Symposium EQ01—Quantum Optical Materials and Devices Based on Impurity Systems

Rationally and precisely controlling the structures of two-dimensional (2D) layered materials allows us to tune the electronic structures and quantum states of matter, discover new physical properties, thus enable new applications. For example, tuning the twist angles between 2D materials results in the formation of moiré patterns and the manipulation of electronic states, which have led to novel quantum phenomena in 2D materials, including unconventional superconductivity, moiré excitons, and tunable Mott insulators. We have shown how the phase, layer stacking and thus physical properties of 2D MX₂ materials can be influenced by screw dislocations. The physical properties of topological insulator and magnetic 2D materials could also be manipulated by introducing topological defects such as screw dislocations. Moreover, I will discuss how to directly synthesize twisted 2D quantum materials with a rational control of interlayer twist angle by combining screw dislocation-driven crystal growth with non-Euclidean (curved) substrates. A general model describing the growth of 2D materials with screw dislocation spirals on non-Euclidean surfaces can lead to the rational realization of continuously twisted layered superstructures. We experimentally demonstrated the growth of twisted spirals of WS₂ and WSe₂ draped over nanoparticles near their centers and microscopic structural analysis confirmed the moiré superlattices between the atomic layers. Preliminary results from the collaborative studies of the emergent properties in these twisting MX₂ materials will also be discussed.

The quest of near-infrared (NIR) solid-state quantum emitters is one of main driving forces in the crossing of computational and experimental materials science and quantum optics (e.g., Ref. [1]). Solid-state quantum emitters in the wavelength region of 1.1-1.6 micrometers have a minimum absorption in living cells, thus would be ideal in vivo probes or biomarkers when they operate at room temperature. Furthermore, coherent emission from the single photon sources in the telecom bands at 1.3-1.6 micrometers could be directly applied for building metropolitan quantum networks through the existing infrastructure of optical fibers. Here we show the latest results about identification and characterization of NIR quantum emitters in diverse systems. We recently identified in a bioinert material, silicon carbide (SiC), room-temperature quantum emitters with high readout contrast associated with divacancy in stacking faults. Furthermore, we carried out in-depth analysis of the fine electronic structure of vanadium defect in SiC which is a promising telecom O-band quantum emitter. Here we mention that the scalability and deterministic creation of quantum emitters are still very challenging issues in SiC, and in general, in threedimensional materials. In twodimensional materials, techniques exist to generate atomic defects on demand. We proposed defects in hexagonal boron nitride (hBN) that can act as quantum bits that have been realized in experiments. However, deterministic creation of those defects is still not in reach. Very recently, activation of single defect spin in tungsten disulfide (WS₂) has been demonstrated at atomistic precision [2] which is a single carbon atom substituting sulfur atom. We show that the carbon defect spin can be initialized, optically readout and coherently manipulated with NIR emission. This result constitutes a scalable quantum bit in a twodimensional material that can be activated at atomistic precision. This work was supported by the Ministry of Innovation and Technology and the National Research, Development and Innovation Office of Hungary (NKFIH) within the Quantum Information National Laboratory of Hungary, the National Quantum Technology Program (NKFIH Grant no. 2017-1.2.1-NKP-2017-00001). A.G. acknowledges the National Excellence Program (NKFIH Grant no. KKP129866), the EU QuantERA project “Nanospin” (NKFIH Grant no. 127902) as well as the support of the European Commission within the Quantum Technology Flagship Project ASTERIQS (Grant no. 820394).
[1] Gang Zhang, Yuan Cheng, Jyh-Pin Chou, and Adam Gali, Applied Physics Reviews 7, 031308 (2020)
[2] Katherine A. Cochrane, Jun-Ho Lee, Christoph Kastl, Jonah B. Haber, Tianyi Zhang, Azimkhan Kozhakhmetov, Joshua A. Robinson, Mauricio Terrones, Jascha Repp, Jeffrey B. Neaton, Alexander Weber-Bargioni, Bruno Schuler, arXiv:2008.12196

The potential impact of active quantum emitters (impurity-based color centers) on creating quantum information systems in the near-term is evident ^1,2. Yet, design and control of robust quantum interfaces to these impurity systems at the nanoscale has remained challenging. In this talk, I will present promising physical mechanisms and device architectures for coupling emitters to other qubit platforms via dipole-, phonon-, and magnon-mediated interactions. I will present our latest work on coupling magnons to emitters as a natural step towards magnon-mediated efficient manipulation of spin-qubit states with applications in sensing and quantum information science. I will start with a scheme that uses magnetic nanoparticles that sustain antenna-like magnon resonances as nanomagnonic cavities for microwave magnetic fields. Here, in recent work we have shown that nanomagnonic cavities can modify the local magnetic environment of spin emitters in the microwave domain, facilitate the magnetic drive of spin transitions, and allow for strong coupling of these emitters with single magnons³. Next, I will discuss how the magnetic fields of nanomagnonic cavities that change spatially on the length scales of single molecular or defect emitters, coupled with descriptions of spin emitters beyond the point-dipole approximation, enables selection rule-breaking of orbital-spin transitions⁴. Such nanomagnonic cavities could pave the way towards magnon-based quantum networks and magnon-mediated quantum gates. Taking this further, I will present some of our recent work in capturing non-Markovian dynamics in open quantum systems (OQSs) built on the ensemble of Lindblad's trajectories approach ^5,6. In the outlook, I will discuss new approaches from quantum chemistry for OQSs that could guide the controllable coupling of active quantum emitters and create robust quantum interfaces to the nanoscale.

^1. Awschalom, D. et al. Development of Quantum Interconnects (QuICs) for Next-Generation Information Technologies. PRX Quantum 2, 017002 (2021).
^2. Head-Marsden, K., Flick, J., Ciccarino, C. J. & Narang, P. Quantum Information and Algorithms for Correlated Quantum Matter. Chem. Rev. (2020) doi:10.1021/acs.chemrev.0c00620.
^3. Neuman, T., Wang, D. S. & Narang, P. Nanomagnonic Cavities for Strong Spin-Magnon Coupling and Magnon-Mediated Spin-Spin Interactions. Phys. Rev. Lett. 125, 247702 (2020).
^4. Wang, D. S., Neuman, T. & Narang, P. Spin Emitters beyond the Point Dipole Approximation in Nanomagnonic Cavities. J. Phys. Chem. C 125, 6222–6228 (2021).
^5. Head-Marsden, K., Krastanov, S., Mazziotti, D. A. & Narang, P. Capturing non-Markovian dynamics on near-term quantum computers. Phys. Rev. Research 3, (2021).
^6. Krastanov, S. et al. Unboxing Quantum Black Box Models: Learning Non-Markovian Dynamics. arXiv [quant-ph] (2020).

Color centers in diamond are among the best performing optically accessible qubits in the solid state. Group IV-vacancy centers in particular can provide highly coherent and stable optical interfaces, a property which is attributed to their inversion symmetry. In this work we investigate the effect of an external electric field on the optical transition of a single tin-vacancy (SnV) center. Our study reveals a vanishingly small permanent electric dipole as well as a suppressed polarizability of the emitter, more than 4 orders of magnitude lower than for an NV center, demonstrating the inversion symmetry protection of a Group IV-vacancy defect in diamond from charge noise. Additionally, we show that by modulating the SnV electric-field-induced dipole we can use the emitter’s linewidth as a nanoscale probe to quantify electric field noise at its location. To gain further insight on the effect of the electric field noise, we also probe the optical transition at different timescales. This allows to resolve the individual spectral jumps of the transition, and to measure the homogeneous linewidth of the emitter without spectral diffusion effects.

Precise control over the electronic and optical properties of defect centers in solid-state materials is necessary for their applications as quantum sensors, transducers, memories, and emitters. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons. Specifically, we investigate three defect types---CH, CB-CB, and CB-VN---in monolayer hexagonal boron nitride (hBN). The lowest-lying electronic excitation of these systems is coupled to an optical cavity where we explore the strong light-matter coupling regime. For all defect systems, we show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. In addition, we find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single-photon emission from defect centers. Finally, we find that initially localized electronic transition densities can become delocalized across the entire material under strong light-matter coupling. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition for both pristine hBN and hBN with defect centers that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.

Recently, as atomically-thin transition-metal dichalcogenides (TMDCs) have advanced to the forefront of quantum information science research, quantum emitters (QEs) have been demonstrated in these materials. However, QEs discovered to date in 2D materials have their operating wavelength confined to the visible spectral range and the technologically important near-infrared (NIR) regime still remains an unconquered territory. Bringing 2D TMDCs into the research of NIR quantum photonics is highly motivated by the potential of creating on-demand telecom-compatible quantum light sources and the possibility of coupling the unique valley pseudo-spin qubits to photons propagating in optical fibers.

Meeting this challenge head on, here we report the first deterministic creation of QEs capable of high purity single photon emission at wavelengths covering entire NIR emission band from 1080 nm to 1550 nm. We achieved this by coupling mono and multi-layer molybdenum ditelluride (MoTe₂) to strain inducing nano-pillar arrays. Our Hanbury Brown and Twiss experiment yielded near-complete photon antibunching (g²(0) < 0.1), unambiguously proving the single-photon nature of the emitters. Polarization-resolved magneto-optical spectroscopies further revealed that while valley symmetry, which is essential for the valley pseudo spin based quantum information processing, is preserved in some QEs, the strain induced anisotropic exchange interaction mixed valley states in other QEs to display cross-linearly polarized doublets with ~1 meV splitting at zero magnetic field. The valley symmetry in these QEs is restored under 8T magnetic field. These findings together point to the exciting potential of accessing valley pseudo-spin through telecom single photons.

Overall our experiment extends the quantum optics research of 2D TMDCs into the technologically important telecom regime for the first time, and establishes layered MoTe₂ as a new platform for quantum communication and transduction research. In contrast to telecom quantum emitters of other material systems such as self-assembled quantum dots and single wall carbon nanotubes, our QEs offer unique opportunities in providing access to valley pseudo-spin degree of freedom and facile integration into quantum-photonic, electronic, and sensing platforms via the layer-by-layer-assembly approach.

Single spin states associated with point defects in solid-state systems are promising candidates for the realization of qubits and novel quantum spintronic devices with applications in communication, sensing, and information processing [1]. Isolated magnetic defects embedded in two-dimensional electron gases (2DEG) with spin-orbit coupling (SOC) can be electrically and optically addressed [2] and coherently manipulated.
We present a theory of circulating current induced by spin-defect in a two-dimensional electron gas (2DEG) with Bychkov-Rashba (α) and linear Dresselhaus (β) spin-orbit couplings [1,2]. We show that the spatial anisotropy dependency on the ratio of the strengths of the spin-orbit fields (α/β) offers a possibility to locally tune spin-orbit induced features of the defect such as the dissipationless circulating current associated with its ground state. The orbital contribution to the magnetization density is sensitive to the spin-orbit ratio, changing sign with respect to the spin magnetization and being null at α=β. The spatial structure of the stray field associated with the current is calculated for different 2D systems and provides a direct way to measure the host spin-orbit fields and the defect spin orientation through nanoscale magnetometry techniques such as NV-centers in diamonds [3].

* This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 721394 and support from US DOE BES Award No. DE-SC0016379.

[1] G. Wolfowicz, et.al. Nat. Rev. Mater., 1-20 (2021).
[2] R. C. Myers, et.al , Nat. Mater. 7 (3), 203-208 (2008).
[3] Bychkov, Y. A. and Rashba, E. I., J. Phys. C 17, 6039 (1984).
[4] Dresselhaus, G., Phys. Rev. 100, 580 (1955).
[5] Casola, F. et. al. Nat. Rev. Mater. 3 (1), 17088 (2018).

Due to the industrial maturity of the host crystal, defects in silicon carbide (SiC) have drawn considerable attention for quantum information and sensing applications. In particular, silicon vacancies in SiC have demonstrated impressive ground state spin properties such as optical spin initialization and readout. This has generated significant interest in room temperature sensing applications, which up until recently have been dominated by nitrogen vacancies in diamond. A key parameter in numerous sensing applications is the inhomogeneous dephasing time T₂^*, which thus far has been limited to a few 100ns in silicon vacancy ensembles. As the nuclear spin bath of the SiC is predicted to be the dominant source of dephasing, we perform isotopic purification of the host crystal leading to an order of magnitude improvement in T₂^*. Furthermore, the S=3/2 ground state quartet of the V_Si allows for a novel choice of basis with increased coherence due to a suppression of strain induced dephasing. A combination of these techniques leads to a nearly 2 order of magnitude improvement in the T₂^* of the defect ensemble even at high defect densities.

The nitrogen-vacancy (NV) defect centre in diamond is a promising candidate in spin-photonic applications such as quantum computing and sensing. The centre's spin-dependant fluorescence transitions allow for a purely optical initialisation and readout of the electron spin states, providing a powerful interface to control a solid state qubit. However, the technological application of the NV centre requires its integration into optical and electronic components requiring a highly accurate and high yield fabrication method.

Ultra-fast laser fabrication has demonstrated the ability to produce coherent NV centres with high accuracy and yield. Vacancies are produced through an initial high energy 300fs seed pulse in a process which drives electrons into the conduction band using multi-photon ionisation. The electrons form a plasma which is able to redeposit its energy into the lattice. Following this initial seed the processed site can be exposed to a low energy ultra-fast pulse train which allows the vacancy to migrate through the lattice without causing additional damage. The formation of an NV centre can be monitored using a confocal fluorescence microscope to observe the centre’s 627-800 nm fluorescence window. Fluorescent traces taking during laser annealing is shown to be used to monitor the creation of NV centres, whilst providing some information on their singularity and orientation. Additionally, we explore the effects of laser pulse parameters, such as pulse energy and width, on these traces.

Within, we use present an experimental and theoretical study of the laser fabrication process. Density Functional Theory (DFT) is used to investigate the interactions between the NV centre and the carbon self-interstitial involved in laser writing and report on simulations which suggest that the intermittent fluorescence spiking is caused by the NV's emission being suppressed by interactions between the two defects. In particular, we investigate simulated strain and electronic interactions between the two defects and the effect of these interactions on the NV centre's band structure and the specific excitation pathway that leads to NV emission.

Electronic interactions between configurations of the NV centre and <100>-split interstitial were simulated by generating all symmetrically unique second and third near neighbour ensembles of the two defects. Some configurations of the <100>-split interstitial and the NV centre demonstrate hybridisation between the defects in these bands. Out of the 48 total configurations simulated, 8 lead to reconstruction and 15 resulted in hybridisation. In such cases hybridisation occurs between one of the degenerate π-orbitals of the <100>-split interstitial and the e-manifold of the NV centre in the ground state, which is involved in fluorescence. This electronic hybridisation may lead to the creation of non-radiative pathways for fluorescence, which would quench the NV emission since the decay through these pathways is typically much faster than the lifetime of the NV centre excited state. Additionally it is demonstrated through a simple Monte-Carlo simulation that the migration of interstitials, around an NV centre, that can suppress fluorescence can lead to similar photon traces to those that are experimentally observed.

Color centers in diamond are widely explored as qubits in quantum networking and sensing due to their exceptional quantum coherence and straightforward optical interface. The further evolution of these quantum technologies will require the integration of diamond – i.e. the quantum state host – into complex device heterostructures, akin to the multi-material heterostructures widely utilized in CMOS technology. However, the material properties of diamond create fundamental difficulties for effective and efficient device integration. Despite recent progress developed to address this material challenge, a fully realized process to reproducibly generate and transfer single-crystal diamond thin films hosting coherent color centers remains outstanding. Here we demonstrate the robust and tunable synthesis of uniform diamond membranes with sufficient quality for advanced quantum technologies. These nanoscale-thick membranes are created based on the "smart cut" technique combined with isotopically (¹²C) purified overgrowth. The membranes have tunable thicknesses (demonstrated 50 nm to 250 nm) and can be deterministically transferred to other substrates via a "dry transfer" technique involving polydimethylsiloxane/polycarbonate (PDMS/PC) stamps. Moreover, the finished membranes have bilaterally atomically flat surfaces (Rq ≤ 0.3 nm) and bulk-diamond-like crystallinity. Furthermore, color centers are able to be synthesized via both implantation and incorporation during growth. Within 110 nm-thick membranes, individual germanium-vacancy (GeV^-) centers exhibit stable photoluminescence at 5.4 K, with average optical transition linewidths as low as 125 MHz. The room temperature spin coherence of individual nitrogen-vacancy (NV^-) centers shows Ramsey spin dephasing times (T₂*) and Hahn-echo times (T₂) as long as 100 μs and 300 μs, respectively. This membrane platform provides a straightforward and deterministic method to integrate diamond into device heterostructures for next-generation quantum technologies and has the potential to supplant bulk diamond in many other quantum applications.

*This work was primarily supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division with support from the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers. A. H. was partially supported by the University of Chicago Materials Research Science and Engineering Center, which is funded by the National Science Foundation under award number DMR-2011854. This work made use of the Pritzker Nanofabrication Facility part of the Pritzker School of Molecular Engineering at the University of Chicago, which receives support from Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), a node of the National Science Foundation’s National Nanotechnology Coordinated Infrastructure. This work also made use of the shared facilities at the University of Chicago Materials Research Science and Engineering Center, supported by the National Science Foundation under award number DMR-2011854. Funding was provided by the Boeing Company and the University of Chicago Joint Task Force Initiative. J. K. and A. B. acknowledge support from the NSF Graduate Research Fellowship under grant no. DGE-1746045.

Ideal quantum photonic chip requires single photon emitters, processors and detectors integrated into a single chip where the single photon sources must have higher degree of brightness, purity and indistinguishability [1]. This requires a precise location of emitting sources in pre-determined positions coupled to nano-photonic structures & nano-electronic devices. Defects in solid state host matrices are promising single photon sources considering their on-demand emission, scalability and room temperature operation [2]. Recently, silicon vacancy point defects in wide band gap material like SiC were demonstrated using focused ion beam (FIB) implantation [3,4]. However, the conversion yield of creating those single vacancy defects using FIB is less than 10%. In order to achieve fully deterministic creation of these single defects in SiC, we integrated our ion implantation system with an in-situ micro-photoluminescence setup sensitive enough for single defect optical characterization. By getting a feedback on the creation of defect in real time through checking the photoluminescence on the irradiated spot, while the sample is still inside the implantation chamber without annealing, we successfully fill an array of single defects following a loop of irradiation maintained at a lower dose level. Considering the nanometer spatial resolution and the versatile energy range of our FIB system, we believe that we can extend our capabilities to other lower dimensional materials and reproduce the precise defect creation onto nano-photonic structures & nano-electronic devices.

[1] Senellart, P., Solomon, G. & White, A. High-performance semiconductor quantum-dot single-photon sources. Nature Nanotech 12, 1026–1039 (2017)
[2] Aharonovich, I., Englund, D. & Toth, M. Solid-state single-photon emitters. Nature Photon 10, 631–641 (2016)
[3] Pavunny, S.P., Yeats, A.L., Banks, H.B. et al. Arrays of Si vacancies in 4H-SiC produced by focused Li ion beam implantation. Sci Rep 11, 3561 (2021)
[4] Wang, J., Zhang, X., Zhou, Y., et al. Scalable Fabrication of Single Silicon Vacancy Defect Arrays in Silicon Carbide Using Focused Ion Beam. ACS Photonics 4, 1054 (2017)

Understanding the spatio-temporal carrier dynamics in semiconductors, especially III-V materials, is critical for improving electronic and optical devices. Here, we develop a new technique, scanning ultrafast electron microscopy (SUEM), that resolves transient carrier densities and surface-sensitive potentials with 15 ps temporal and 100 nm spatial resolution. SUEM is used to quantify the surface defect density and the recombination of surface trap states in III-V semiconductors, n-GaAs and p-InAs: two parameters which often limit electro-optical device performance. The high temporal resolution probing of transient carrier densities reveals photosaturation at high laser fluences (0.2 and 8 μJ/cm², for p-InAs and n-GaAs, respectively) due to surface band flattening. Surprisingly, our SUEM signal is more sensitive to the local surface potential and the potential in the vacuum at short distances above the surface as opposed to the conduction band electron density itself. Our novel interpretation of SUEM contrast mechanisms opens doors for the carrier dynamics involved in the practical design of defect-driven microelectronic devices.

Photogenerated molecular spin systems hold great promise for applications in quantum information science because they can be prepared in well-defined spin states at modest temperatures, often exhibit long coherence times, and their properties can be tuned by chemical synthesis. We will discuss molecular spin systems composed of organic chromophores such as porphyrins and perylenediimides (PDIs) covalently linked to stable organic radicals. Following photoexcitation of the chromophore, femtosecond optical spectroscopy shows that the exchange interaction between the electrons on the chromophore and the unpaired electron spin of the radical results in quartet state formation in >10¹¹ s^-1. The linkers between the chromophores and the radicals are used to control the spin-spin exchange interaction between the chromophore and the stable radical. Time-resolved pulsed electron paramagnetic resonance experiments show that the resulting quartet state is strongly spin-polarized, having spin lattice relaxation times > 100 μs and relatively long coherence times of ~2-4 μs at temperatures as high as 80 K. Using microwave pulses to manipulate the quartet spin sublevel populations is accompanied by chromophore optical changes. These systems function in a manner analogous to defect centers in solids, such as NV centers, because they can be optically pumped to a highly spin-polarized state, manipulated using microwave pulses, and read out by monitoring optical changes making them viable for quantum information studies.

Single-molecule magnets are compounds of transition metals or lanthanides that display slow relaxation of magnetization in the absence of a static magnetic field. SMMs may also show magnetic hysteresis and, hence, magnetic bistability, which has led to proposals that SMMs could be used in magnetic information storage. Significantly, the properties of SMM originate from within individual molecules with dimension of less than 1 nm rather than from cooperative interactions across much larger magnetic domains.

The problem with SMMs is that their magnetic bistability is typically only observed with liquid helium cooling. Recently, we reported the first SMM to function above the boiling point of liquid nitrogen. Molecules of this SMM consist of a single dysprosium atom sandwiched between two organic (i.e. cyclopentadienyl) rings, a structural arrangement that creates excellent conditions for addressing the electronic structure of the lanthanide.

In this presentation, new results describing the impact of incorporating phosphorus and arsenic atoms into the cyclopentadienyl rings on the SMM properties will be described.

Three-dimensional epitaxially fused quantum dots superlattices are theoretically predicted to exhibit band-like charge transport behavior while still retaining tunable semiconductor properties, making them promising candidates for future energy materials. Realization of the predicted emergent electronic properties has remained due in part to defective interdot epitaxial connections. These defects include edge dislocation, screw dislocations and stacking fault that contribute to the tilting and rotational misalignment between the atomic lattices of neighboring connected dots. Existing imaging techniques such as Scanning Transmission Electron Microscopy allow for direct observation of local attachment in atomic scale and characterizing two-vector-based local atomic orientations with the help of scanning diffraction techniques like the 4D-STEM. However, it is still not fully understood enough on how a collective of these geometric frustration in the atomic scale will affect the structural regularity of the local or even long-range superlattice structure variations, hence, preventing our access to defect-free superlattice attachment, especially for a three-dimensional film. Here, we present the combination of using full-tilt HAADF Electron Tomography with Geometric phase analysis in 3D to characterize and understand structural features spanning two orders (0.1 ~ 10 nm) of magnitude in spatial length scale throughout a multi-layer superlattice film. We apply 3D windowed diffraction calculations based on the tomogram to extract phase information of desired lattice planes, with which allow us to obtain in 3D the local atomic lattice orientations, vectorial displacement field, local strain tensor field and dislocation lines that propagate in-between particles. With this vast amount of information of atomic structural details, quantitative analysis can be conducted on thousands of quantum dot pairs to understand the relation between atomic dislocations and QD orientational misalignment as well as how these geometric frustrations are reflected in strain field. The insights we gained from this particular QD SL material promises a step toward minimizing structural fault in the oriental attachment and hence further approaching the goal of charge delocalization. We also believe that the analytical method we present here provides a novel way of comprehensively understanding structural variations with multi-dimensional quantifications, which we expect, to become increasing important in the rise of complex materials.

Colloidal quantum dot (CQD) superlattices with semiconductor properties promise many exciting properties for organic opto-electronic devices including the delocalization of electronic bands with tunable properties. The scaling up of these devices would allow low-cost high-throughput methods to make organic photovoltaics and other opto-electronic devices but this is only possible through higher quality processing methods that achieve these delocalized band structures. Current characterization methods like transmission electron microscopy (TEM) and grazing-incident small angle x-ray scattering (GISAXS) do not probe the exact the structure of the superlattice in three dimensions. We have applied novel data analysis techniques to tomographic reconstruction data from STEM images to determine the center of mass of each quantum dot, the number of necks each of the PbSe quantum dots has to its nearest neighbors, the thickness of the necking, the in-plane and out-of-plane misorientation angles, and local strain measurements in the CQD superlattice. This presentation will highlight data used and its applications in improving the superlattice structure to achieve next-generation opto-electronic properties.

Optically addressable spins in solids are an area of intense research interest in quantum information science with applications in quantum networks, computing and sensing. Due to recent advances in nanophotonic cavity design and fabrication, detection and control of single rare-earth ions via their highly coherent 4f-4f optical transitions have recently been demonstrated. We focus on the 171 isotope of Yb doped into a YVO4 crystal due to low electric and magnetic field noise sensitivity facilitated by no 1st-order DC stark shift combined with optical and spin clock transitions. In this talk I will introduce this platform by characterizing the optical and spin transitions. Subsequently, we leverage the high-fidelity control and long coherence times to explore the interaction of single ¹⁷¹Yb ions with a small local ensemble of vanadium lattice spins. We propose using collective modes of this ensemble as a secondary quantum register, a key resource for building quantum repeaters.

We discuss recent progress in investigating the electronic structure [1] and coherence properties [2] of spin defects in three- and two-dimensional materials using first principles electronic structure calculations (DFT and many body perturbation theory), and spin Hamiltonians [3]. In particular we will present results for defects in diamond, SiC, and MoS₂.

Supported by AFOSR and DOE-BES (MICCoM)

[1] H. Ma, M. Govoni and G. Galli, npj, Comput. Mat., 6 (85), (2020); H. Ma, N. Sheng, M. Govoni and G. Galli
J. Chem. Theory Comput., 17, 2116-2125 (2021).Yu Jin et al. 2021 arXiv:2106.08608
[2] M. Onizhuk and G. Galli, Appl. Phys. Lett., 118, 154003 (2021); M. Onizhuk et al.
PRX Quantum, 2, 010311 (2021)A. Bourassa et al., Nat. Mat. 2020.
[3] K. Ghosh, H. Ma, V. Gavini and G. Galli, Phys. Rev. Mat. 3, 043801 (2019).

Optically addressable spins in materials are important platforms for quantum technologies, such as repeaters and sensors [1]. Identification of such systems in two-dimensional (2d) layered materials offers advantages over their bulk counterparts, as their reduced dimensionality enables more feasible on-chip integration into devices.

Hexagonal boron nitride (hBN) is a two-dimensional van der Waals crystal that was recently shown to host a plethora of defects that display photoluminescence (PL) spectra ranging from 580 nm to 800 nm. Here, we show that previously identified [2] carbon-related single defects in 2d hexagonal boron nitride (hBN) show ODMR at room temperature [3].

We show that single-defect ODMR contrast is up to 100x stronger than that of hBN ensembles and displays a magnetic-field dependence with positive or negative sign per defect. By measuring hundreds of defects we can identify a yield of spin active defects of ~5%. Further, the ODMR lineshape comprises a doublet resonance, indicating a S=1 state with low but finite zero-field splitting. Our results offer a promising route towards realising a room-temperature spin-photon quantum interface in hexagonal boron nitride.

[1] Awschalom, D. D., Hanson, R., Wrachtrup, J. & Zhou, B. B. Quantum technologies with optically interfaced solid-state spins. Nat. Photonics. 12, 516–527 (2018).
[2] Mendelson, N., Chugh, D., Reimers, J.R. et al. Identifying carbon as the source of visible single-photon emission from hexagonal boron nitride. Nat. Mater. 20, 321–328 (2021).
[3] Stern, H.L., Jarman, J., Gu, Q., Barker, S.E., Mendelson, N., Chugh, D., Schott, S., Tan, H.H., Sirringhaus, H., Aharonovich, I., Atature, M. Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride. Under review, arXiv preprint: 2103.16494 (2021).

The realization of integrated quantum optical circuits (QOCs) to enable single and high dimensional entangled photons for quantum communication, robust quantum teleportation and quantum sensing with quantum limited resolution has been a long-sought goal in the field of quantum information processing (QIP). All these applications explore the effect of quantum interference and entanglement between different on-demand single photon sources (SPSs) in a scalable architecture. The lack of a starting ordered array of SPSs so far has stood as a major obstacle to realizing compact quantum information technologies.

In this talk I will present our proposition and continued work on demonstration of such a starting platform of SPS arrays based on a unique class of spatially-ordered arrays of surface-curvature driven mesa-top single quantum dots (MTSQDs) [1-3]. These AlGaAs/InGaAs MTSQDs exhibit spectral uniformity as low as 1.8nm across 5×8 arrays including clusters of 6 as-grown QDs having emission energy within 250μeV (within on-chip electrical tuning range) and single photon purity > 99.5%. For the usage of this unique class of ordered spectrally uniform MTSQD SPSs to create interconnection between different SPSs for quantum circuits or networks via monolithic or hybrid integration, we also demonstrate planarization of the MTSQDs without compromising their spectral uniformity and single photon emission characteristics [3]. The highly pure single photons from such planarized MTSQD are also indistinguishable with as-measured indistinguishability of 0.5±0.12 at 18.5k (without Purcell effect), limited by the spectral diffusion and phonon induced dephasing at this elevated based temperature of 18.5K of our cryostat at hand. Potential unity indistinguishability can be reached at low temperatures ~4K. Such planarized MTSQDs can have single photon emission from 700-1300nm using our AlGaAs/InGaAs material system that can be further extended to 1550nm using Phosphorus based material system to cover the full range of wavelength needed for quantum communication, teleportation, and sensing in space.

Such planarized ordered array of highly spectrally uniform SPSs generating highly pure and indistinguishable photons offers the long-sought platform for on-chip deterministic integration with light manipulating structure using either hybrid, conventional 2D PhC or dielectric metastructures [4,5] to realize QOCs generating needed multiphoton entangled states for the aforementioned applications. Work on integration of MTSQD with light manipulating units and study of photon interference/entanglement is undergoing.

The work is supported by the US Army Research Office (W911NF1910025) and the Air Force Office of Scientific Research (FA9550-17-1-0353).

[1] Jiefei Zhang et al., Jour. App. Phys. 120, 243103 (2016)
[2] Jiefei Zhang, et al., App. Phys. Lett 114, 071102 (2019)
[3] J. Zhang, et al., APL photonics 5, 116106 (2020)
[4] S. Chattaraj and A. Madhukar, Jour. Opt. Soc. America B. 33, 2414 (2016).
[5] S. Chattaraj, et al., IEEE Jour. Quant. Elec. 56, 1 (2019).

Single-photon emitters (SPEs) associated with point-defects and ions in semiconductors are currently considered a significant resource for the solid-state implementation of quantum repeaters and fiber-based long-distance quantum networks, and interconnecting quantum computers. Specifically, non-classical single-photon light sources emitting in the near-infrared region of the electromagnetic spectrum around 1.5 μm, falling in the lowest loss wavelength range of fiber optics networks, are critical chip-scale building components for the development of fiber-based quantum networks. The realization of scalable on-chip quantum devices, such as single-photon sources and quantum memories, requires novel nanostructured materials that must be compatible and can be integrated with existing electronic circuits, waveguide architectures, and current chip-scale and silicon process technology. We present nanophotonic structures composed of arrays of silicon carbide (SiC) nanowires (NWs) based on a novel and fab-compatible nanofabrication process. Furthermore, these NW-based structures enable the positioning of erbium (Er³⁺) ions with an accuracy of 10 nm, an improvement on the current state-of-the-art ion implantation processes, and are pivotal in engineering the Er³⁺-induced 1.54 μm emission. An approximately 22-fold increase of the room-temperature Er³⁺ PL emission is observed in these optical nanostructures compared to their thin-film analog at saturation, while using 20 times lower pumping power. These nanostructures demonstrate broadband and efficient excitation characteristics for Er³⁺, with an absorption cross-section (~2×10⁻¹⁸ cm²) two-order larger than typical benchmark values for direct absorption in rare-earth-doped quantum materials. Additionally, we have explored the modeling foundations for plasmonic cavity engineering of the Er³⁺ emission to significantly enhance its spontaneous emission rate. Finally, we benchmark and optimize our state-of-the-art near-infrared single-photon detector system to study single-photon behavior from these Er:SiC NW telecom nanophotonic structures.

For various applications in the field of quantum information processing stationary qubits are required, providing long-lived spin coherence and suitable level schemes for coherent control and optical read out. In addition, transferring the spin information to indistinguishable single photons is necessary e.g. to distribute entanglement in quantum networks. Similarly, quantum computing with linear optics (LOQC) inherently relies on bright light-matter interfaces that provide single indistinguishable photons.
Color centers in diamond, more specifically the group-IV-vacancy centers, have emerged as promising candidates among solid state qubits. They exhibit favorable features such as individually addressable spins with long coherence times and bright emission of single, close to transform limited photons. While for silicon-vacancy centers (SiV) long spin coherence times are only achieved at milliKelvin temperatures, recent experiments have shown that the negatively charged tin-vacancy center (SnV) [1] overcomes this drawback. Its large ground state orbital splitting results in the suppression of phonon induced dephasing processes of the spin states, allowing long spin coherence times at significantly higher temperatures (>1K) [2]. Among its good spectral qualities such as bright single photon emission and transform limited linewidths [3,4] the indistinguishability of the emitted single photons remains a missing cornerstone to be demonstrated.
By means of Hong-Ou-Mandel interferometry we here investigate the indistinguishability of single photons consecutively emitted by an off-resonantly excited single SnV-center. We find high Hong-Ou-Mandel visibilities, being a direct measure for high indistinguishability of the single photons. We compare the experimental results with the predictions of a theoretical model [5] and extract the magnitude of spectral diffusion potentially affecting single photon indistinguishability in the present system. Furthermore we estimate the timescale of spectral diffusion by repeating the experiment with various delays between emissions of the interfering photons.

[1] Iwasaki T. et al., Phys. Rev. Lett. 119, 253601 (2017).
[2] Debroux R. et al., arXiv:2106.00723 (2021).
[3] Trusheim M. E. et al., Phys. Rev. Lett. 124, 023602 (2020).
[4] Görlitz J. et al., New J. Phys. 22, 013048 (2020).
[5] Kambs B. and Becher C., New J. Phys. 20, 115003 (2018).
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On-chip quantum optical circuits (QOCs) comprising ordered single photon sources (SPSs) coupled to network of light manipulating elements (LMEs) that allow manipulation of the emitted photons in a single or few photon regime to enable on-chip photon interference and photon mediated emitter-emitter entanglement is a major goal for scalable quantum information processing (QIP). The path to realization of such QOCs is recently enabled by the new class of spectrally uniform on-chip integrable mesa top single quantum dot (MTSQD) [1-3] single photon sources that provide a platform for indistinguishable photon emission with >99% purity from ordered and spectrally uniform QDs that are also integrable in the monolithic or hybrid integration techniques for codesign with LMEs to interconnect different SPSs. The LMEs must provide the needed light manipulating functions of (1) enhancement of emission rate of the emitters, (2) enhanced emission directionality (3) on-chip propagation, (4) splitting and (5) recombining of single photons to enable efficient extraction of photon, photon interference and photon mediated emitter-emitter coupling in a horizontal architecture for quantum entanglement.
The conventional approach to implementation of such LMEs has been photonic crystal and ridge waveguides-based cavities and waveguides that demand mode-matching between components and hinders scalable implementation of all the above functions. In contrast, we have recently introduced [4,5] the approach of Mie resonance of subwavelength scale dielectric building blocks (DBBs) that exploits a single collective mode of the entire circuit to provide all the five above mentioned functions- thus eliminating mode mismatch between the components. We have demonstrated via finite element method-based simulations of the EM fields [4] that such Mie resonance based DBB metastructures can provide simultaneously a Purcell enhancement ~5-10, emission directionality resulting in 50% photon coupling to the Mie mode, and at the same time propagation, splitting and combining using the same collective magnetic dipole mode of the DBB array- thus enabling realization of quantum optical circuits without any impedance mismatch.
Importantly, the Mie mode of such MTSQD-DBB quantum optical circuits allow coupling between distant emitters- spontaneously resulting in emitter-emitter entanglement over large on-chip distance, critical for applications in quantum metrology, sensing and computation. We present theoretical work to understand the emergence of emitter-emitter entanglement of a system of two MTSQDs coupled via the collective magnetic dipole mode of a DBB based back-to-back nanoantenna waveguide unit [4]. We show a ~1.7 times faster decay rate of the coupled emitter system compared to single emitter- a signature of super radiance. Owing to the lossless propagating nature of the Mie mode, the super-radiant state can be formed over large ~10-100 μm on-chip distances. The emergence of such super radiant states has been explored by employing Von-Neumann Lindblad approach to account for the coherent and incoherent processes involved in the general Hilbert space involving multiple excitons and photons. Further, using the same approach, emergence of a positive degree of entanglement between the distant emitters is shown starting from a purely product state. Spontaneous creation of entanglement between distant on-chip emitters in such MTSQD-DBB integrated optical circuits provides a way to realize architectures for quantum metrology and computation.
Further work on optical studies on such DBB based light manipulating structure is ongoing.
We acknowledge support by ARO Grant (W911NF-19-1-0025) and AFOSR Grant (FA9550-17-01-0353).

1. J.Zhang et.al, J.Appl.Phys.120,243103(2016)
2. J.Zhang et. al, App. Phys. Lett. 114, 071102(2019)
3. J.Zhang, et. al, APL photonics 5, 116106 (2020)
4. S.Chattaraj, et. al, IEEE J.Quant.Elec. 56, 1, 1(2019)
5. S.Chattaraj, et. al, J.Opt.Soc.Am.B. 33, 12(2016)

Topological quantum computation based on Majorana bound states may enable new paths to fault-tolerant quantum computing. Several recent experiments have suggested that the vortex cores of topological superconductors, such as iron-based superconductors, may host Majorana bound states at zero energy. \textcolor{red}{However, quantum computation with these zero-energy vortex bound states requires a precise and fast manipulation of individual vortices which is difficult to do in a scalable manner. To address this issue, we propose a control scheme based on local heating via, for example, scanning optical microscopy to braid vortex-bound Majorana zero modes in a two-dimensional topological superconductor. First, we derive the conditions required for transporting a single vortex between two defects in the superconducting material by trapping it with a hot spot generated by local optical heating. Equipped with critical conditions for the vortex motion, we then establish the ideal material properties for vortex braiding and describe how transition errors resulting from finite speed and/or temperature can be minimized. Our work paves the way toward optical or microscopic control of zero-energy vortex bound states in two-dimensional topological superconductors. *This work has been conducted by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Solid state quantum light sources are emerging as promising candidates for many applications in quantum technologies [1-3]. Among these sources, optically active point defects in hexagonal boron nitride (hBN) are attracting considerable attention due to their extreme brightness, and high Debye Waller factor which means the majority of the photons are emitted into the zero phonon line (ZPL) . While final defect assignments are still under debate , a number of recent experiments and theoretical papers hint at carbon related defects adjacent to a vacancy site in the hBN lattice [4,5]. In addition, numerous recent studies have shown that several defects in hBN exhibit spin dependent optical transitions, and exhibit optically detected magnetic resonance (ODMR), which is vital for their employment as solid state qubits and quantum sensors at the nano-scale [6,7].
For practical quantum photonic applications, where photon interference and generations of indistinguishable photons are required , it is important to characterize the coherent properties of the emitted photons. Specifically, studies of dephasing mechanisms, coherence and line broadening effects underpin the applicability of quantum emitters for photon interference experiments. Previous studies of hBN quantum emitters have revealed the emissions in hBN are broadly affected by spectral diffusion. Preliminary resonant excitation experiments showed that observation of Fourier Transform limited lines is possible, but rather rare, as compared to other solid state emitters, such as diamond [28,29]. Some of the challenges stemmed from the fact that the level structure of the emitters is still poorly understood, and environmental effects in layered materials are strongly sample-dependent.
In this presentation, we will report on spectroscopy of hBN defects at cryogenic temperature to study dephasing mechanism of quantum emitters in details. Importantly, our work focuses predominantly on coherent excitation (i.e. the excitation laser is on-resonance with the hBN emission). First we characterize the significant dephasing and spectral broadening mechanisms in an hBN single photon emitter under resonant excitation. We find that the resonant linewidth, even at cryogenic temperature, is dominated by phonon broadening and results in linewidths of ~ 1 GHz. We also see that degenerate electronic states and strain do not play a significant role in phonon dephasing. We further showed that spectral diffusion can be minimized by employing excitation powers well below saturation. Overall, the brightness of the emitters exemplify that emission rate in excess of 200 MHz with bandwidth of less than 1 GHz [8]. Next we employed a weak non-resonant laser at a wavelength between 500 nm and 532 nm, in addition to the resonant laser that drives the system coherently to stabilise the optical transition of single defects in hBN. The secondary laser acts as an additional excitation pathway and re-initializes the system into its bright state. The two laser repumping scheme enables the observation of brighter resonant fluorescence [9]. We provide an in-depth analysis of the photodynamics of the system, and discuss its applicability for an improved coherence. Our results provide an important analysis and required modification for future experiments on two photon interference with quantum emitters in hBN.

[1] I. Aharonovich, et al., Nat. Photonics 10, 631-641, (2016).
[2] A. W. Elshaari, et al., Nat. Photonics, (2020).
[3] M. Atatüre, et al., Nature Reviews Materials 3, 38-51, (2018).
[4] N. Mendelson, et al, Nature Materials 20, 321-328, (2021).
[5] C. Jara, et al, The Journal of Physical Chemistry A 125, 1325-1335, (2021).
[6] A. Gottscholl, et al., Nature Mater. 19, 540-545, (2020).
[7] N. Chejanovsky, et al., Nature Materials, (2021).
[8] S. J. White, et al., https://arxiv.org/abs/2105.11687 (2021).
[9] S. J. U. White, et al., Physical Review Applied 14, 044017, (2020).

Single photon emitters are fundamental resources of quantum optics and quantum information technologies. Recently, the emergence of single photon emission in atomically thin materials such as hexagonal boron nitride (h-BN) has triggered tremendous interests in 2D material based single photon sources. For full exploitation of 2D single photon emitters for quantum technologies, however, the ability to control each atomic defect individually is critical. In this talk, we introduce methods to generate and manipulate single photon sources in 2D h-BN crystals using local crystalline geometries and electrical controls. First, we show the role of strain for the creation and orientation of the single photon emitters in h-BN. Second, we demonstrate that the photon energy of single photon sources in h-BN can be controlled using an electric field by fabricating 2D van der Waals heterostructures composed of h-BN and graphene layers. A diverse trail of Stark shifts is observed, providing information on crystallographic ground states of defect structures. Lastly, we will discuss the electrical switching of the quantum emitters enabled by the Fermi level tuning of graphene electrodes.

Hexagonal boron nitride (h-BN) has been recently found to host a variety of quantum point defects, which are promising candidates as single-photon sources for solid-state quantum nanophotonics applications. Most recently, optically addressable spin qubits in h-BN have been the focus of intensive research due to their unique potential in quantum computing, communication, and sensing. However, the number of high-symmetry high-spin defects that are desirable for developing innovative spin qubits in h-BN is highly limited. Here, we combine density functional theory (DFT) and quantum embedding theories (QET) to show that out-of-plane X_NY_i dimer defects (X, Y=C, N, P, Si) form a new class of stable C_3v spin-triplet defects in h-BN. We find that the dimer defects have a robust ³A₂ ground state and ³E excited state, both of which are isolated from the h-BN bulk states. We show that ¹E and ¹A shelving states exist and they are positioned between the ³E and ³A₂ states for all the dimer defects considered in this study. To support future experimental identification of the X_NY_i dimer defects, we provide an extensive characterization of the defects in terms of their spin and optical properties. We predict that the zero-phonon line of the spin-triplet X_NY_i defects lies between the infrared (~1.0 eV) and the visible (~1.8 eV) range. We compute the zero-field splitting of the dimers’ spin to range from 1.79 GHz (Si_NP_i⁰) to 29.5 GHz (C_NN_i⁰). Our results broaden the palette of high-spin defect candidates that would be useful for the development of spin-based quantum technologies in two-dimensional hexagonal boron nitride.

Colour centres in two-dimensional hexagonal boron nitride (hBN) have emerged as a promising platform for quantum photonic applications, as they emit single photons at room temperature and can be readily integrated into nanoscale devices thanks to their reduced dimensionality. Recently, we showed the first demonstration of optically-detected magnetic resonance (ODMR) from single spin defects in a van der Waals material at room temperature,¹ with ODMR contrasts of up to 30%, demonstrating the potential of hBN for applications in quantum information and quantum networks. Interestingly, single defects with similar spectral properties can show negative, positive or no ODMR contrast, indicating rich photo- and spin dynamics in the material. Integration of these colour centres into devices such as cavities to enhance emission or improve collection efficiencies will lead to a stronger platform to investigate and exploit their fundamental physics, and to evaluate their potential as solid-state qubits or nanoscale quantum sensors. In this contribution, we present our latest results towards integrating these individually optically addressable spin defects into tunable devices to enable a room-temperature single spin-photon interface in a two-dimensional material.

¹ H. L. Stern, J. Jarman, Q. Gu, S. Eizagirre Barker, N. Mendelson, D. Chugh, S. Schott, H. H. Tan, H. Sirringhaus, I. Aharonovich, M. Atatüre Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride (2021) arxiv:2103.16494

Quantum technologies are attracting attention worldwide due to their projected benefits over existing technologies. Modern devices such as computers, communication devices, and sensors could benefit from quantum technologies because quantum devices will have the ability to collect data at a precise accuracy and a record-breaking speed. One of the quantum technologies is the NV center in CVD diamond. In this technology, NV centers, or nitrogen vacancy centers, occur when two carbon atoms are removed from a diamond and replaced with a single nitrogen atom that has a vacancy next to it. When a sufficiently high energy photon (a green light (532nm) ) is directed on diamond with an NV center, the diamond re-emits a red light (665nm), and the strength of the re-emitted red light depends on the spin of the NV center. The spin of the NV center can be altered by exciting a microwave at a specific frequency, resulting in the strength of the red light emitted by the diamond to change. The ability to measure the change of strength of the light re-emitted by NV centers is often used in supercomputers, radars, and other quantum technologies. NV centers can also be used for a single photon emitter. Most importantly, NV centers can be formed in lab grown CVD diamonds by generating a plasma in a Microwave Chemical Vapor Deposition (microwave CVD). Hydrogen, nitrogen, and methane gases can be flowed into the microwave CVD and nitrogen can be introduced into the growing diamond structures to create the NV centers. Using this method, we have grown CVD diamond films with varying concentrations of NV centers. The effect of various process parameters such as deposition power, gas ratios, substrate temperatures, and growth pressure have been studied on NV center formation, density, quality, and in-situ doping of nitrogen. Quantum structures have been characterized using Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. In-situ laser reflectivity was also implemented to investigate growth characteristics of CVD diamond layers. In this paper, we will discuss the dependence of process conditions and characterization of NV centers and highlight potential applications and sensor development.

One of the grand challenges in imaging defects in 2D materials is a lack of spatial structures to resolve, which complicates the interpretation of the resolution in our system. In addition, there is still very little control over the type of defects. One approach taken in diffraction-limited PL imaging exploited the strain imposed by nanopillar arrays that were spaced at least 6 microns apart. To be able to characterize better the existing SMLM setup, we will use variable spacing arrays. Currently, to induce defects, we still treat pristine exfoliated material with Oxygen, Nitrogen or Hydrogen plasma typically, obtaining defects at a random location. To address the main challenge of engineering defects deterministically at various densities, Focus ion beam (FIB), naimley electrically discharged, Xe⁺ of several kV that will be focused on the sample, allowing the creation of the defects in a predefined location. Given the spatial limitations linked to the substrate charging and instrument drift, achieving sub 100 nm spacing of the defects stil presents a challenge.

We present laser synthesis and processing of novel quantum nanostructures of diamond related materials. With our recent breakthrough in nonequilibrium laser processing, we have shown than amorphous carbon layers can be converted into phase-pure graphene, diamond or Q-carbon (new allotrope of carbon) by increasing the undercooling before melt quenching. Carbon layers can be converted into graphene, graphene oxide or reduced graphene oxide. Parallel results are obtained for the conversion of amorphous BN into phase-pure h-BN, c-BN or Q-BN. We will focus here on nanodiamonds, epitaxial thin films, and nanoneedles, which can be grown epitaxially by domain matching epitaxy. Through epitaxial growth, nanodiamonds are grown with a controlled orientation, such as <111> on <0001> sapphire. These nanodiamonds and nanoneedles can be doped with single NV or SiV centers, in addition to higher concentrations in macrodiamonds and thin films. Pure (undoped) Q-carbon is ferromagnetic, and it becomes superconducting upon doping with boron with record BCS transition temperature of over 57K so far. We discuss the properties and applications of these graphene, diamond and Q-carbon related nanostructures with a wafer-scale integration in nanosensing, quantum computing and quantum communication.

Optically addressable point-defects in wide band-gap semiconductors are a versatile platform for quantum information science (QIS). Such qubits are intrinsically sensitive to their local crystalline, charge, and nuclear environments. These variations directly affect the coherence, charge stability, and the optical transition frequency of the defects, limiting their use as scalable platforms for nanoscale sensing and quantum communication. Interestingly, with recent advances in host growth, nanofabrication, and defect synthesis, these environments can be engineered via control of crystal structure, isotopic purity, and host dimensionality. Herein, we will present work on the optimization of these defect systems via isotopically controlled synthesis and localization of various defect systems in silicon carbine and diamond. An emphasis will be placed on exploring how advanced, synchrotron-based characterization techniques provide a direct means to better understand the local crystal environment surrounding the defects and offers a pathway for improving the defect-based quantum technologies. Finally, recent advances in synthesising low-dimensionality (nano-scale particles and membranes) defect hosts for deterministic integration with other classical or quantum systems will be presented.

Color center is a promising platform for quantum technologies, but their application is hindered by the typically random defect distribution and complex mesoscopic environment. Employing cathodoluminescence, we here demonstrate that an ultraviolet-emitting single photon emitter can be readily activated and controlled on-demand at the twisted interface of two hexagonal boron nitride flakes. The brightness of the color center can be enhanced by two orders of magnitude by altering the twist angle. Additionally, a brightness modulation of nearly 100% of this color center is achieved by an external voltage. Our ab-initio GW and GW-BSE calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron-hole recombination. This mechanism is further exploited to draw nanoscale color center patterns using electron beams.

Colour centres in two-dimensional hexagonal boron nitride (hBN) have emerged as a promising platform for quantum photonic applications, as they emit single photons at room temperature and can be readily integrated into nanoscale devices thanks to their reduced dimensionality. Recently, we showed the first demonstration of optically-detected magnetic resonance (ODMR) from single spin defects in a van der Waals material at room temperature,¹ with ODMR contrasts of up to 30%, demonstrating the potential of hBN for applications in quantum information and quantum networks. Interestingly, single defects with similar spectral properties can show negative, positive or no ODMR contrast, indicating rich photo- and spin dynamics in the material. Integration of these colour centres into devices such as cavities to enhance emission or improve collection efficiencies will lead to a stronger platform to investigate and exploit their fundamental physics, and to evaluate their potential as solid-state qubits or nanoscale quantum sensors. In this contribution, we present our latest results towards integrating these individually optically addressable spin defects into tunable devices to enable a room-temperature single spin-photon interface in a two-dimensional material.

¹ H. L. Stern, J. Jarman, Q. Gu, S. Eizagirre Barker, N. Mendelson, D. Chugh, S. Schott, H. H. Tan, H. Sirringhaus, I. Aharonovich, M. Atatüre Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride (2021) arxiv:2103.16494

At the core of most quantum technologies is the development of homogeneous, long lived qubits with excellent optical interfaces, and the development of high efficiency and robust optical interconnects for such qubits. To overcome inhomogeneities in semiconductor spin qubits and in their connections, we have been relying on fast photonics inverse design and on optimization (Floquet engineering) of the qubits themselves. We illustrate this approach to scalable semiconductor quantum systems with our results on quantum photonics based on diamond and silicon carbide.

We will present an overview of Sandia’s focused ion beam (FIB) implantation capability as it pertains to the fabrication of single defect center-based devices via direct write nanofabrication in a wide range of devices and substrates. Sandia’s Ion Beam Laboratory (IBL) has seven main accelerators including three high energy (250 keV to ~50 MeV) FIB systems with micron-scale resolution and three low energy (2 keV to 200 keV) FIB systems with nanometer scale resolution. In this talk we will highlight both the A&D nanoImplanter and the Raith Velion. These are both multi-species kV energy level FIB systems with a minimum spot size of <10 nm with both mass resolution using an ExB filter and single ion implantation using fast blanking. The combination of high spatial resolution, variable energy and the ability to implant a wide range of elements from the periodic table makes these versatile machines for a range of topics from device modification including the deterministic seeding of TaOx memristor devices, detector testing using high resolution ion beam induced charge collection (IBIC) to probe the structure of defect cascades, device fabrication including deterministic single donor devices for quantum computing research, to the formation of individual defect centers in wide bandgap substrates including diamond, SiC, hBN, etc…. Here we concentrate on FIB implantation to create defect centers in diamond making use of in-situ counting to reduce the error associated with the Poisson distribute and in SiC using a newly developed in-situ photoluminescence (PL) setup to create near deterministic array formation.

Using in-situ diamond detectors combined with FIB implantation we have demonstrated the fabrication of SiV arrays with a factor of two reduction in the error for single defect center formation as compared to standard timed implantation. This allows us to develop an improved understanding of the defect center activation under high temperature annealing. In the same accelerator we have demonstrated in-situ PL that allows for the near deterministic formation of Vsi arrays in SiC via an implant, measure PL and repeat process.

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

Single photon emitters (SPE) in wide bandgap semiconductors play a crucial role quantum sensing, computing, and communication. For utilization of SPEs for these quantum applications, it is crucial to locally place a precise number of ions in the host semiconductor for the highest possible yield of devices. Various techniques have been proposed for the deterministic creation of SPEs, including vacancy generation through pulsed laser, electron irradiation, and localized annealing by laser heating. However, none of these techniques can introduce non-naturally occurring impurity-type emitters, which are especially suitable for deterministic SPE creation, enabling near background free SPEs. Vacancy-centers in diamond have attracted great interest for the use in quantum information science.
The silicon vacancy center (SiV) in diamond is an ideal candidate for the use as a SPE in quantum communication because of its potential for high photon emission rate, small phonon sideband emission, large Debye-Waller factor, and microwave-controllable electronic spin. Creation of SiV through a combination of ion implantation and high-temperature annealing is routinely realized, but to enable fabrication of large-scale devices a high confidence in the number of created SiV , implying control over the number of implanted Si ions, is required. Typical ion implantation experiments rely on measurement of the beam current and timing the ion beam implantation to obtain implantation of a specific number of ions. Since the number of ions implanted by an ion beam is determined by Poisson statistics, the uncertainty on the number of ions is large when only a few ions are required.
In this presentation, we present an in-situ ion counting experiment which can reduce the uncertainty in the number of implanted Si ions, which is especially important for small total number of implanted ions. We show that for implantation of 30 ions, required to generate on average a single SiV, the uncertainty on the number of ions per implantation is reduced to 1/4^th the uncertainty in a timed implantation. We determine the number of SiV per implantation spot through using Hanburry Brown Twiss interferometry and find that the yield is approximately 3% for both timed and counted implantation techniques, demonstrating our ability to count implanted Si ions. The activation rate of 3% for implanted Si ion to SiV conversion prevents us from fully realizing the potential for counting implants, as the yield is dominated by the Poisson statistics of activation. However, in future work, implementing our counted implant technique combined with defects with high activation rates, will bring us closer to truly deterministic SPE placement.
Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the DOE’s National Nuclear Security Administration under Contract No. DE-NA0003525. This work was funded by the Laboratory Directed Research and Development Program and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science.

Developing novel methods for the synthesis, processing, and characterization of quantum optical materials for quantum sensing and communication applications remains an outstanding challenge. This talk will present recent results in designing and creating quantum defects in insulators and wide-gap diamond semiconductors using extreme (GPa) pressures and temperature (>1800K) within a diamond anvil cell (DAC). Molecular dopants including tetraethyl orthosilicate (TEOS) and hexamethylenetetramine (HMT) can be chemically doped within amorphous carbon aerogels to create the negatively charged silicon split-vacancy (SiV) as well as both neutral and negatively-charged nitrogen-vacancy (NV) point defects. Pressure-dependent optical spectroscopy of the SiV center correlates well with computational predictions based on ab initio / DFT quantum cluster calculations. Recently the use of lithium fluoride (LiF) as a pressure transmitting medium has enabled the recovery of novel quantum optical materials for ex -situ characterization at cryogenic temperatures via time-correlated single photon counting. High-pressure has also been used to tune the crystal field energies of lanthanide point defects within insulating ceramic materials that have recently demonstrated the first experimental measurements of solid-state laser refrigeration at GPa pressures.

The understanding and design of carrier dynamics and their interactions with defects at a buried insulator/semiconductor interface is essential for achieving optimum performance in modern electronics. We report on the use of scanning ultrafast electron microscopy (SUEM) as a non-contact and non-destructive technique to remotely probe the sub-nanosecond dynamics of excited carriers at a silicon surface buried below a 1 μm thermal silicon oxide. Our measurements illustrate a novel SUEM contrast mechanism, whereby optical modulation of the space-charge field in the semiconductor modulates the electric field in the thick oxide, thus affecting its secondary electron yield. By analyzing the SUEM contrast temporally, spatially and as a function of laser fluence we demonstrate the diffusion mediated capture of excited carriers by interfacial traps.

Van der Waals materials have emerged over the last decade as the new playground for quantum photonics devices. Among them, hexagonal boron nitride (hBN) is an interesting candidate, mainly because of its crystallographic compatibility with many different 2D materials, but also because of its ability to host optically active spin defects. We have recently reported [1] the optically detected magnetic resonance (ODMR) of spin-triplet negatively charged boron vacancies (V_B^-) in hBN and determined their spin-Hamiltonian parameters. Furthermore, we demonstrated the coherent control of V_B^- at room temperature and determined the relevant spin-relaxation times. In this respect, sensors based on such color centers embedded in an intercalated hBN layer may be particularly attractive, since the distance between the sensor and the object to be sensed can be quite small. The influence of external stimuli (magnetic field, temperature and pressure) on this spin defect will be also discussed.[2]
[1] A. Gottscholl et al., Nat. Mater. 19, 540–545 (2020)
[2] A. Gottscholl et al., Nat. Commun. (accepted) (2021)

Atomically thin transitional metal dichalcogenides (TMDs), have recently come to the forefront of research in materials physics. This is largely due to the ease with which they can be combined into artificially engineered heterostructures that exhibit emergent electronic and optical properties. Optical excitations in semiconducting TMDs, viz., excitons, can be localized in potential wells resulting in quantum emitters. Unlike their monolayer counterparts, excitons in heterobilayers, such as MoSe₂/WSe₂, feature a permanent electric dipole which can be used to tune their emission energy electrically. Moreover, the few-body interaction between dipolar quantum emitters can be exploited for quantum nonlinearity. Furthermore, many-exciton dipolar interactions result in effective magnetic fields of ~ 6 Tesla. Thus, dipolar quantum emitters are a promising platform to study quantum optical phenomena and to realize strongly interacting phases of optical excitations in 2D materials.

Single-photon emitters (SPEs) in hexagonal boron nitride, emitting in the 1.6-2.1 eV spectral region, have attracted a lot of experimental and theoretical activity since their discovery [1]. Many defects have been proposed to explain the observed emission. However, the jury is still out regarding the chemical origin of these emitters. Despite intense theoretical investigations, no single defect seems to describe the experiments in a consistent manner. In a recent work [2], it was confirmed that the presence of carbon increases the concentration of SPEs. However, it remains unclear whether carbon participates in the structure of the SPEs directly or helps create SPEs indirectly.

In this work I discuss a first-principles computational study of the thermodynamics of carbon defects in hexagonal boron nitride (hBN). The defects considered are carbon monomers, dimers, trimers and larger carbon clusters, as well as complexes of carbon with vacancies, antisites, and substitutional oxygen. Our calculations show that monomers (C_B, C_N), dimers, trimers, as well as C_N-O_N pairs are the most prevalent species under most of growth conditions. In comparison to these defects, complexes of carbon with vacancies and antisites occur at much smaller concentrations (< 10¹⁴ cm^-3). Taking into accound the emission wavelength of these defects, our results show that carbon trimers are the only carbon-related defects that can likely explain the results of Ref. [2]. This finding is in agreement with recent calculations that showed that optical lineshapes of trimer defects are in very good agreement with experimental spectra [3], affirming the potential role of carbon trimers in single-photon emission in hexagonal boron nitride.

The work has been performed with M. Maciaszek and L. Razinkovas.

[1] T. T. Tran et al., Nature Nanotechnology 11, 37 (2016)
[2] N. Mendelson et al., Nature Materials 20, 321 (2020)
[3] C. Jara et al., Journal of Physical Chemistry A 125, 1325 (2021)

Quantum technologies are attracting attention worldwide due to their projected benefits over existing technologies. Modern devices such as computers, communication devices, and sensors could benefit from quantum technologies because quantum devices will have the ability to collect data at a precise accuracy and a record-breaking speed. One of the quantum technologies is the NV center in CVD diamond. In this technology, NV centers, or nitrogen vacancy centers, occur when two carbon atoms are removed from a diamond and replaced with a single nitrogen atom that has a vacancy next to it. When a sufficiently high energy photon (a green light (532nm) ) is directed on diamond with an NV center, the diamond re-emits a red light (665nm), and the strength of the re-emitted red light depends on the spin of the NV center. The spin of the NV center can be altered by exciting a microwave at a specific frequency, resulting in the strength of the red light emitted by the diamond to change. The ability to measure the change of strength of the light re-emitted by NV centers is often used in supercomputers, radars, and other quantum technologies. NV centers can also be used for a single photon emitter. Most importantly, NV centers can be formed in lab grown CVD diamonds by generating a plasma in a Microwave Chemical Vapor Deposition (microwave CVD). Hydrogen, nitrogen, and methane gases can be flowed into the microwave CVD and nitrogen can be introduced into the growing diamond structures to create the NV centers. Using this method, we have grown CVD diamond films with varying concentrations of NV centers. The effect of various process parameters such as deposition power, gas ratios, substrate temperatures, and growth pressure have been studied on NV center formation, density, quality, and in-situ doping of nitrogen. Quantum structures have been characterized using Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. In-situ laser reflectivity was also implemented to investigate growth characteristics of CVD diamond layers. In this paper, we will discuss the dependence of process conditions and characterization of NV centers and highlight potential applications and sensor development.