long radiative lifetimes for coherent control.

The findings demonstrate that DAPs in wide-bandgap semiconductors offer a viable platform for engineering optically addressable long-range interactions between point defects. The large, optically switchable dipole moments, tunable ZPLs, and sufficiently long radiative lifetimes make them suitable building blocks for quantum technologies. The experimental confirmation of the strong electron-phonon coupling in B-N pairs in diamond underscores the importance of selecting shallow defects to minimize this effect and achieve well-resolved spectral lines. The success with Al-N pairs in SiC suggests that similar strategies could be applied to other semiconductor systems. The predicted longer radiative lifetimes in SiC compared to diamond are advantageous for coherent control. The ability to isolate and manipulate individual DAP shells with increasing dipole moments opens avenues for stronger, longer-range interactions.

This research proposes and validates a novel quantum platform using donor-acceptor pairs (DAPs) in wide-bandgap semiconductors. Calculations and experiments demonstrate the feasibility of creating optically addressable, long-range interactions based on DAPs' large, tunable dipole moments and sufficiently long radiative lifetimes. The study highlights the importance of choosing shallow defects to minimize electron-phonon coupling and improve spectral resolution. Future research could explore additional semiconductor materials, optimize nanofabrication techniques for controlled placement of donor and acceptor atoms, and investigate advanced readout strategies.

The study focused on a limited set of defects in two host materials, which may not be fully representative of the broader range of possibilities. The one-dimensional configurational coordinate model for calculating photoluminescence spectra is an approximation and may not fully capture the complex interactions in the system. Experimental results were limited, primarily focusing on B-N pairs in diamond to validate DFT findings. Further experimental work is needed to fully explore the potential of different DAPs and to address the impact of non-radiative processes on the lifetimes of DAPs.