Highlights ✍
Our group member, Gergő Thiering, has been honored by receiving the János Bolyai Research Grant from the Hungarian Academy of Sciences. (2020) Congratulations!
Our group member, Dávid Beke, has been honored by receiving the János Bolyai Research Grant from the Hungarian Academy of Sciences. (2019) Congratulations!
Our group member, Viktor Ivády, has been honored by receiving the Premium Postdoctoral Fellowship from the Hungarian Academy of Sciences. (2018) Congratulations!
Defects with associated electron and nuclear spins in solidstate materials have a long history relevant to quantum information science that goes back to the first spin echo experiments with silicon dopants in the 1950s. Since the turn of the century, the field has rapidly spread to a vast array of defects and host crystals applicable to quantum communication, sensing and computing. From simple spin resonance to longdistance remote entanglement, the complexity of working with spin defects is fast increasing, and requires an indepth understanding of the defects’ spin, optical, charge and material properties in this modern context. This is especially critical for discovering new relevant systems for specific quantum applications. In this Review, we expand upon all the key components of solidstate spin defects, with an emphasis on the properties of defects and of the host material, on engineering opportunities and on other pathways for improvement. This Review aims to be as defect and material agnostic as possible, with some emphasis on optical emitters, providing broad guidelines for the field of solidstate spin defects for quantum information.
Nat. Rev. Mater. pp 120 (2021). DOI:10.1038/s4157802100306y
A plethora of singlephoton emitters have been identified in the atomic layers of twodimensional van der Waals materials. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic gefactor of ~2 and zerofield splitting below 10 MHz. The photokinetics of one type of defect is compatible with groundstate electronspin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of inplane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, outofplane delocalized πorbital electron, probably originating from substitutional impurity atoms. We extracted spin–lattice relaxation times T1 of 13–17 μs with estimated spin coherence times T2 of less than 1 μs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride.
Nat. Mater. , pp. 16. (2021). DOI:10.1038/s41563021009794
In collaboration with the [Nathalie P. de Leon Research group] we report the realization of optically detected magnetic resonance and coherent control of SiV(0), enabled by efficient optical spin polarization via previously unreported higherlying Rydberglike excited states. We assign these states as bound exciton states using group theory and density functional theory. These bound exciton states enable new control schemes for SiV(0) as well as other emerging defect systems.
Phys. Rev. Lett. 125, 237402 (2021). DOI:10.1103/PhysRevLett.125.237402
In this paper, we analyze the numerical aspects of the inherent multireference density matrix renormalization group (DMRG) calculations on top of the periodic Kohn–Sham density functional theory using the complete active space approach. The potential of the framework is illustrated by studying hexagonal boron nitride nanoflakes embedding a charged single boron vacancy point defect by revealing a vertical energy spectrum with a prominent multireference character. We investigate the consistency of the DMRG energy spectrum from the perspective of sample size, basis size, and active space selection protocol. Results obtained from standard quantum chemical atomcentered basis calculations and planewave based counterparts show excellent agreement. Furthermore, we also discuss the spectrum of the periodic sheet which is in good agreement with extrapolated data of finite clusters. These results pave the way toward applying the DMRG method in extended correlated solidstate systems, such as point defect qubit in wide band gap semiconductors.
J. Chem. Theory Comput. 17, 2, 1143–1154 (2021). DOI:10.1021/acs.jctc.0c00809
Defectbased quantum systems in wide bandgap semiconductors are strong candidates for scalable quantuminformation technologies. However, these systems are often complicated by chargestate instabilities and interference by phonons, which can diminish spininitialization fidelities and limit roomtemperature 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 densityfunctional theory and experimental synchrotron Xray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide as a nearstacking fault axial divacancy and show how this model explains these defects’ robustness against photoionization and room temperature stability. These results provide a materialsbased solution to the optical instability of color centers in semiconductors, paving the way for the development of robust singlephoton sources and spin qubits.
Nature Communications 10, 5607 (2019). DOI:10.1038/s41467019134956
The product Jahn–Teller effect may occur for such coupled electron–phonon systems in solids where single electrons occupy double degenerate orbitals. We propose that the excited state of the neutral XV splitvacancy complex in diamond, where X and V labels a groupIV impurity atom of X = Si, Ge, Sn, Pb and the vacancy, respectively, is such a system with eg and eu double degenerate orbitals and Eg quasilocalized phonons. We develop and apply ab initio theory to quantify the strength of electron–phonon coupling for neutral XV complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective SiV(0) quantum bit, and reveals the complex nature of the excited states of neutral XV color centers in diamond.
npj Comput Mater 5, 18 (2019). DOI:10.1038/s41467019134956
We use densityfunctional theory calculations to see if other groupIV elements—namely, germanium, tin, and lead—in a vacancy center exhibit similar optical properties to silicon but with improved spin properties that permit an economical cooling system. We develop a new theory for understanding the interaction of light and magnetic fields with these color centers, and we explore how the dynamical motion of the atoms strongly affects the magnetooptical properties of the color center. By combining our theory with densityfunctional theory calculations, we find that leadvacancy color centers should have favorable optical and spin properties for operating at room temperature.
Phys. Rev. X 8, 021063 (2018). DOI:10.1103/PhysRevX.8.021063
Our ab initio simulations determined the magnetooptical properties of the neutral divacancy in silicon carbide that contributed to understanding its qubit operation carried out by Prof. David D. Awschalom group.
Phys. Rev. X 7, 021046 (2017). DOI:10.1103/PhysRevX.7.021046
Our ab initio study identified the nitrogen terminated (111) diamond surface to host shallow nitrogenvacancy centers for quantum sensing. In cooperation with Alex Retzker we showed, that special quantum simulations can be carried out with the nitrogen nuclear spins in this system
Nano Letters 17 (4), pp 22942298. (2017) DOI:10.1021/acs.nanolett.6b05023
We investigated the electronphonon coupling in small diamond cages called diamondoids. These quantum systems are mostly modelled within the BornOppenheimer approximations, where the coupling between the electrons and vibration modes is not fully taken into account. In our study we shown that the severe electronphonon coupling is a key point to properly describe the overall lineshape of the experimental photoemission spectrum. Our method goes beyond the typical BornOppenheimer approximation, where the electrons are pure electronlike quasiparticles. In our model, we deduced a link between the manybody perturbation approach of the electronphonon coupling and the wellknown JahnTeller effect.
Nature Communications 7: 11327. (2016). DOI:10.1038/ncomms11327
