Session F49: Optically Active Spins - Photonic Integration and Emerging Materials

Interfacing solid-state quantum emitters into real-world photonic platforms

Solid-state quantum emitters have attracted much attention as integrated sources of photonic and spin qubits, which are basic elements for a range of quantum applications. Recent advances in the generation, manipulation, and integration of these emitters demonstrate a variety of quantum resources: bright quantum light sources, quantum memories, and spin-photon interfaces. In particular, integrating quantum emitters with photonic cavities or waveguides enables scalable quantum interactions between multiple photons and emitters. Given their high performance and scalability, quantum emitters are taking the next stages towards scalable, integrated quantum systems on photonic integrated circuits or fiber optics. Therefore, all quantum operations are efficiently possible in real-world photonic platforms. To achieve this goal, several technologies, including high-efficiency photonic interfaces, precise hybrid integration, and local frequency control, must be implemented in a practical photonic platform. In this talk, I present recent races and future challenges in scalable, integrated quantum photonics.   

Towards quantum technologies with VSi centers in c-plane silicon carbide

Optically active solid-state defects have enabled the pursuit of a wide range of quantum technologies. The V_Si center in silicon carbide is a promising next-generation candidate, boasting excellent optical- and spin coherence within the technologically mature silicon carbide platform. I will present our progress on studying the optical coherence and spectral stability of V_Si centers in samples diced from a commercially available, 4-inch c-plane silicon carbide wafer. To enhance the limited collection efficiency, we fabricate light-guiding nanopillars and work towards integrating V_Si centers in nanophotonic 'alligator' cavities, which are especially well-suited for the c-plane defect orientation. Enhancement of the V_Si center's optical properties in commercially available wafers might open up new opportunities for its use in large-scale quantum technologies.   

First-principles study of point defects and doping limits in CaO

The simple oxide CaO is a promising quantum-defect host due to its ultrawide band gap and scarcity of spinful nuclei in the crystal lattice. Here, we present results of HSE hybrid-functional calculations for intrinsic point defects in CaO. We discuss the electronic behavior of calcium and oxygen vacancies, calcium and oxygen interstitials, and calcium-oxygen antisites. We identify the dominant defects, and determine the Fermi-level pinning energies of CaO. Calcium and oxygen vacancies are found to be major electron and hole killers, respectively. Interestingly, we find that most band-gap regions of CaO are allowed to place the Fermi level, except those very close to the band edges. We have additionally investigated hydrogen impurities in CaO, considering that wide-band-gap oxides are prone to lack of control of doping due to presence of unintentional hydrogen impurities.   

High-throughput computationally-driven discovery and experimental realization of a new quantum defect in WS2

Point defects in semiconducting hosts have been proposed as the building blocks for future quantum technologies. Two dimensional hosts, including boron nitride and transition metal dichalogenides like WS₂ are especially appealing as they promise accessible surface for quantum sensing and long spin coherence time. However, identifying which points defects in a 2D host (e.g., WS₂) offer the most advantageous optoelectronic and spin properties is still unclear. This presentation will show searching for quantum defects in WS₂ can be accelerated through high-throughput computational screening. By constructing a database of over 1000 charged defects in WS₂, utilizing a combination of Density Functional Theory (DFT) and hybrid functionals, we pinpoint defects with the most promising characteristics. We will discuss the general trends in our dataset that will serve as a guideline for further computational and experimental work. Importantly, we will report on the synthesis and scanning tunneling microscopy and spectroscopy of one of our quantum defect candidates; demonstrating good agreement with the theoretical high-throughput prediction and confirming the discovery of an entirely new quantum defect with high potential for applications in WS₂.   

Symmetric carbon tetramers forming chemically stable spin qubits in hexagonal boron nitride

Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy related defects, such as the NV center in diamond and the VB− in hexagonal boron nitride (hBN), are at the forefront of high spatial resolution and low dimensional sensing. On the other hand, vacancies' reactive nature and instability at the surface limit further developments. Here, we present the symmetric carbon tetramers (C4) in hBN and propose them as a chemically stable spin qubit for sensing in low dimensions. We utilize periodic-DFT and many-body quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect, such as radiative and nonradiative lifetimes and photoluminescence spectra. We show that the nitrogen centered symmetric carbon tetramer gives rise to spin state dependent optical signals with strain sensitive intersystem crossing rates.   

Oral: Pseudo Jahn-Teller effect at the intrinsic defects in hexagonal boron nitride

Since the 2016 discovery of defect-based quantum emitters in hexagonal boron nitride (hBN), significant theoretical and experimental efforts have been directed towards identifying their chemical nature(s). Most of the theoretical works themselves focus on the high-symmetry reference structures for these defects and have not considered the possibility of the pseudo Jahn-Teller effect (PJTE), which arises from the vibronic coupling between the ground state and excited states. Using density functional theory-based calculations, we explore the effects of pseudo Jahn-Teller distortion on the structural, electronic and optical properties of intrinsic defects in hBN. Using intrinsic defects as prototype defects, we demonstrate the consequences of PJTE on the observable properties of the defects and show that one cannot rule out PJTE at the defects a priori.   

Tunneling spectroscopy of VOPc encapsulated in Van der Waals heterostructures

Small molecules have gained recent interest as qubit candidates due to their tunability via chemical modification. Molecular qubits have been studied extensively optically but electrical studies are hampered by the lack of a robust mechanism for integration into a solid-state architecture. In this work, we present a method for encapsulating small molecules, vanadyl phthalocyanine (VOPc), into a mechanically exfoliated heterostructure to generate a graphite/hBN /VOPc/hBN/graphite tunnel junction and probe the electronic states of the molecule via tunneling spectroscopy. We observe resonances in the tunneling spectra of these devices that quantitatively agree with spectra obtained via scanning tunneling microscopy (STM) of graphite/hBN/VOPc half-stacks. This result paves the way for further studies of the electrical and spin properties of many a wide variety of molecules, adatoms, and point defects encapsulated in 2D tunnel junctions.   

Microwave spin control of a van der Waals solid-state defect ensemble

Color centers in solid-state materials serve as the foundation for qubits that can be addressed optically, and spin-photon interfaces contribute to advancements in constructing quantum networks. Extensive research has focused on these defects in diamond, silicon, and silicon carbide. However, a burgeoning interest lies in investigating similar properties in emerging and innovative platforms.

In this study, we present findings concerning a specific group of defects found in a van der Waals material, specifically boron vacancies in hexagonal boron nitride (hBN). We utilize phonon sideband emission at approximately 850 nm as an effective optical readout method to examine the spin characteristics. We conduct a comparative analysis of spin qubits in naturally occurring hBN and isotopically purified hBN (B10 and N15). Our focus involves comprehensive mapping of the spin Hamiltonian as a function of the magnetic field as well as demonstrating coherent Rabi oscillations induced by microwave drive (~1-3 GHz). Additionally, we measure coherence times under different dynamical decoupling sequences, offering valuable insights into various sources of dephasing. Enhancing coherence represents a potential avenue for constructing high-precision quantum sensors and memories.   

Defect-assisted electron tunneling in 2D Van der Waals heterostructures

Defect-assisted resonant electron tunneling in hexagonal boron nitride (hBN) is demonstrated via electron transport measurements of graphite/hBN/graphite tunnel junctions. This study reveals distinct resonance peaks in the differential conductance (dI/dV) corresponding to native defect states in hBN. We investigate tunneling in devices fabricated from three sources of hBN and see substantial variation in defect density and electronic activity depending on the source and synthesis method. Further, hBN grown with excess carbon in the reaction chamber shows the emergence of sharp features typically associated with tunneling through point defects. These features are stable on sweep up/down and their temperature dependence reveals thermal broadening that allows for the inference of a zero-temperature inhomogeneously broadened linewidth of 9 meV. These results validate this modular platform as a tool for investigating tunneling spectroscopy of atomic defects in 2D materials and given recent interest in these systems as potential qubits, has a significant potential impact for emerging applications in quantum information.   

Nuclear Spin Polarization and Control in hexagonal boron nitride

Spin defects in van der Waals materials have emerged as important resources in condensed matter physics, spintronics, and quantum sensing. So far, nuclear spins in 2D materials have not been well studied. We report optical polarization and control of nuclear spins in a van der Waals material, hexagonal Boron Nitride (hBN), at room temperature. We show optical polarization characterization by means of ODMR spectra and show Rabi oscillations of the nuclear spins at the level anticrossing. Our work opens up new avenues for the manipulation of nuclear spins in van der Waals materials for quantum information science and technology.   

Spin relaxation due to Spin-phonon coupling in Molecular spin qubit

A promising molecular spin qubit, comprising a Lu(II) compound characterized by a large clock transition and an extended electron spin lifetime, has garnered considerable attention in recent years. A huge hyperfine interaction plays an important role in this spin qubit. We investigate how changes in the hyperfine interaction between the electron and the Lu nucleus, induced by phonon motion, impact spin relaxation. We employ first-principles calculations to obtain the variance in hyperfine interaction resulting from phonon motion. The phonon dispersion and modes are treated using the frozen phonon method. Subsequently, we solve the Redfield theory equation of motion in a second quantization framework to determine the electron spin relaxation time. Temperature dependence of the spin relaxation time is also obtained. The calculated relaxation time is in good agreement with experimental data at low temperatures.   

Computational study of all-electrical quantum operations on molecular spin qubits

We solve the problem of a single electron scattering from two decoupled spin-1/2 magnetic impurities with a single-particle Green's function treatment. We show that this treatment can map the scattering problem onto a quantum operation on the two impurities, and that for certain forms of the scattering potential this operation can be made unitary in order to realize two-qubit gates. We demonstrate that when the scattering potential is the sd exchange model, we can achieve a SWAP gate in a completely time-independent manner and without ever directly coupling the two impurities by appropriate location of a real-space barrier voltage term. We subsequently apply the same treatment to a SWAP^1/2 gate. We next examine whether even when the barrier location is fixed the momentum of the scattered electron can modulate the nature of the gate in a time-independent manner. We find that this more experimentally attainable scheme suffers from reduced gate fidelity. Finally, we explore beyond the single-particle picture by performing a many-body quantum mechanical simulation of our system of interest.