Session A43: First-Principles and Quantum Embedding Simulations for Point Defects in Semiconductors

Current state of Embedding techniques for Quantum Computing

In this talk, I will review embedding techniques on quantum computers for materials and chemistry. Embedding is a method of combining classical and quantum computing techniques to explore problems which typically have a natural center or centers of strongest or more correlated interactions, and this part/parts are done on the quantum computer, while the rest is done with a classical system. The goal is usually either greater accuracy, greater speed, or greater opportunity for analytical insight into the system. What distinguishes embedding techniques is usually the proportion and frequency of interchange between quantum and classical techniques. Some key embedding techniques will be represented by other speakers in this session, and I will let them describe their own work, and only put their work in the context of others. Other techniques will not appear except in this talk, and to these I will devote more attention. In particular I will describe one technique that has been used with quite some success by a team I am on, and which has appeared in a recent arxiv posting.   

First principles simulation of neutral excitations in materials

The simulation of light activated processes in materials for energy sustainability and quantum information science requires a robust description of neutral excitations in complex heterogeneous systems. I will present a hierarchical modeling approach that enables us to simulate neutral excitations in materials with increasing complexity. First, I will discuss the simulation of excitons in large systems using density matrix perturbation theory, where the dielectric screening is evaluated from first principles with a finite field method or approximated by machine learning models. Second, I will discuss the simulation of neutral excitations in the presence of structural relaxations using the Huang-Rhys theory. Calculated photoluminescence spectra of point-defects are presented for diamond and silicon carbide, and carefully validated against experiment. Third, I will present the calculation of strongly-correlated neutral excitations of deep defects, e.g., color centers in diamond. A quantum embedding method that relies on input from density functional theory is used to generate an effective Hamiltonian that describes the low-lying excitations of the defect, and whose many-body eigenstates are obtained using configuration interaction. Finally, I will discuss opportunities for electronic structure calculations that are driven by emerging trends in the high-performance computing landscape, which include strategies to leverage exascale and quantum computing.   

Computational spectroscopy for point defects

First-principles calculations based on density functional theory (DFT) play a crucial role in the identification and characterization of point defects in materials. However, the miniaturization of devices, use of new types of semiconducting and wide-band-gap materials in electronics, and advancement of quantum technologies motivates the further development of computational methods for defects. Specifically, the utility of such calculations can be enhanced by: (i) improving the general quantitative accuracy of the methods, (ii) increasing the range of defect properties that can be addressed, and (iii) including aspects that make calculations more directly comparable to experiment. This talk will discuss several examples of recent theoretical developments aimed at each of these directions. I will give an overview of methodologies for determining experimentally observable properties based on radiative and nonradiative defect-related transitions, including how the effects of temperature can be included for direct comparison to electrical measurements, and how these properties relate to defects for quantum technologies. Also, I will discuss how quantum embedding has emerged as a promising way of combining DFT with many-body methods for correlated excited states of defects.   

Unconventional defect configurations in aluminum oxide

Electronic structure calculations have been instrumental in characterizing point defects in materials, including formation energies, diffusivities, optical properties, and more. Accurate results rely on accurate atomic configurations, which are not always straightforward to obtain: complex total energy surfaces with several local minima at distinct defect geometries challenge first-principles predictions for, e.g., metal oxides. In this work, we evaluate several approaches for sampling candidate defect sites within density functional theory, including Bayesian inference and a method based on the Voronoi decomposition of the pure crystal. We also investigate the influence of exchange and correlation, spin polarization, and different schemes for correcting finite size effects for charged defects. We discover a series of complex and unconventional defect geometries in aluminum oxide, in some cases identifying lower energy interstitial geometries than previously known, and in other cases finding that rearranging a point defect into a defect cluster is energetically favorable. Sometimes, the lowest energy geometry even depends on defect charge state. These findings demonstrate the necessity for thorough and systematic defect geometry optimization and have profound implications for poorly understood defect migration paths and diffusion processes in aluminum oxide, including the possibility of mechanisms mediated by charge-transfer processes.   

Theoretical magneto-optical spectroscopy of solid state defect quantum bits

We live in the era of second quantum revolution in which solid state defect quantum bits play a significant role. An exemplary solid state defect quantum bit is the nitrogen-vacancy center in diamond which can be effectively initialized and readout at room temperature. We show how theoretical magneto-optical spectroscopy on nitrogen-vacancy center explained its optical spinpolarization loop which is the key mechanism in the initialization and readout. To this end, methods to calculate highly correlated electronic states and levels embedded in the itinerant solid state electron system with thousands of electrons has been developed [1] which is often called "quantum embedding" method or can be viewed as a multiscale method where the itinerant electron system is treated by density functional theory whereas the Coulomb-interaction between the strongly interacting orbitals in the system is directly calculated, i.e., so called configurational interaction theory. To our knowledge, there is no rigorous theory about the interface of the two approaches, i.e., the double counting term, therefore, we have recently started to use density matrix renormalization group wavefunction methods based on density functional theory ground state calculations which produce promising results for defect spins in hexagonal boron nitrides [2,3]. We show that understanding the optical spinpolarization loop requires the exploitation of dynamical effects due to the enhanced electron-phonon interaction. In this regard, we show the power of Jahn-Teller theorem when combined with density functional theory calculations of few thousands of electrons system [4,5]. In particular, we show the extension of Herzberg-Teller theorem from the optical transition to intersystem crossing [6] which is the key of quantum bit operation of nitrogen-vacancy center and related quantum systems.