Session W72: Quantum Chemistry Techniques for Quantum Systems

Quantum algorithms for non-unitary time evolution of quantum systems

Open quantum system evolution in the presence of an environment is crucial to understanding and improving many processes including the communication of quantum information and the transfer of energy. Quantum computing platforms have emerged as a promising route to modelling and predicting the behaviour of such systems. However, mapping inherently non-unitary dynamics into the unitary framework of gate-based quantum algorithms is a challenging task. Here, I will discuss two different density matrix gate-based quantum algorithms to predict the dynamics of open quantum systems. I will discuss the theory behind these algorithms, their extension from the Markovian to the non-Markovian regime, and applications relevant in chemistry and physics.   

Stability and molecular pathways to the formation of spin defects in silicon carbide

Spin defects in wide-bandgap semiconductors provide a promising platform to create qubits for quantum technologies. Their synthesis, however, presents considerable challenges, and the mechanisms responsible for their generation or annihilation are poorly understood. Here, we elucidate spin defect formation processes in a binary crystal for a key qubit candidate—the divacancy complex (VV) in silicon carbide (SiC). Using classical and ab initio molecular dynamics with enhanced sampling techniques, we characterize the formation mechanism of VV. We then predict the conditions favoring the formation of divacancies over the competing process of mono-vacancy formation. Moreover, we identify pathways to create new spin defects and determine their electronic properties using hybrid density functional calculations. The detailed view of the mechanisms that underpin the formation and dynamics of spin defects presented here may facilitate the realization of qubits in an industrially relevant material.   

Spin-Photon Interface and Optical Readout of Spin Defects from First-Principles Calculations

For initialization and readout of spin states, an optical interface is generally desired for its practical ease of use and isolating spin qubits. An efficient spin-photon interface requires knowledge of key optical parameters, including photoluminescence (PL) spectra, quantum efficiency, and spin-dependent optical contrast (PL contrast) through optically detected magnetic resonance (ODMR). In particular, high quantum efficiency and large PL contrast ensure reliable spin information readout. First-principles tools of predicting readout efficiency for spin qubits are highly desirable as such predictions can be directly compared to experiments and used as target parameters for qubit optimization. However, prediction of PL contrast requires knowledge of all kinetic processes, which need to include interactions among spin, photon, phonon, electron and exciton for spin defects. In this talk, we show our recent development on spin-dependent PL contrast and ODMR including all radiative, nonradiative and intersystem crossing rates, with defect-exciton, electron-phonon couplings and spin-orbit interactions, entirely from first-principles calculations [1]. We show that our method provides excellent agreement with experimental PL contrast and ODMR spectra for NV center in diamond. We then apply it to predict spin-related optical properties in carbon defects in hexagonal Boron-Nitride, an emerging material platform for hosting spin qubits. Our work provides the computational platform for optimizing spin-photon interface and predicting optical readout efficiency of spin qubits from first-principles. [1] T. Smart et al, npj Comput. Mater. 7, 59, (2021).   

Manipulating Quasi-Bound States in a Photonic Crystal with Periodic Impurities to Store Quantum Information

We analytically model a one-dimensional lattice with periodic impurities representing a photonic crystal. We then investigate bound states in the continuum by computing the transmission coefficient and the reflection coefficient. It turns out that when there are more impurities in our system there exist more wave numbers where the particles in our system  become essentially trapped. Due to this window of wavenumbers, in theory, quantum information could be encoded in our system by constructing differently shaped wave packets that are bound by the tuning of parameters in our system.   

Numerical Modeling of Multi-Defect Spin Dynamics in a Hyperfine Field

Spins in solid state defects represent promising building blocks for quantum information applications. For these purposes, maintaining the desired quantum state of these spins is crucial, however it is generally not straightforward, as spins can interact with various degrees-of-freedom of the solid state host. In many cases, hyperfine interactions between the electronic spin of the defect and the nuclear spins of the host lattice dominate decoherence. In these scenarios, the cluster-correlation expansion (CCE) method has proven to be a useful numerical tool in qualitatively and quantitatively capturing the effective spin coherence of the combined spin-bath system, and more recent generalizations have included population dynamics as well. In this work, we discuss extending this generalized CCE method to capture the spin dynamics of two qubit spins coupled with a common bath of nuclear spins. Using this framework, we evaluate the spin dynamics in a variety of defect qubit candidate systems, where we also consider different defect pair separations, magnetic field strengths, and pulse sequence schemes.  We study the interplay of the two qubits and the role of the hyperfine spin bath and highlight particularly important quantum-dynamical processes involved in these various regimes.   

Novel ways of probing local spin environments in complex materials

Characterizing nuclear quadrupole interactions presents a valuable method to describe the symmetry of local spin environments because the interaction imprints this information onto the dynamics of the spins. In traditional magnetic resonance, this information is encoded into the spectrum of the nuclear spins but can potentially be obscured by mutiple sources of spectral broadening. Here, we compare the use of coherent spins states and traditional thermal states as initial states for the experiment, and explore how quadrupolar interactions are encoded into the respective dynamics. It is known that coherent spin states evolve into spin-squeezed states under the quadrupolar interaction. We also compare how different sources of spectral broadening influence the observed dynamics. We compare how different sources of spectral broadening influence the observed dynamics with implications for the observation and manipulation of correlated magnetic phases.   

First-principles simulations of the divacancy formation and annealing in 3C-SiC

The divacancy (VV) in silicon carbide (SiC) is a promising spin qubit candidate for quantum technology applications. However, the formation and annealing mechanisms of the VV in SiC remain poorly understood at the microscopic level, thus hindering its controlled fabrication, manipulation and integration. Here, we combine first principles molecular dynamics (FPMD) with enhanced sampling simulations and nudged elastic band calculations to investigate the energetics of the VV formation and migration in cubic SiC. We predict pathways for the VV formation and annealing, and identify the critical intermediate configurations required for the correct interpretation of atomic transformation processes. Moreover, we characterize the impact of charge transfer, spin-flip and charge states in determining preferred pathways.   

Effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional materials

Point defects  in two-dimensional wide band-gap materials are potential hosts for emerging quantum properties such as single-photon emitters and spin quantum bits, as building blocks for quantum information technology devices. Despite the experiment efforts on characterization of such defects, their exact chemical composition remains unknown.  Meanwhile strain and environmental screening can strongly modify the optical signature, which may be responsible for observed variation of photoluminescence in different samples.

In this work, we studied environmental screening by varying layer thickness and substrate as well as effects of strain on optoelectronic properties of quantum defects in h-BN. We apply many-body perturbation theory including accurate electron correlation and electron-hole interaction to study quasiparticle energies, optical excitation and exciton lifetime of these quantum defects. In particular, we used our reciprocal-space interpolation method for summing-up effective polarizability[1] to include substrate screening effect at GW/BSE level. Our study provides important guidance on interpolation of experimental optical signature and manipulation of optoelectronic properties of single photon emitters in layered materials.

[1] C. Guo et al, Phys. Rev. B, 102, 205113 (2021)   

An integrated protocol for first-principles calculations of spin defects in silicon carbide

Point defects in wide band-gap semiconductors have shown great potential as spin qubits for quantum information technologies. Here we focus on silicon carbide—a commercially mature material— and on specific defects, the nitrogen vacancy complex, and the di-vacancy in hexagonal SiC. We present a series of calculations to predict multiple properties of these defects, including zero-phonon lines, spin Hamiltonian parameters for the calculations of coherence times [1], and charge transition processes in divacancies [2]. The latter are promising processes providing an avenue to realize single-shot readout via spin-to-charge conversion thus enabling the realization of numerous quantum protocols.

This work is supported by AFOSR FA9550-19-1-0358.

 

[1] Zhu, Y. et al, Phys. Rev. Materials 5, 074602 (2021).

[2] Anderson, C. et al, arXiv. 2110.01590 (2021).   

A solid state test bed for quantum time keeping and quantum weak values

In this work, we propose the use of the magnetoresistance phenomenon for quantum time keeping via a solid-state device realization of the Larmor clock. Specifying the spin dependent tunneling process as a generalized measurement, we describe how to utilize standard magnetoresistance measurements on this set up to distill the embedded information on the quantum weak values that describe the traversal time inside the barrier. Our results using the Keldysh non-equilibrium Green's function technique are synergetic with the Larmor clock in the coherent limit. We further analyze the proposed transport signatures and the resulting interpretations of the traversal time and the measurement back action in the limit of weak phase breaking decoherence effects.  Furthermore, in the limit of weak Zeeman fields, and a suitably engineered pre-selection, we demonstrate how to minimize the measurement back action. The device ideas that we suggest here can thus be extended to open up a spintronics test bed to investigate quantum weak value amplification and possibly facilitate the use of magnetoresistance effects in quantum metrology.    

Electronic structure and epitaxy of InSb-CdTe core-shell nanowires

Indium antimonide (InSb) nanowires are considered prime candidates for the emergent semiconducting-superconducting hybrids used for the topological Majorana particles research. A common concern in current nanowire devices is the overly strong superconducting-semiconducting coupling, which tends to overwhelm the intrinsic properties of the semiconducting nanowire. It has been proposed that adding a tunnel barrier at the nanowire-superconductor interface modulates this coupling.

Here, we add cadmium telluride (CdTe) shells around the InSb nanowires and investigate their suitability to serve as a tunnel barrier between a superconductor and InSb nanowires based on the electronic structure of the InSb-CdTe interface, the growth, and epitaxy of the CdTe on InSb. We use density functional theory to extract the individual bandgaps of InSb and CdTe as well as the band alignment at the interface. Moreover, we demonstrate that these insulating, lattice matched CdTe shells can be grown epitaxially on the InSb nanowires without interfacial strain or defects. The obtained defect-free and epitaxial interfaces along with the control over shell thickness we achieve indicate the potential of these CdTe shells to act as a control knob that tunes the superconducting-semiconducting coupling.   

Analysis of population dynamics and amplification of electron paramagnetic resonance signal of nitrogen-vacancy spins in diamond under laser illumination

Optical defects in diamond such as the negatively charged nitrogen vacancy centers are promising candidates for gain medium in the microwave region for continuous operation at room temperature owing to their ability of room temperature polarization and long spin lifetimes. Preferential polarization of its ground state, m_s = 0, sublevel with off-resonant laser illumination results in population inversion under suitable magnetic fields (>100mT). We will present our data on signal enhancement of continuous wave cavity detected EPR signals under low laser and microwave power conditions. We aim to discuss our quantitative analysis of this signal enhancement for the various EPR peaks in the spin system. The high field peak shows a sign flip verifies inversion which can be used to achieve low noise amplification and masing beyond a threshold pump power and cavity Q.