Symposium QT07—Atomic and Molecular Quantum Systems and Defect Engineering for Quantum Technologies

Quantum technologies have become a top research priority. Point defects in semiconductors provide a platform that combines the environmental isolation necessary to maintain coherence with the ability to perform electrical and optical manipulation [1]. The NV center in diamond has been widely studied as an individually-addressable quantum system; our goal is to identify centers in other materials that exhibit similarly favorable properties. Building on the first-principles methodology for point-defect calculations [2], we have developed the capability to predict transition energies, lineshapes of optical transitions [3,4], and rates of radiative [5] and nonradiative [6] transitions, relevant for intersystem crossings. I will illustrate these developments with examples for AlN [7] and hexagonal boron nitride (h-BN) [8,9]. Single-photon emission has been observed in h-BN at ~2 eV and at 4.1 eV, but microscopic identification of the underlying point defects has proved elusive. We attribute the single-photon emission at ~2 eV to boron dangling bonds [8], and the 4.1 eV emission to a C-C dimer [9].

Work performed in collaboration with A. Alkauskas, L. C. Bassett, C. E. Dreyer, A. Janotti, J. Lyons, M. Maciaszek, M. Mackoit, L. Razinkovas, M. Turiansky, J. Varley, J. Weber, and Q. Yan, and supported by DOE and NSF.

[1] C. E. Dreyer et al., Annu. Rev. Mater. Res. 48, 1 (2018).
[2] C. Freysoldt et al., Rev. Mod. Phys. 86, 253 (2014).
[3] A. Alkauskas et al., New J. Phys. 16, 073026 (2014).
[4] L. Razinkovas et al., Phys. Rev. B 104, 045303 (2021).
[5] C. E. Dreyer et al., Phys. Rev. B 102, 085305 (2020).
[6] M. E. Turiansky et al., Comput. Phys. Commun. 267, 108056 (2021).
[7] J. B. Varley et al., Phys. Rev. B 93, 161201 (2016).
[8] M. E. Turiansky et al., Phys. Rev. Lett. 123, 127401 (2019).
[9] M. Mackoit-Sinkeviciene et al., Appl. Phys. Lett. 115, 212101 (2019).

Nowadays, quantum information processing (QIP) gets more and more important regarding the increasing demand for secure communication. For QIP a quantum platform is required which is able to perform computations and transfer entanglement over long distances.
Among others, negatively charged group IV defects in diamond gained increasing interest as promising candidates due to their efficient spin photon interface and the long coherence times they can provide. The best known example is the SiV [1], for which it was furthermore demonstrated that neighboring nuclear spins can be accessed and manipulated [2]. Exploiting the dipolar coupling individual ¹³C can be addressed, forming a quantum register. However, at T>500mK in the ground state phonon mediated relaxation has a severe detrimental effect on the electron spin coherence. This makes the operation in expensive dilution refrigerators or dedicated control over phononic coupling via strain or geometry necessary.
Point defects with a substitutional group IV atom of a higher atomic number could serve as an alternative as they provide higher ground state splitting, requiring less low temperatures to get rid of this limiting effect. Recently, the focus especially has shifted to SnV as it was shown to have long coherence times already at liquid helium temperatures [3]. However, it is still an outstanding challenge to produce decent samples with good optical properties, as the heavy atom can cause strain or even damage to the diamond crystal during implantation. Thus, these samples are often lacking spectral or charge state stability and showing a broad inhomogeneous distribution.
Here we report on recent progress of GeV operated in the mK regime. The smaller linewidths of the GeV [4] allow for separation of the spin selective transitions at lower magnetic fields compared to the SiV. This results also in smaller microwave frequencies required for the coherent control. As the transmission gets better at lower frequencies, this leads to less losses, which enabled us to drive the spin with up to 5MHz. Moreover, we were able to measure a coherence time of 160µs.
As the Ge atom is smaller, it can be incorporated in the diamond lattice more easily leading to a higher number of indistinguishable single photon emitters. Furthermore, compared to the SiV the GeV has a higher quantum yield resulting in a higher fluorescence. These features can be beneficial considering entanglement distribution schemes.
Moreover, by transferring the techniques of addressing surrounding nuclear spins developed for the SiV, this could provide a nice hybrid quantum register, which can be considered as a platform for quantum information processing.

[1] Sukachev, Denis D., et al. "Silicon-vacancy spin qubit in diamond: a quantum memory exceeding 10 ms with single-shot state readout." Physical review letters 119.22 (2017): 223602.
[2] Metsch, Mathias H., et al. "Initialization and readout of nuclear spins via a negatively charged silicon-vacancy center in diamond." Physical review letters 122.19 (2019): 190503.
[3] Görlitz, Johannes, et al. "Coherence of a charge stabilised tin-vacancy spin in diamond." arXiv preprint arXiv:2110.05451 (2021).
[4] Siyushev, Petr, et al. "Optical and microwave control of germanium-vacancy center spins in diamond." Physical Review B 96.8 (2017): 081201.

Spins in solid state defects represent promising building blocks for quantum information applications. For these purposes, maintaining the desired quantum state of these spins is crucial, however it is generally not straightforward, as spins can interact with various degrees-of-freedom of the solid state host. In many cases, hyperfine interactions between the electronic spin of the defect and the nuclear spins of the host lattice dominate decoherence. In these scenarios, the cluster-correlation expansion (CCE) method [1,2] has proven to be a useful numerical tool in qualitatively and quantitatively capturing the effective spin coherence of the combined spin-bath system, and more recent generalizations have included population dynamics as well [3]. In this work, we discuss extending this generalized CCE method to capture the spin dynamics of two qubit spins coupled with a common bath of nuclear spins [4]. Using this framework, we evaluate the spin dynamics in a variety of defect qubit candidate systems, where we also consider different defect pair separations, magnetic field strengths, and pulse sequence schemes. We study the interplay of the two qubits and the role of the hyperfine spin bath and highlight particularly important quantum-dynamical processes involved in these various regimes.

1. Yang and Liu, PRB, 78, 085315 (2008)
2. Yang and Liu, PRB, 79, 115320 (2009)
3. Yang et al, Annals of Physics, 413, 168063 (2020)
4. Ciccarino and Narang, in preparation.

Nuclear spins interact weakly with their environment, and therefore exhibit long coherence times. This has led to their use as memory qubits in quantum information platforms, where they are controlled by electromagnetic waves. Scaling up such platforms comes with challenges in terms of power efficiency, as well as cross-talk between devices. Here, we demonstrate coherent control of a single nuclear spin using surface acoustic waves. We use mechanically driven Ramsey and spin echo sequences to show that the nuclear spin retains its excellent coherence properties. We estimate that this approach requires 2–3 orders of magnitude less power than more conventional control methods.Furthermore, this technique is scalable due to the possibility of guiding acoustic waves and reduced cross-talk between different acoustic channels. This work demonstrates the use of mechanical waves for complex quantum control sequences, and offers an advantageous alternative to the standard electromagnetic control of nuclear spins.

In direct band-gap semiconductors, the donor-bound exciton provides a photonic link to the donor electron spin. Here we demonstrate high optical homogeneity and coherent population trapping of donor spins in bulk ZnO and single ZnO nanowires. We conclude with an outlook toward single donor isolation and donor nuclear spin access.

We report on studies of defect evolution and color center formation in diamonds and single crystalline silicon. In diamond we are developing a method to create self-aligned NV-centers along the tracks of a swift heavy ions (Au, 1 GeV). As we reported in [1], we found that NV centers are formed along the tracks of swift heavy where the formation is dominated by energy deposited by electronic stopping of the ions and do not correlate with the end-of-range of the ions where mostly vacancies are created. Here we will report on our experimental effort to extract single ion tracks in a lift out process and to characterize the NV centers along the track. Our earlier measurements indicate an average spacing of NV centers on the order of a few nanometers and chain length of tens or more NV centers. Chains with this spacing and length can become resources for quantum sensing applications and for studies of spin coupling. However, alignment and quality of the NV center arrays have to be investigated.
Color centers with photon emission in the telecommunication bands promise to enable large-scale integration of quantum communication between local quantum computer nodes. Attractive candidates are color centers in silicon including the W-, G-, or T-centers. These might have long electron spin coherent times and narrow linewidths in 28Si enriched SOI-wafers, which we are exploring. Potential application of such color centers for quantum networks can leverage the established silicon-based manufacturing processes of integrated electronics and photonics platforms. We report on experimental measurements and theoretical calculations (density functional theory) of color centers formed in silicon with intense ion pulses that locally heat and simultaneously dope silicon samples.

The work is supported by the U.S. Department of Energy Office of Science, Office of Fusion Energy Sciences, under Contract No. DEAC02-05CH11231. We acknowledge support through coordinated research project F11020 of the International Atomic Energy Agency (IAEA).

Refs:

1. R. E. Lake, A. Persaud, C. Christian, E. S. Barnard, E. M. Chan, A. A. Bettiol, M. Tomut, C. Trautmann, T. Schenkel, Direct formation of nitrogen-vacancy centers in nitrogen-doped diamond along the trajectories of swift heavy ions. Appl. Phys. Lett. 118, 084002 (2021).

The neutral divacancy (VV0) in silicon carbide (SiC) exhibits robust spin coherence and a high-quality near-infrared spin-photon interface in a material compatible with mature fabrication techniques. Here, we make use of this scalable semiconductor host and design electronic devices to manipulate embedded isolated quantum systems [1]. Applying gigahertz ac electric fields to these SiC devices produces coherent interference in the form of Landau-Zener-Stückelberg fringes, arising from interactions between microwave and optical photons [2]. In this platform, we demonstrate lifetime-limited optical coherence and clock-like spin transitions with increased robustness against magnetic noise. Electrical driving of excited-state electron orbitals offers advantages over spin-based coupling and points towards new types of hybrid quantum systems.

We then discuss various strategies to extend the coherence of these spin qubits including isotopic purification, clock transitions, pulsed dynamical decoupling, and continuous driving to engineer a decoherence protected subspace. These subspaces are surprisingly decoupled from the major sources of noise for spin qubits, resulting in an over 10,000 times improvement in coherence [3]. Moreover, by exploiting a novel spin-to-charge mapping technique, we demonstrate single-shot readout of the quantum state and measure further extension of the single spin coherence by over two orders of magnitude to more than five seconds [4]. Finally, we demonstrate the control and entanglement of a single nuclear spin with an electron spin in SiC. This class of nuclear memories can further extend coherence and enable multi-qubit quantum registers [5]. These protocols require few key platform-independent components, suggesting that substantial coherence improvements can be achieved and deployed in a wide selection of quantum architectures.

[1] C. P. Anderson*, A. Bourassa* et al., Science 366, 6470, 1225 (2019).
[2] K. C. Miao et al., Science Advances 5, 11, eaay0527 (2019).
[3] K. C. Miao et al., Science 369, 1493 (2020).
[4] C. P. Anderson*, E. O. Glen*, et al., submitted (2021); arXiv 2110.01590.
[5] A. Bourassa* and C. P. Anderson* et al., Nature Materials 19, 1319 (2020).

A key advantage of spin-defect based quantum technologies is the potential to engineer the defect properties by modifying the atomic environment. Systems which exhibit coupling between magnetic and electric degrees of freedom provide a route to manipulate spins by applying an electric-field, suggesting the prospect of localized spin-control via electrostatic gating.

We have elucidated the fundamental limit of magnetoelectric coupling by electric-field control of magnetic dopants in ferroelectric hosts, demonstrating through combined first principles calculations and electron-paramagnetic resonance measurements that the spin directionality evolves following a switching path coupled to polarization switching.[1]

We further leverage the versatility of the ferroelectric oxide platform by modifying the crystal field environment of the defect through epitaxial strain, low symmetry hosts and emergent topological polarization textures. Results demonstrating spin-control in these systems provide an enhanced understanding of spin-charge coupling in defect systems and offer novel routes to tailored spin-control in ferroelectric crystals.

[1] J. Liu, V. V. Laguta, K. Inzani, W. Huang, S. Das, R. Chatterjee, E. Sheridan, S. M. Griffin, A. Ardavan, R. Ramesh, Coherent electric field manipulation of Fe³⁺ spins in PbTiO₃. Science Advances, 7 : eabf8103 (2021).

Microscopes based on solid-state quantum sensors have recently emerged as powerful tools for probing material properties and physical processes in regimes not accessible to classical sensors, especially on the nanoscale [1]. Quantum microscopy has been used to image a variety of phenomena, from magnetism and charge transport in nanoscale devices to remanent magnetic fields from ancient rocks and biological organisms [1-3]. However, applications of these microscopes have so far mainly relied on spin defects hosted in a rigid, three-dimensional crystal which limits their ability to closely interact with the sample under study. In this context, a long-standing goal has been to identify a spin defect in a two-dimensional van der Waals (vdW) material suitable for quantum microscopy. In this work [4], we demonstrate a versatile and robust quantum microscope using spin defects embedded within a vdW material, hexagonal boron nitride (hBN) [5]. To showcase the multi-modal capabilities of this platform, we assemble vdW heterostructures incorporating a quantum-active hBN layer, and demonstrate simultaneous temperature and magnetic imaging of a vdW ferromagnet, as well as of an operating graphene device. By enabling intimate proximity between sensor and sample, potentially down to a single atomic layer, the hBN quantum sensor opens new frontiers for nanoscale quantum sensing and microscopy. Moreover, given the ubiquitous use of hBN in modern materials and condensed matter physics research, the technique is well-placed to find widespread use in these fields. Preliminary results also showcase the potential for the use of more advanced quantum sensing protocols to sense fluctuating signals, with applications in the physical and life sciences envisaged.

[1] F. Casola, et al Nat. Rev. Mater. 3, 17088 (2018)
[2] L. Thiel, et al Science 364, 973-976 (2019)
[3] M. J. Ku, et al Nature 583, 537-541 (2020)
[4] A. Healey, at al arXiv:2112.03488 (2021)
[5] A. Gottscholl, et al Nature Materials 19, 540 (2020)

Chemistry enables the construction of atomically precise units which are tunable and chemically identical. We harness this structural control for the synthesis of materials for quantum information science. Within quantum information science an area with particular promise for molecular chemistry is quantum sensing. Sensing is inherently analyte specific, and molecules can be tuned towards different analytes and environments - for example imbuing a molecule with water solubility for biocompatibility. Recent results on molecules for quantum information science will be presented, in particular research on optical read-out of spin information in molecules - the molecular analogue of color centers.

Magnons are receiving interest in quantum information science because they can interact with point defects as well as coherently couple to spins, photons, and phonons. Highly coherent magnons require a low-loss magnetic material, such as the molecule-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]_x, x~2). The best V[TCNE]_x thin films grown using chemical vapor deposition (CVD) demonstrate ultra-low Gilbert damping α = (3.98 ± 0.22) × 10^-5 at room-temperature. Low damping in V[TCNE]_x persists at cryogenic temperatures, with linewidths at 5K comparable to room temperature (2 G at 9.4 GHz). Furthermore, V[TCNE]_x can easily be patterned using established e-beam lithography and liftoff techniques and incorporated into devices while maintaining low damping. Realizing the full potential of V[TCNE]_x for quantum information science requires a deeper understanding of the relationship between the coordination compound structure and the magnetic properties.

This talk will discuss recent studies on the microscopic structural changes that lead to an increase in magnetic damping as V[TCNE]_x ages. This deeper understanding informs a new generation of CVD growth. For example, Raman spectroscopy reveals that as V[TCNE]_x ages and oxidizes, structural changes in chemical bonds correlates with a suppression of magnetic order [1]. A parallel study uses electron energy-loss spectroscopy to validate the changes in bonding and in the vanadium coordination as V[TCNE]_x ages [2]. These advances in understanding the structure-function relationship have offered insight to the design of a new furnace for CVD growth of V[TCNE]_x.

[1] H. F. H. Cheung et al. J. Phys. Chem. C 2021 125 (37), 20380-20388. https://doi.org/10.1021/acs.jpcc.1c04582

[2] Personal communication; A. Trout et al. Microsc. Microanal. 2020, 26, 3112−3114.

Open quantum system evolution in the presence of an environment is crucial to understanding and improving many processes including the communication of quantum information, the transfer of energy, and decoherence dynamics in quantum technologies. Quantum computing platforms have emerged as a promising route to modelling and predicting the behaviour of such systems. However, mapping inherently non-unitary dynamics into the unitary framework of gate-based quantum algorithms is a challenging task. Here, I will discuss two different density matrix gate-based quantum algorithms to predict the dynamics of open quantum systems, along with their application to fermionic molecular systems.

Scanning Tunneling Microscopy (STM) can be combined with electron spin resonance [1]. The major advantage of spin resonance is the fact that the energy resolution is independent of the temperature and thus can be much higher than a Fermi-function limited spectroscopy technique such as STM tunneling. In ESR-STM we apply a microwave-frequency electric field to the STM tunnel junction and convert this AC electric field into a driving field for the ESR. We find an energy resolution, which is about 1,000 to 10,000 times better than low-temperature STM. Two advantages of ESR-STM over ensemble-averaging techniques are first, the obvious fact that individual spin systems are measured and second, that this can be combined with precise atom manipulation to build engineered nanostructures.
We will begin with a simple example of ESR-STM by utilizing the atomic spin of Ti-H molecules which are adsorbed on thin MgO films supported on Ag metal substrates. Ti-H is a beautiful example since it has a spin of S=1/2 in this configuration together with a rich isotope distribution including nuclear spins [2].
In the second example we will take ESR-STM from the coherent manipulation of a single spin to the resonant control of 2 coupled spins. This important step allows us to perform double resonance experiments (ELDOR), in which we independently and coherently drive both spins in a weakly coupled dimer. A model reveals the complex spin dynamics and allows us to extract the Rabi rate and the spin relaxation time for the remote spin as well as for the one under the tip [under review]. Taken together with the recent advance towards pulsed ESR-STM, it might be possible to implement quantum information processing on spins on surfaces.
ESR-STM is just in its infancy with many groups joining this research effort. I believe that this technique will occupy a bright corner of quantum-coherent nanoscience.

Susanne Baumann, William Paul, Taeyoung Choi, Christopher P. Lutz, Arzhang Ardavan, Andreas J. Heinrich, “Electron Paramagnetic Resonance of Individual Atoms on a Surface”, Science 350, 417 (2015).
Philip Willke, Yujeong Bae, Kai Yang, Jose L. Lado, Alejandro Ferrón, Taeyoung Choi, Arzhang Ardavan, Joaquín Fernández-Rossier, Andreas J. Heinrich and Christopher P. Lutz, “Hyperfine interaction of individual atoms on a surface”, Science 362, 336 (2018)

Support from Institute for Basic Science (IBS-R027-D1) is gratefully acknowledged.

The ongoing research on materials, which show interesting magnetic behaviors at cryogenic temperatures e.g., superconducting materials/monolayers, require locally precise and also sensitive magnetic sensors, which are functional under these conditions.
As they are already known for their high sensitivity and good robust optical properties at room temperature, Nitrogen Vacancy Centers (NV) seem to be a suitable candidate. Nevertheless, performing measurements at cryogenic temperatures also opens the door for the usage of other defects like the Silicon Vacancy center (SiV). Whereby the optical properties of the NV do not improve at these temperatures, the SiV shows excellent optical properties and also an ODMR contrast up to 100% is achievable.
To utilize the SiV for this purpose, the dynamics of the electron spin in an external magnetic field have to be investigated in detail, described by the Zeeman term of the Hamiltonian. Pioneering work was already done by Hepp et. all, but limited to a certain direction to the SiV principal axis. For a complete understanding these dynamics have to be further investigated in detail by determining the g-tensor of the SiV. Here we report on recent progress, using magnetic spectroscopy measurements on the SiV to obtain its g-tensor and probe the applicability as a magnetic sensor. Furthermore, these insights can be transferred to other group IV defects allowing for precise alignment of the magnetic field to the quantum axis, which is required for gate operations with high fidelity.

The generation of single photons is one key component for many quantum applications. It was shown, that resonance fluorescence from a quantum two-level system, e.g. a semiconductor quantum dot, enables single photon generation with high optical quality. Furthermore, excitation in the weak driving or the so called Rayleigh regime leads to an emission that inherits the excellent linewidth and coherence properties of the laser. In addition, autocorrelation measurements of the coherently scattered light have suggested that it also gains the single photon statistics from the two-level system. However, we present measurements that prove that the simultaneous observation of sub-natural linewidth and antibunching is not possible. High-resolution spectroscopy reveals the sharp spectral feature of the Rayleigh regime with a vanishing component of incoherently scattered light that is limited in its linewidth by the lifetime of the excited state of the two-level system. Filtering the emission in the order of the Fourier limited linewidth leads to the loss of antibunching in the correlation measurement. Our theoretical model identifies two-photon interference between the coherent and incoherently scattered light as the origin of antibunching of the unfiltered emission. This prefigures schemes to achieve a source of single photons with sub-natural linewidth.

Cyanine dyes attached to DNA scaffolds have been shown to exhibit molecular aggregation and exciton delocalization. The molecular aggregation and exciton delocalization of dyes are useful for the development of excitonic devices, such as quantum computers utilizing molecular exciton wires. The functionality of excitonic devices is determined by the dynamics of excitons on the aggregate, such as exciton exchange and exciton-exciton interaction. Thus, it is necessary to understand how dyes orient and interact electronically when aggregated. The orientation and electronic properties of cyanine dyes when bound to DNA scaffolds were studied using density functional theory (DFT), time dependent (TD-) DFT, and molecular dynamics (MD) methods. Using DFT and TD-DFT, dye monomer static dipole differences and transition dipole moments, both of which contribute to exciton dynamics, were calculated. MD simulations of the dyes covalently bound to DNA scaffolds were done to examine the orientations of the dyes when aggregated. Using the static dipole differences and transition dipole moments together with dye orientations, excitonic coupling and interaction energies were calculated.

Quantum embedding theories offer a means to combine multiple levels of electronic structure theory in order to predict accurate electronic properties of correlated materials. These methods can correct errors in low-level theories, such as the delocalization error in density functional theory or the lack of electron correlation in Hartree-Fock theory. In this work, we develop a purely wavefunction based approach to quantum embedding in order to compute correlated electronic states of systems with weak to strong correlations within spatially localized embedded fragments. Furthermore, we benchmark various active space wavefunction methods (CASSCF, CCSD, etc.) for quantum defects in solids.

Background
Quantum sensor is known as an emerging tool to detect magnetic/electric fields, temperature and pressure with high accuracy and spatial resolution within various targets ranging from inorganic material to living cells. Negatively charged nitrogen-vacancy complex defect in diamond known as NV center is a typical spin defect utilized for quantum sensor. Negatively charged silicon vacancy (V_Si^-) in silicon carbide (SiC) can also act as quantum sensor to measure magnetic fields and temperatures. We have been developing silicon vacancy quantum sensors for direct measurement inside SiC device [1,2]. In this work, we report on a current-induced magnetic fields inside the device detection.

Experiment
The SiC devices used in this study were planar p⁺pn⁺ diodes with an opposite electrode. The devices were fabricated by implanting Al and P ions into a p-type 4H-SiC epitaxial film with 5.6 µm thickness on n-type substrate. V_Si^- was introduced into the device by particle beam writing (PBW) (H, 2 MeV, beam spot = Φ1 µm, fluence = 1×10⁶ H⁺/spot). V_Si^- dot (5×5 µm²) patterns were formed around the opposite electrode. The depth of V_Si^- was estimated to be ~30 µm from SRIM simulation [3]. The current-induced magnetic field was measured by optical detection magnetic resonance (ODMR) using a confocal scanning fluorescence microscope (CFM). An external applied magnetic field of B_z = 1 mT was applied to split two ODMR peaks (43 and 95 MHz) to determine each ODMR peak position precisely. The current-induced magnetic field was generated by forward current of 10 mA.

Results
Measurements were carried out at five points. One (point 1) was center between the opposite electrode. The others (point 2-5) were located ±50 and 100 µm away from that position perpendicular to the opposite electrode. Two main ODMR peaks (43 and 95 MHz) were observed at point 2-5 because the B_curr was almost parallel to the c-axis, in other words θ~0°. On the other hand, at the point 1, three very weak ODMR peaks positioned ~15 (we could not determine a precise peak position due to slightly out of measurement range), 82 and 124 MHz were observed in addition to the two main peaks that were split by Zeeman effect caused by B_z. As the current-induced magnetic field (B_curr) had only in-plane component (⊥B_z) in this position, the combined magnetic field vector (B_z + B_curr) deviated from the c-axis, meaning polar angle θ ≠ 0°. In this situation, theoretical calculation predicts that ODMR peak can split into six peaks at maximum. Comparing with experimental values of peak positions at point 1 and theoretical values, B_curr and θ were estimated to be 0.07~0.17 mT and 4~10°, respectively.

Acknowledgement
This study was partially supported by JSPS KAKENHI Nos. 21H04553 and 20H00355 and MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395. This study was carried out within the framework of IAEA CRP F11020.

References
[1] Y. Yamazaki et al., Appl. Phys. Lett. 118 021106 (2021).
[2] T. M. Hoang et al. Appl. Phys. Lett. 118 044001 (2021).
[3] SRIM program : http://www.srim.org/.

The potential impact of active solid-state quantum defects, frequently referred to as artificial atoms in solids, on creating quantum information systems in the near-term is evident ^{1,2 3} . Yet, design and control of multiqubit systems and robust quantum interfaces to these defects at the nanoscale has remained challenging. While there are computational methods and measurement techniques that can study a single solid-state spin qubit based on defects in solids, understanding the interaction between them remains a major challenge. In this context, I will introduce the crucial chemical degree of freedom to the state-of-the-art in quantum defects by demonstrating how these “artificial atoms” become “artificial molecules” when placed within a few angstroms of each other. At these length scales, the host lattice is not only a passive dielectric that screens interactions between defects in complexes, but actively participates in structure relaxation and determines configuration-dependent interactions. Armed with this physical understanding, I will demonstrate how this chemical degree of freedom can be used to tune quantum defects. Next, I will present promising physical mechanisms and device architectures for coupling such quantum defects to other qubit platforms via dipole-, phonon-, and magnon-mediated interactions. Specifically I will share our latest work on coupling magnons to magnetic emitters as a natural step towards magnon-mediated efficient manipulation of spin-qubit states with applications in sensing and quantum information science. I will discuss 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 magnons⁴. I will show 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 transitions⁵. 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 ^6,7. In the outlook, I will discuss new approaches from quantum chemistry for OQSs that could guide the controllable coupling of active quantum defects 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. Philbin, J. P. & Narang, P. Computational materials insights into solid-state multiqubit systems. PRX Quantum 2, (2021).
4. 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).
5. 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).
6. Head-Marsden, K., Krastanov, S., Mazziotti, D. A. & Narang, P. Capturing non-Markovian dynamics on near-term quantum computers. Phys. Rev. Research 3, (2021).
7. Krastanov, S. et al. Unboxing Quantum Black Box Models: Learning Non-Markovian Dynamics. arXiv [quant-ph] (2020).

Future quantum networks require end nodes that combine excellent qubit control and coherence with efficient spin-photon interfaces. Optically active spin qubits in diamond represent an auspicious building block. Among these, the group-IV-vacancy qubits are emerging promising candidates: thanks to inversion symmetry these systems are first-order insensitive to charge noise at surfaces, opening the path towards integration in nanophotonic photonic crystal waveguides and cavities.
The fabrication of such nanophotonic devices in diamond itself is challenging, as there is no known wet-processing technique that would allow to fabricate free-hanging structures starting from bulk diamond material.

Here, we present our recent results on the realization of all-diamond nanophotonic photonic waveguides and crystal cavities. We demonstrate the fabrication process flow, report on the optical properties of the resulting structures, and show that such structures can be fabricated employing a quasi-isotropic crystal plane dependent reactive-ion-etch.

Diamond photonics is emerging as a promising platform in the fields of quantum sensing and quantum information. The main driver is given by the existence of several color centers in both natural and synthetic diamond, being the Nitrogen Vacancy (NV) center the most promising one [1]. The NV center is a point defect in the diamond lattice structure, in which two adjacent carbon atoms are substituted by a nitrogen and a vacancy. In particular, its negatively charged state (NV^-) is an optically active defect, which presents long room temperature spin coherence times, an appealing property for nanoscale magnetic field sensing [2] and for quantum information processing systems [3].
However, to produce a functional quantum photonic device such color centers have to be created in a controlled and deterministic manner, as single defects or in ensembles, and interconnected through optical waveguides in the bulk of diamond. Several techniques have been exploited for this purpose such as ion implantation-assisted lift-off and ion beam implantation, which are mostly suitable for surface modifications and are intrinsically related to damage of the material, resulting in reduced optical performance of the created color centers and waveguides.
We propose femtosecond laser writing as an alternative technique, which enables the direct formation in the volume of the crystal of waveguides and NV^- centers through a simple and non-invasive approach [4]. Quantum grade diamond (2 x 2 x 0.3 mm, <5 ppb nitrogen content, MB optics) was laser written through the use of an Yb:KGW system (Pharos, Light Conversion) with a 230-fs pulse duration, exploiting its second harmonic at 515 nm, focused in the bulk of the crystal with a 1.25-N.A. oil immersion lens with 100× magnification. For the fabrication of waveguides, the type II geometry was employed [5], consisting of two modification lines separated by 13 μm produced by focusing the laser pulses at 25 μm depth at 500 kHz repetition rate, with 60 nJ pulse energy and 0.5 mm/s scan speed, resulting in single-mode guiding in the visible.
For the creation of NV^- centers, single-pulse exposures with 28 nJ energy were applied in the volume of the crystal to induce vacancies in the lattice. When the sample was subsequently annealed at 1000 °C for 3 h in a nitrogen atmosphere, these vacancies migrated and combined with the nitrogen atoms existing in the bulk of diamond and formed NV^- centers. We have also demonstrated the integration of these two building blocks within a proof-of-concept quantum photonic device, fabricated in a single processing step through femtosecond laser writing. With the same femtosecond laser system, the waveguides were first fabricated and afterwards single laser exposures within the guiding volume were inscribed. After the annealing, the waveguide quality was not compromised, while photoluminescent spectra demonstrated the creation of single NV^- centers within the waveguide. Considering additional trials in other waveguides, the single NV^- formation probability was calculated to be 31 ± 9%. Crucial for the coupling of the NV^- centers with the waveguide mode is their alignment close to the center of the waveguide, which has been estimated to be of the order of 1 μm in both longitudinal and lateral direction. Intensity autocorrelation measurement g⁽²⁾(τ) demonstrated single-photon emission. Finally, the possibility to guide excitation light at 532 nm through the waveguide to the NV^- center and the waveguide-coupled excitation was demonstrated through confocal microscopy measurements.
In conclusion, a proof-of-concept integrated quantum photonics chip in diamond has been entirely fabricated through the femtosecond laser writing technique.
[1] I. Aharonovich, et al., Nat. Photonics, 5, 397 (2011).
[2] H. Clevenson, et al., Nat. Phys., 11, 393 (2015).
[3] B. Hensen, et al., Nature, 526, 682 (2015).
[4] J. P. Hadden, et al., Opt. Lett., 43, 3586 (2018).
[5] B. Sotillo, et al., Sci. Rep., 6, 35566 (2016).

Efficient on-chip integration of single-photon emitters imposes a major bottleneck for applications of photonic integrated circuits in quantum technologies. Resonantly excited solid-state emitters are emerging as near-optimal quantum light sources, if not for the lack of scalability of current devices.
A promising scalable platform is based on two-dimensional (2D) semiconductors. However, resonant excitation and single-photon emission of waveguide-coupled 2D emitters have proven to be elusive. Here, we show a scalable approach using a silicon nitride photonic waveguide to simultaneously strain-localize single-photon emitters from a tungsten diselenide (WSe₂) monolayer [1,2] and to couple them into a waveguide mode [3]. We demonstrate the guiding of single photons in the photonic circuit by measuring second-order autocorrelation of g⁽²⁾(0)=0.150±0.093 and perform on-chip resonant excitation yielding a g⁽²⁾(0)=0.377±0.081 [4]. These results open up the way towards coherent control of 2D quantum emitters and more elaborate two-dimensional emitter based on-chip quantum photonic circuits.

[1] A. Branny, et al., Nat. Commun., 8, 15053, (2017).
[2] C. Palacios-Berraquero, et al., Nat. Commun. 8, 15093, (2017).
[3] F. Peyskens, et al., Nat. Commun., 10, 4435, (2019).
[4] C. Errando-Herranz, et al., ACS Photonics, 8, 4, 1069–1076 (2021).

Transition metal dichalcogenides (TMDCs) are of great interest for quantum information science (QIS). TMDC color centers have advantages for single photon emission by curtailing decoherence channels via dimensionality and natural low abundance of non-zero nuclear spin isotopes. They also offer direct probing of the defect electronic states with scanning tunneling microscopy (STM) and a unique way of benchmarking first principles defect computations. Recent experimental work has shown that STM can be used to also control defect formation in TMDCs. The experimental ability to control the filling of a sulfur vacancy with a substitutional metal is an especially exciting new development.
We will report here on a combined theory and experimental study of the Co-filled sulfur vacancy in WS₂. Using beyond-DFT techniques (hybrid functionals and GW), we study what charge states are energetically favorable and their single-particle levels. We directly compare our theoretical results with experimental data obtained through atomically-precise scanning tunneling spectroscopy (STS) using an ultrastable, low temperature, and ultrahigh vacuum STM system. Our work demonstrates the importance of treating appropriately the amount of exact exchange used when performing hybrid computations on 2D systems. We finally discuss the properties of this Co-filled sulfur vacancy defect and its interest for QIS applications.

The quest of solid-state quantum emitters for specific applications 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. Divacancy spins in silicon carbide (SiC) are such centers. We have demonstrated that such divacancy spins can be engineered into ultrasmall cubic SiC nanoparticles by synthesis methods with introducing stacking faults [2]. Stacking faults in 4H SiC mediate to stabilize SiC divacancy color centers with producing high readout contrast at room temperature comparable with that of nitrogen-vacancy center in diamond [3,4]. Silicon is another technologically mature material that can host single photon centers. We have identified single photon emitters in Si by first principles calculations that are promising in quantum communication applications [5,6].
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 which includes boron-vacancy defects and carbon defects. 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 [7] 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 near-infrared 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. A.G. acknowledges the National Excellence Program (NKFIH Grant no. KKP129866), EU H2020 FETOPEN project QuanTELCO (Grant no. 862721).

[1] Gang Zhang, Yuan Cheng, Jyh-Pin Chou, and Adam Gali, Applied Physics Reviews 7, 031308 (2020)
[2] Dávid Beke, Jan Valenta, Gyula Károlyházy, Sándor Lenk, Zsolt Czigány, Bence Gábor Márkus, Katalin Kamarás, Ferenc Simon, and Adam Gali, The Journal of Physical Chemistry Letters 11 1675-1681 (2020).
[3] Viktor Ivády, Joel Davidsson, Nazar Delegan, Abram L. Falk, Paul V. Klimov, Samuel J. Whiteley, Stephan O. Hruszkewycz, Martin V. Holt, F. Joseph Heremans, Nguyen Tien Son, David D. Awschalom, Igor A. Abrikosov and Adam Gali, Nature Communications 10, 5607 (2019).
[4] Qiang Li, Jun-Feng Wang, Fei-Fei Yan, Ji-Yang Zhou, Han-Feng Wang, He Liu, Li-Ping Guo, Xiong Zhou, Adam Gali, Zheng-Hao Liu, Zu-Qing Wang, Kai Sun, Guo-Ping Guo, Jian-Shun Tang, Hao Li, Li-Xing You, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo, National Science Review, on-line (2021). DOI:10.1093/nsr/nwab122
[5] Péter Udvarhelyi, Bálint Somogyi, Gergo Thiering, Adam Gali, arXiv:2106.13578, accepted to Physical Review Letters, and references therein
[6] Yoann Baron, Alrik Durand, Péter Udvarhelyi, Tobias Herzig, Mario Khoury, Sébastien Pezzagna, Jan Meijer, Isabelle Robert-Philip, Marco Abbarchi, Jean-Michel Hartmann, Vincent Mazzocchi, Jean-Michel Gérard, Adam Gali, Vincent Jacques, Guillaume Cassabois, Anaïs Dréau, arXiv: 2108.04283
[7] 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

Vacancy-related color centers in silicon carbid represent quantum bits or can be employed as single photon emitters. The excited defect states and the photons emitted in transitions between them and the groundstate alongside spin-selective, non-radiative transitions via intermediate low-spin states are pivotal parts of the mechanism underlying qubit applications. Optical excitation of the qubit may also lead to an ionization into other charge states in which the qubit is silent. The ability to control and deliberately switch the charge state is pivotal for applications and has recently been explored experimentally for qubit centers in SiC [1, 2]. However, the charge states actually involved in the switching and their optical properties are often not clear. Furthermore the electrical detection of spin states, as demonstrated for the NV-center in diamond[3], involves emission of defect spins into the conduction or valence band in a two-photon process. How the optical ionization yield and the spin contrast depend on the photon energy is an open question.
Here we investigate the optical ionization of the divacancy, the silicon vacany and other vacancy-related color centers in SiC within the frame work of the CI-CRPA approach [4]. This approach allows for the embedded description of highly correlated multiplet states, such as the important low spin states. We shine light onto ionizing single and two-photon processes relevant for optical charge state switching and electrical detection of the spin states. Our results show that an enhanced ionization cross section can occur for photon energies well above the ionization thresholds. This determines the charge state yielded by the ionization once the ionization threshold of competing ionization channels are overcome.

D. A. Golter and C. W. Lai, Sci. Reports 7, 13406 (2017).
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M. Bockstedte, F. Schütz, Th. Garratt, V. Ivady, A. Gali, njp Quantum Materials 3, 31 (2018).

Optically active point defects in semiconductors are promising for quantum sensing close to room temperature and quantum communication at temperatures as low as ~4 K. One system that has shown great promise are point defects in 4H-SiC, including V⁰_Si,V⁰_C, and (V_C+V_Si)⁰. However, most current research uses optical excitations to control the spin states of these point defects, which generally lacks flexibility in controlling wavelength when trying to specifically excite different defects. Motivated by the success of single ion implantation, we aim to explore an alternate method to initialize/write these optically active defects via proton irradiation. Here we utilize real-time time-dependent density functional theory (RT-TDDFT) to examine the process of channeling protons exciting V⁰_Si,V⁰_C, and (V_C+V_Si)⁰ defect states in 4H-SiC. Specifically, we monitored the interaction between a proton with velocities of [1.0, 0.2, 0.15, 0.1, 0.05 a.u.] and vacancy defect states through electronic stopping power, occupation number of the Kohn-Sham states, and e-h pair count. From these simulations, we found the proton to be able to transfer its energy into the local electron density of the defect and excite each type of vacancy in 4H-SiC. Additionally, the amount of excitation varied based on the velocity of the proton and which defect state it was acting on, opening up the opportunity for enacting control over the excitation of each defect. Furthermore, similar to optical inversion, we found the proton to generate varying amounts of e-h pairs in the defect states depending on its velocity, which can potentially be used to control the charge state of each defect by locally changing the quasi-Fermi level. Last but not least, we note that the ion beam method might suffer from the fact that it takes longer for ions to travel in comparison to photons, which will give rise to slower writing speed.

Quantum emitters in solid-state materials can be utilized as the basic unit of quantum informaiton technologies. We 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. Our ab-initio GW calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron-hole recombination. The brightness of this emitter can be tuned by three ways: (1) electron beam irradiation, (2) twist angle at the interface, and (3) an external voltage. In addition, ways to tune the spectrum of the emitters at the interface will also be discussed.

Imbuing materials with useful quantum properties requires not only creating defects and defect-dopant complexes with predefined chemical constituents, structure, and bonding configuration, but also placing these defects at exact locations and reliably in pairs or arrays. This required level of precision and repeatability exceeds what is currently possible by traditional methods of nanoscale fabrication, such as photolithography, which has and continues to underpin information technology. Potential new avenues for achieving the needed atomic scale precision for fabrication of engineered quantum defects may be found by leveraging atomic resolution imaging instruments as platforms for atomic manipulation. Scanning tunnelling microscopy has been demonstrated as one such viable path to construct atomic scale structures. More recently, the combination of the highly focused, energetic, and maneuverable beam of the scanning transmission electron microscope (STEM), with emerging capabilities to modulate local environments in operando utilize novel feedback and control schemes, offers a way to controllably transform and modify material at the atomic scale to build defects with useful quantum properties.
Presented here are recent results in which STEM is used to controllably create individual defects in suspended single layer and twisted bilayer graphene, and further dope these defects with a range of elements. A critical aspect to achieving atomic scale fabrication is the need better understand how individual defects and ensembles of defects are created and how they behave within 2D materials. We will show how temperature plays an important role in vacancy dynamics and defect chain assembly and present a model which describes many of the somewhat counterintuitive observations made while drilling and shaping 2D materials with the electron beam. We will also show the results of using several different approaches to locally evaporate dopant materials in-situ and in close proximity to where defects are created and the subsequent insertion of dopant atoms into vacancies and how these provide a path to deterministically create arrays of defect-dopant complexes. Results of theoretical studies undertaken to understand defects dynamics and predict the properties that emerge from these defects will also be presented.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and was performed at the Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy, Office of Science User Facility.

Solid-state single-photon emitters (SPEs), or quantum emitters (QEs), are central building blocks for a number of emerging photonic quantum technologies. Recently, 2D layered materials have been intensively studied as a novel non-classical light emission source. As SPE research in 2D materials to date has primarily focused on the visible spectral range, it is highly desirable to extend the emission range of 2D SPEs to the technologically important near-infrared (NIR) regime. While most 2D semiconductors have inherent electronic band structures that limit the operating wavelength to the visible spectral range, 2D heterobilayers with type-II band alignment open the door to the engineering of the excitonic states and the emission energy, holding promise for achieving 2D NIR SPEs.

I will present the site-controlled creation of NIR SPEs in 2D transition metal dichalcogenide (TMDC) MoX₂/WX₂ heterobilayers. The emission of non-classical light is induced through the deterministic implantation of localized strain using a scalable, nanopillar-based, strain engineering technique. The interlayer excitons trapped by the strain-induced potential wells produce bright, narrow-bandwidth photoluminescence (PL) with emission wavelength ranging from 900 nm to 1 μm. Our photoluminescence excitation spectroscopy reveals that NIR emission originates from trapped interlayer excitons. Our Hanbury Brown and Twiss (HBT) experiment yielded near-complete photon antibunching, unambiguously proving the quantum nature of the emitters. The high-purity single-photon emission can be maintained at liquid nitrogen temperature without a significant decrease in the emission intensity. Photon lifetime and pump-power dependent PL measurements were conducted to understand the carrier dynamics of the localized emitters. No photon bleaching, blinking, or spectrum diffusion was observed in the time domain, suggesting the quantum emitters are photon stable. Circularly polarized emission with a large degree of circular polarization was observed in magneto-PL experiments, revealing a valley-sensitive selection rule. First-principles studies were conducted to understand the physics of the NIR SPEs. Our interlayer quantum emitters are the first demonstration of site-controlled non-classical light emission in the 900 nm - 1 mm NIR regime. These newly discovered interlayer quantum emitters together with our MoTe₂ telecom-wavelength QE work¹ extend the emission range of 2D SPEs to the NIR regime and offer new opportunities for the development of next-generation fiber-based quantum communication and quantum information applications.

1. Zhao, H., Pettes, M. T., Zheng, Y., & Htoon, H. (2021). Site-Controlled Telecom Single-Photon Emitters in Atomically-thin MoTe2. arXiv preprint arXiv:2105.00576.

Room-temperature quantum sensors based on nitrogen-vacancy (NV^-) centers in diamond provide a platform to achieved unparalleled performance in magnetic navigation, neuroimaging, and materials characterization. The formation of high-quality spin qubits requires precise control of diamond growth properties, such as nitrogen doping density and defect formation. Plasma-enhanced chemical vapor deposition (PE-CVD) is a promising technique to achieve controlled substitutional doping of N, but coherence times are often limited by N-N dipolar interactions, poor (< 10%) N to NV^- conversion efficiencies, and charge state instability. Here, we overcome these challenges using a unique combination of PE-CVD paired with high energy implantation. Specifically, we grow and process diamonds with controlled fractions of multiple donor species (group V and VI) that strongly impact the formation of NV^- centers. We directly measure how these changes in the surrounding chemical environment impact critical quantum sensing properties such as the NV^- coherence time, Ramsey fringe contrast, creation yield, and charge state stability. These results provide new insights into how diamond chemical composition directly impacts NV^- performance and, ultimately, quantum sensor sensitivity.

Due to their excellent optical properties, semiconductor quantum dots are promising systems for the on-demand generation of single photons. A wealth of different excitation schemes have been developed, each with their specific advantages and disadvantages. Resonant excitation allows for the generation of highly indistinguishable photons, while the single-photon purity is limited by reexcitation [1]. In contrast, two-photon excitation of the biexciton suppresses reexcitation, resulting in ultra-low multi-photon errors [2]. However, the indistinguishability of emitted photons is inherently limited by the cascaded decay [3].

Here, we theoretically model and experimentally demonstrate a single-photon generation scheme that combines all the advantages of previously established excitation methods [4]. The scheme is based on the resonant two-photon excitation of the biexciton followed by a precisely-timed stimulation of the biexciton-exciton transition, which effectively prepares the system in an exciton state. On the one hand, the two-photon excitation of the biexciton suppresses re-excitation and enables ultra-low multi-photon errors. On the other hand, the precisely timed stimulation pulse prepares the system in the exciton state at a well-defined time. This strongly reduces timing jitter in the preparation of the exciton state caused by the biexciton population lifetime, and consequently, reestablishes the high indistinguishability for photons emitted from the exciton to ground state decay.
The scheme allows to deterministically program the polarization of the emitted photon (H or V) via the polarization of the stimulation pulse. Moreover, this allows to obtain all emission of interest to occur in the polarization of the detection channel enabling higher brightness than cross-polarized resonant excitation. Experiments and simulations exploring the system dynamics are in very good agreement and confirm the validity of the theoretical model.

[1] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017)
[2] L. Hanschke et al., npj Quantum Information 4, 43 (2018)
[3] E. Schöll et al., Physical Review Letters 125, 233605 (2020)
[4] F. Sbresny et al., arXiv:2107.03232 (2021)

Self-assembled InGaAs quantum dots are amongst the most performant solid-state quantum emitters. At the same time, they can host electron or hole spin qubits which can be manipulated using ultrafast laser fields. The electron spin coherence time has been previously shown to be limited by strong contact hyperfine coupling to the noisy nuclear spin environment. A straightforward approach to mitigate the influence of nuclear spin noise involves encoding the qubit on the spin of a valence band hole which, compared to the electron, has much weaker, albeit anisotropic hyperfine coupling. Here, we probe the inhomogeneous dephasing time of a single hole spin qubit (T₂^*) using all optical ultrafast pump-probe techniques and obtain T₂^*>200ns at zero B-field, a value two orders of magnitude longer than for electrons. Ramsey interference measurements performed for various transverse magnetic fields reveal hole spin coherence times in the nanosecond regime and a characteristic nonlinear decrease of the hole spin coherence time with increasing magnetic field. This observation is attributed to the influence of the electrical noise on the spin via the electric field dependent hole g-factor.
To determine the decoherence dynamics and its underlying physical processes, we perform spin echo measurements of the single hole spin which reveal a strong spin echo envelope modulation with two discrete frequencies. A fast oscillation of the spin echo signal (4.8MHz/T) is identified as being due to the effect of quadrupolar coupling on the longitudinal nuclear field component. Uniquely to the hole spin, we observe an additional slow oscillation (0.8MHz/T) that is identified as stemming from a bimodal g-factor behavior, potentially induced by the high intensity rotation laser pulse changing the local electric environment in the vicinity of the quantum dot.

Optical spectroscopy is one of the most powerful tools used to understand nanostructured materials such as Quantum Dots (QDs). However, there are many parameters that influence the observed emission energies, including both the energies of confined states and many-body interactions. The situation is made even more complex by the fact that QDs are often inhomogeneous - small variations in size, shape, and composition lead to different values for every parameter in each unique nanostructure. Developing scalable quantum devices and systems that leverage the unique properties of QDs requires a method for analyzing the characterization data to extract values for the many parameters that contribute to the observed features. However, fits to too many free parameters have no scientific meaning.
We develop and present a Markov Chain Monte Carlo (MCMC) method for performing statistically-meaningful fits to optical spectroscopy data of QDs. The system we use to develop our MCMC method is a vertically-stacked InAs Quantum Dot Molecule (QDM) grown on GaAs by molecular beam epitaxy. The QDM is embedded in a n-i-Schottky diode that allows the application of electric fields that tune the relative energies of the discrete states of the QDs. We analyze the photoluminescence (PL) from a single QDM as a function of applied electric field. Using the MCMC method, we simultaneously fit all experimental data to analytical expressions of the Hamiltonians for the system. The algorithm explores a range of values for each observable and, after millions of iterations, returns a histogram of values that generate an acceptable fit. We show that that MCMC approach extracts values for important physical parameters that are significantly different from those extracted using traditional methods. This approach can be extended to similar analyses of a wide range of nanostructured systems described by complex Hamiltonians.

Tremendous research activity worldwide has focused on attempting to harness the exotic properties of quantum physics for new applications in sensing, computation, and communications - a push to develop “engineered quantum systems”. Color centers in diamond have proven to be promising candidates for not only quantum computing but also for quantum sensing. Amongst the various color defects, nitrogen-vacancy centers (NV centers) could provide a platform for precision magnetometry allowing for nanoscale magnetic imaging. In this talk I will give an overview of our research towards the development of an imaging tool for mapping neuronal signals from mammalian brain cells. While the diamond quantum microscope is routinely used for measuring static magnetic fields in a wide field of view with diffraction limited spatial resolution, dynamic widefield magnetometry for the measurement of temporally varying magnetic fields has been very challenging. I will describe the first demonstration of dynamic widefield magnetometry using Nitrogen vacancy centers in diamond and its applications.

We report [https://arxiv.org/abs/2107.13518] experimental detection and control of Schrodinger’s Cat like macroscopically large, quantum coherent state of a two-component Bose-Einstein condensate of spatially indirect electron-hole pairs or excitons. These excitons are artificial atom like quasi particles within solids which can be generated with optical and/or electrical excitations. As a result, achieving BEC within an optoelectronic device can provide access to millions of excitons as qubits to allow efficient, fault-tolerant quantum computation. In this work, phase coherent periodic oscillations in photo generated capacitance as a function of applied voltage bias and light intensity over a macroscopically large area are measured. Periodic presence and absence of splitting of excitonic peaks in the optical spectra based on photocapacitance point towards tunneling induced variations in capacitive coupling between quantum well and quantum dots. Observation of negative ‘quantum capacitance’ due to screening of charge carriers by the quantum well indicate Coulomb correlations of interacting excitons in the plane of the sample. Coherent resonant tunneling in this well-dot heterostructure restricts the available momentum space of the charge carriers within this quantum well. Consequently, the average electric polarization vector of the associated indirect excitons collectively orients along the direction of applied bias and these excitons undergo Bose-Einstein condensation below ~100 K. Generation of interference beats in photocapacitance oscillation even with incoherent white light further confirm the presence of stable, long range spatial correlation among these indirect excitons. Moreover, some of these photo generated excitons can be addressed independently of others using applied bias over a localized region. In addition, collective Rabi oscillations of these macroscopically large, ‘multipartite’, two-level, coupled and uncoupled quantum states of excitonic condensate as qubits are also observed. Therefore, our study not only brings the physics and technology of Bose-Einstein condensation within the reaches of semiconductor chips, but also opens up experimental investigations of the fundamentals of quantum physics using similar techniques. It is also obvious that most of the DiVincenzo’s criteria for quantum computation are also fulfilled in this system in principle.
Operational temperature of such excitonic BEC can be raised further using similar 0D-2D heterostructure having more densely packed, ordered array of QDs in the x-y plane and/or using materials having larger excitonic binding energies. However, fabrications of single crystals of 0D-2D heterostructures using 2D materials (e.g. transition metal di-chalcogenides, oxides, perovskites etc.) having higher excitonic binding energies are still an open challenge for semiconductor optoelectronics. Even now these 0D-2D heterostructures can be scaled up for mass production of miniaturized, portable quantum optoelectronics devices using the existing III-V and/or Nitride based semiconductor fabrication technologies. Such feasibilities of fabricating quantum devices, however, certainly difficult to implement for most other current approaches being developed for quantum computation outside the boundaries of a sophisticated research laboratory. It can bring in a paradigm shift in optoelectronics for faster and wider adaptation of quantum technologies in terms of miniaturization and portability.

Rare-earth ion (REI) dopants in solid-state host crystals are gaining interest as an attractive platform for quantum information processing due to long coherence times and potential scalability. Recent efforts to improve coherence times in REI dopants have focused on finding suitable host crystals with minimal sources of decoherence (e.g., nuclear spin, oxygen vacancies, and strain variation). Molecular beam epitaxy (MBE) offers advantages in this regard, as it can grow epitaxial films using highly pure elemental sources and isotopically purified materials. In addition, MBE growth can provide these materials in thin film and heterostructure forms that offer flexible and scalable host crystal environments.

Here, we report on the MBE growth of Er-doped anatase TiO₂ thin films on LaAlO₃ (001) substrates with varying doping concentrations. We characterize them using x-ray diffraction and photoluminescence (PL) measurements to investigate how factors such as the Er concentration, buffer layer and delta-doping affect the optical inhomogeneous linewidths of Er³⁺ ions. Initial results show Er³⁺ PL from two different sites in TiO₂ and one site in LaAlO₃, with PL intensities consistent with the doping concentration.

*This work is supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Co-design Center for Quantum Advantage (C2QA) under contract number DE-SC0012704.

Monovacancy defects are known to induce magnetization in graphene. However, the understanding of the effects of defect-defect interaction on magnetization under applied strain remains elusive. In this talk, we will present the effects of symmetry-breaking strain on magnetization in multidefect graphene. We used the density functional theory simulation under the generalized gradient approximation for the exchange-correlation energy to investigate the magnetic consequence of strain applied along different directions relative to the defect pair. Our results show that the spin magnetic moment increases with increasing strain. Decomposition of the total magnetic effects into the indiviudal effects of the orbitals reveals that the $p_z$ orbital dominates the change in the total magnetic moment, while the net change of the local moment in $p_x$ and $p_y$ orbitals are the same but with opposite signs. Nonetheless, for a pair of monovacancis, the strain has no effect on the spin magnetic moment when the inter-defect distance reaches a critical value. For longer inter-defect distance, the magnetic moment increases with increasing strain, and the combined effects can be well-approximated as a linear superposition of their individual effects. Additionally, if the defects are magnetically isolated, there exists a critical strain whereat each of the defects experiences a second-order Jahn–Teller reconstruction (JTRR) that switches the orientation of the defect and alsters the magnitude of the magnetic moment. These results are expected to shed new light of controlling the magnetic behavior of single or multi-defect system as a function of the separation distance and the intensity of mechanical strain.

Silicon is one of the most established materials in modern electronics and integrated circuits, with mature technology for the construction of integrated photonics. Though, it has received less attention as a host of color centers, mainly due to its relatively small bandgap making deep centers emitting near telecom range less likely. However, recently, studies on radiation damage centers have found that the interstitial carbon and hydrogen cluster, called the T-center, not only emits in the telecom O-band, but also retains an optically addressable electron and nuclear spin with considerable coherence times[1]. The implications for future silicon-based quantum devices warrant a thorough investigation of the physics of this defect. Here, we present a first-principles characterization of the T-center in silicon. We examine the electronic structure of the defect, including its optical and spin properties in addition to stability and vibrational properties of the emitter ground state, and discuss this in the light of the recent experimental developments.

[1] L. Bergeron, C. Chartrand, A. T. K. Kurkjian, K. J. Morse, H. Riemann, N. V. Abrosimov, P. Becker, H.-J. Pohl, M. L. W. Thewalt, and S. Simmons, PRX Quantum 1, 020301 (2020).

Realizing quantum networks requires quantum registers next to optically addressable qubits. A variety of
approaches have been reported including recent work on nuclear-spin pairs and single electron spins
surrounding nitrogen-vacancy centers [1, 2]. Here, we investigate a qubit encoded in a pair of electron spins
formed by two P1 centers. We find long inhomogeneous dephasing times of tens of milliseconds, due to a
combination of a decoherence-free subspace and a clock transition. By observing the spin pairs’ temporal
evolution and utilizing RF pulse sequences to address various transitions, we characterize the internal
interactions and resolve the characteristic degrees of freedom of the P1 centers; the Jahn-Teller distortion and
nitrogen state. Using these extra degrees of freedom, we find the P1 centers’ relative position. This work
provides a novel qubit with potential applications in quantum networks and sensing, whilst enriching the
understanding of electronic spin-pair dynamics.

1.Bartling, H. P. et al. Coherence and entanglement of inherently long-lived spin pairs in diamond.
arXiv:2103.07961 [quant-ph] (2021).
2.Degen, M. J. et al. Entanglement of dark electron-nuclear spin defects in diamond. Nature
Communications 12, 3470 (2021).

Deterministic placement of dopant atoms using STM-based hydrogen depassivation lithography enables atom-based device structures with applications ranging from novel low dimensional quantum materials to devices for quantum information processing. This technique enables fabrication of planar atomic-scale dopant profiles with extremely high density and abrupt interfaces. For quantum applications it is often required to be able to provide well-aligned micron scale support structures for atomically defined devices such as antennas (for coherent manipulation) and top gates (for efficient chemical potential control). In this talk we present a single electron quantum device with a central island and a set of two in-plane gates used to capacitively control the flow of current between source/drain-leads and the quantum dot island. We demonstrate successful integration of a low-thermal budget top gate using an e-beam grown HfO2 dielectric layer while maintaining the functional integrity of the buried atomic device. Compared with the STM-patterned in-plane gates, we demonstrate expanded gating range and improved tunability of the SET island tunnel-coupled few-dopant quantum dot using the top gate and find good agreement between the experimental results and capacitance simulations. We will discuss the potential applications of using a top gate to globally address scaled-up arrays of donor-dots as well as future directions of top-gate designs. We will also present new alignment strategies that improve back-end gate alignment to atomic-scale STM patterned devices.

Point-defects in silicon carbide (SiC) have been shown to be compelling analogues of diamond NV centers and have been increasingly recognized as a candidate platform for quantum information science (QIS) and nanoscale sensing. Polytypism in SiC presents a materials design parameter that is not available in diamond and has the potential to create and control new solid-state quantum systems. We used first-principles modeling techniques based on the density functional theory to compute the spectroscopic features of divacancies in SiC that are in close proximity to interfaces between 3C, 2H, 4H and 6H polytypes. We will discuss pathways to harness polytypism in SiC for QIS applications where growth processes may be used to embed spin-defects with tailored properties in optoelectronic devices.

Polyoxometalates (POMs) with localized spins have potential as molecular qubits for quantum computing applications. POMs may incorporate magnetic atoms such as V in their structures, producing novel molecules with promising electro/magneto-optical properties. Nevertheless, for eventual applications, molecular qubits need to be arranged in optically-addressable arrays which imposes unavoidable interactions with underlying supports and adjacent POMs. Specifically, spin-lattice (phonon) coupling is an influential decoherence mechanism that remains insufficiently understood for supported molecular qubits. Herein, we synthesized W-based POMs with different numbers of V atoms and transferred them into the gas phase using electrospray ionization. Ion soft landing, a versatile surface modification technique, was used to deliver mass-selected POMs with predetermined V-composition to different self-assembled monolayer surfaces. Alkylthiol, hydrophobic perfluorinated alkylthiol, and hydrophilic carboxylic-acid terminated surfaces on gold were selected as well-defined model supports with which to characterize POM-substrate and POM-POM interactions. Infrared reflection absorption spectroscopy, scattering-type scanning near-field optical microscopy (s-SNOM), and density functional theoretical calculations provide insight into the properties of supported POMs, how they are influenced by V-doping, and how they are perturbed by interaction with the different supports. Spatially-resolved s-SNOM results reveal the VPOM distribution on the supports and the effect of surface coverage on the POM-SAM and POM-POM interactions. The electronic properties of the bare POMs are determined by negative ion photoelectron spectroscopy and cyclic voltammetry, while the supported POMs are investigated by electrostatic force microscopy and scanning Kelvin probe microscopy. Our results provide fundamental insight into how substrate and intermolecular interactions influence the properties of supported molecular qubits, which is central to manipulating their coherence times for quantum computing applications.

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, using the NV center for nanoscale imaging and spectroscopy requires the stabilization of coherent NV centers within nanometers of the surface, as well as the chemical immobilization of target molecules to the surface. Diamond surfaces are inert and sterically hindered, making wet chemical functionalization challenging. Typical surface modification techniques involving plasma and high temperature processing can lead to surface damage that degrades NV properties, and the harsh conditions severely limit the scope of target moieties. I will describe our recent work to develop a surface reaction discovery pipeline, which has resulted in an extensive library of wet chemical functionalization reactions that utilize gentle conditions, expanding the scope of molecules that can be attached to the diamond surface. We also show that these chemistries are compatible with coherent, shallow NV centers, paving the way to nanoscale magnetic resonance imaging and spectroscopy with NV centers in diamond.

Spin-phonon (s-ph) decoherence governs the intrinsic limit of spintronic and quantum devices, so its understanding is a pressing matter in spin-based technologies. First-principles calculations enable precise predictions of s-ph interactions. However, understanding spin decoherence and relaxation due to phonons remains an open challenge. There are two main s-ph decoherence mechanisms in solids – the Elliott-Yafet (EY) and D’yakonov-Perel’ (DP) – with distinct physical origins and theoretical treatments. Here we present a unified many-body approach for s-ph decoherence, which encompasses both the EY and DP mechanisms. Our method combines the ab-initio s-ph interactions with a diagrammatic approach to treat interacting spins. We apply our method to various centrosymmetric and noncentrosymmetric materials, where the EY and DP mechanisms are dominant, respectively. In both cases, our computed spin relaxation times are in excellent agreement with experiments. We provide evidence that our diagrammatic approach captures both the EY and DP spin decoherence, regardless of material symmetry. Our work demonstrates a widely applicable approach for quantitative analysis of spin decoherence in various qubits and in their bulk or 2D host materials.

Understanding the effects of vibrational modes on solid-state qubits proves a major challenge in developing robust spin-photon interfaces for semiconductor quantum computing architectures. Donor spins in silicon are known to exhibit remarkably long-coherence times, making them attractive candidates for qubits, however the semiconductor environment introduces strong electron-phonon interactions which adversely affect the fidelity of the spin-photon interface, and therefore our ability to entangle qubits and perform quantum gate computations. In order to better understand the role of electron-phonon couplings in these systems, we study a microscopic model that captures the physical mechanisms inherent to these interactions in semiconductor substrates. We report on calculated fluorescence emission spectra that closely reproduce experimentally observed phonon sideband formation, as well as zero-temperature optical transition linewidth broadening due to non-local electron-phonon couplings. We explore a range of parameters permitted by the model and discuss implications for experimental design.

The realisation of a large-scale error corrected quantum computer relies on our ability to reproducibly manufacture qubits that are fast, highly coherent, controllable and stable. The promise of achieving this in a highly manufacturable platform such as silicon requires a deep understanding of the materials issues that impact device operation. In this talk I will demonstrate our progress to engineer every aspect of device behaviour in atomic qubits in silicon. This will cover the use of atomic precision lithography to achieve fast, controllable exchange coupling [1], fast, high fidelity qubit initialisation and read-out [2]; low noise all epitaxial gates allowing for highly stable qubits [3]; and qubit control [4] that provide a deep understanding of the impact of the solid-state environment [5] on qubit designs and operation. I will also discuss our latest results in analogue quantum simulation.

[1] Y. He, S.K. Gorman, D. Keith, L. Kranz, J.G. Keizer, and M.Y. Simmons, “A fast (∼ns) two-qubit gate between phosphorus donor electrons in silicon”, Nature 571, 371 (2019)
[2] D. Keith, M. G. House, M. B. Donnelly, T. F. Watson, B. Weber, M. Y. Simmons, “Microsecond Spin Qubit Readout with a Strong-Response Single Electron Transistor”, Physical Review X 9, 041003 (2019); D. Keith, S. K. Gorman, L. Kranz, Y. He, J. G. Keizer, M. A. Broome, M. Y. Simmons, “Benchmarking high fidelity single-shot readout of semiconductor qubits”, New Journal of Physics 21, 063011 (2019).
[3] L. Kranz S. K. Gorman B. Thorgrimsson Y. He D. Keith J. G. Keizer M. Y. Simmons, “Exploiting a Single-Crystal Environment to Minimize the Charge Noise on Qubits in Silicon”, Advanced Materials 32, 2003361 (2020).
[4] L. Fricke, S.J. Hile, L. Kranz, Y. Chung, Y. He, P. Pakkiam, J.G. Keizer, M.G. House and M.Y. Simmons, “Coherent spin control of a precision placed donor bound electron qubit in silicon”, Nature Communications 12, 3323 (2021).
[5] M. Koch, J.G. Keizer, P. Pakkiam, D. Keith, M.G. House, E. Peretz, and M.Y. Simmons, “Spin read-out in atomic qubits in an all-epitaxial three-dimensional transistor”, Nature Nanotechnology 14, 137 (2019 – with cover article).

Transition metal dichalcogenides (TMDC) have recently emerged as potential candidates to host coherent qubits to realize quantum information technologies in two-dimensional materials platforms [1]. Prospective qubit candidates include quantum dots, valley qubits, and spin defects [2,3,4]. Notably, a recent experimental study reported the creation of a localized electron spin by using a carbon radical ion in WS₂, which could be further developed to a spin qubit system [5]. In this study, we theoretically investigate the decoherence time of spin defects in MX₂ TMDC materials (M=Mo, W and X=S, Se, Te). We compute the Hahn-echo decoherence time (T₂) of an electron spin associated with carbon radical ions in TMDC by combining a cluster correlation expansion method (CCE) and density functional theory [6]. We found that the T₂ time of TMDC materials ranges from 1.6 ms to 36 ms, and the longest T₂ time was found for WS₂. We also discuss the microscopic mechanism of the decoherence in TMDC materials by analyzing their spin Hamiltonian terms. Our results show that TMDCs are promising materials to host robust spin qubits with long coherence time, which would be crucial for their potential applications in quantum sensing and quantum information processing.

[1] Liu, X., Hersam and M.C., Nat. Rev. Mater. 4, 669–684 (2019).
[2] Ye, M., Seo, H. and Galli, G., npj Comp. Mater. 5, 44 (2019).
[3] Onizhuk, M. and Galli, G., Appl. Phys. Lett. 118, 154003 (2021).
[4] Kormányos, A. et al., Phys. Rev. X 4, 011034 (2014).
[5] Cochrane, K. A. et al., arXiv:2008.12196 (2020).
[6] Seo, H. et al., Nat. comm. 7, 12935 (2016).

Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units to develop a wide range of solid-state quantum technologies [1]. For such applications, the long coherence time of NV centers is crucial. Numerous previous studies identified that the NV’s decoherence is often governed by the magnetic noise produced by the 13C nuclear spin bath and the nitrogen (P1) electron spin bath in a diamond. While the 13C-induced decoherence has been well understood, the understanding of the P1-driven decoherence is still incomplete. In this study, we aim at a systematic investigation on the P1-driven decoherence of NV ensembles with varying P1 concentrations from 1ppm to 100ppm by combining cluster correlation expansion theory and density functional theory. We find an excellent agreement between our theoretical results and previous experimental results [2,3]. We also discuss the microscopic mechanism of the P1-induced decoherence of the diamond NV center in detail. Our results provide a key reference, which can be employed to optimize the NV’s performance in various quantum applications such as NV-based magnetometry and NV-based quantum sensors.

[1] G. Wolfowicz et al., Nat. Rev. Meter. 6, 869 (2021).
[2] E. Bauch et al., PRB 102, 134210 (2020).
[3] V. Stepanov et al., PRB 94, 024421 (2016).

Negatively charged boron vacancy (V_B^-) in hexagonal boron nitride (h-BN) has recently emerged as a new promising spin qubit candidate in 2-dimensional materials hosts for solid-state quantum applications [1]. Their spin coherence time (T₂), however, was measured to be very short limited to a few microseconds [2], which is mainly due to their interaction to the inherent dense nuclear spin bath of the h-BN host [3]. In this study, we theoretically propose ways to enhance the quantum coherence of the V_B^- spin in h-BN by using isotopic and strain engineering. We combine density functional theory and cluster correlation expansion to compute the decoherence of V_B^- spins induced by the dense nuclear spin bath of h-BN. We show that inhomogeneous strain can create spatially varying nuclear quadrupole interaction in h-BN, which can significantly suppress the nuclear spin flip-flop dynamics in the bath. In addition, we find that the coherence time of the V_B^- spin can be effectively engineered by adjusting the ratio of ¹⁰B and ¹¹B isotopes in h-BN. We show that the combination of the two methods could increase the T₂ time by 4.5 times larger than the T₂ time in a pristine h-BN bulk. Our results provide not only a fundamental understanding of the decoherence of V_B^- spins in h-BN, but pave the way to engineer their T₂ time, which is crucial for their practical applications.

[1] A. Gottscholl et al., Nat. Mater. 19, 540-545 (2020)
[2] A. Gottscholl et al., Sci. Adv. 7, eabf3630 (2021).
[3] M. Ye, H. Seo, and G. Galli, npj Comp. Mater. 5, 44 (2019).

Engineering robust solid-state quantum systems is amongst the most pressing challenges to realize scalable quantum photonic circuitry. While several 3D systems (such as diamond or silicon carbide) have been thoroughly studied, solid state emitters in two dimensional (2D) materials are still in their infancy.
In this presentation I will discuss the appeal of an emerging van der Waals crystal – hexagonal boron nitride (hBN). This unique system possesses a large bandgap of ~ 6 eV and can host single defects that can act as ultra-bright quantum light sources. In addition, some of these defects exhibit spin dependent fluorescence that can be initialised and coherently manipulated.
Throughout the presentation I will focus on exploring the crystallographic origin of these defects, and potential means to engineer them on demand. I will then discuss on best pathways to integrate these defects with optical cavities and resonators to realise hBN quantum photonic circuitry. I fill also present challenges and opportunities in engineering hBN devices and will frame it more broadly in the growing interest with 2D materials nanophotonics.

Atomically thin, semiconducting, transition metal dichalcogenides, with their band gap being direct in monolayer have intriguing opto-electronic properties like valley selective circular dichroism, spin-valley locking, chiral phonons at high symmetry points of honeycomb lattice. Single photon emitters have been observed in these materials which are believed to be excitons trapped in defect potential. These quantum emitters inherit the two-dimensional nature and valley degree of freedom from the host material. I will talk about these localized excitons, where we observed entanglement between photons emitted from these quantum dots and chiral phonons in the host material ^[1]. For the charged localized excitons having an excess single spin, we observed that we can initialize a single spin optically ^[2], taking the advantage of valley selective circular dichroism and spin-valley locking of the host material. I will conclude by discussing the advantage that the two-dimensional nature of these materials presents in easily forming a van der Waal heterostructure, where the localized excitons have a permanent dipole moment and their emissions can be tuned electrically ^[3].
[1] Chen, X., Lu, X., Dubey, S. et al., Nature Physics 15, 221 (2019)
[2] Lu, X., Chen, X., Dubey, S. et al., Nature Nanotechnology 14, 426 (2019)
[3] Li, W., Lu, X., Dubey, S., Devenica, L. and Srivastava, A., Nature Materials 19, 624 (2020)

Quantum bit as the heart of quantum information technology brings unprecedented capability of computation that is expected to transform science and society in unimaginable ways. While solid-state defects in three-dimensional hosts such as diamond-NV centers have demonstrated as a promising qubit, two-dimensional (2D) materials offer a new paradigm for the realization of patterned qubit fabrication and operation at room temperature. Using high-throughput atomistic simulations and a symmetry-based hypothesis, we identify six neutral anion-antisite defects in transition metal dichalcogenide (TMD) monolayers that host a paramagnetic triplet ground state. The optical transitions and triplet-singlet intersystem crossings in the qubit providing a complete cycle for initialization, manipulation and redout of the qubit are discussed. As an illustrative example, the operational principles of the antisite qubit in WS₂ are discussed in details. The key characters of host materials that give rise to defects with a triplet ground state are discussed, which suggests a feasible strategy for continued discovery of promising defect qubits in diverse classes of 2D materials. Our study opens a new pathway for creating scalable and controllable spin qubits in 2D TMDs.

The microscopic understanding of defect-related modifications in 2D materials requires correlation between atomic structure and resulting macroscopic electronic, optical or excitonic properties. Combing controlled defect engineering with optical spectroscopy as well as atomic imaging and ab-initio theory, we identify the optical signature of pristine chalcogen vacancies in single layer MoS₂. [1] Vacancies introduce a narrow optical emission, markedly different from previously observed broad luminescence bands. Comparing annealed vs. He-ion treated MoS₂, we establish that the recently discovered single-photon emitters in He-ion irradiated MoS₂ originate from chalcogen vacancies. Using focused ion beam irradiation, the latter can be created site-selectively [2] with a spatial precision better than 10 nm [3], which is important for a prospective integration of defect-based single photon emitters into quantum photonic circuits.
[1] E. Mitterreiter et al., Nat. Commun. 12, 3822 (2021).
[2] J. Klein et al. ACS Photonics 8, 669-677 (2021).
[3] E. Mitterreiter et al., Nano Lett. 20, 4437 (2020).

Atomically thin transition metal dichalcogenides (TMDs), including WS2, WSe2, MoS2, MoSe2, have attracted considerable interest due to their promising applications in electronic and optoelectronic devices.[i],[ii],[iii] Defects in TMD monolayers (MLs) can serve as excitonic traps, leading to exciton localization and even single photon emission.[iv],[v],[vi] In this study, we systematically investigate the influence of strain profiles on the emission properties of WSe2 MLs by suspending them onto trenches with various widths. A red shift in the photoluminescence spectra can be observed as the trench width increases, indicating strain-induced bandgap narrowing in the WSe2 MLs. Abundance of defects were created in the MLs at specific regions suspended above the trenches, consistent with the local strain profiles obtained from numerical simulations. Noteworthy, photon antibunching, a hallmark of single photon emission, was only observed for defects created under certain strain amplitudes. A further increase in the strain amplitude reduces the exciton localization degree. These findings would help elucidate future design of quantum defects in TMDs for quantum optics applications.


[i]. Mak, K., Shan, J. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nature Photon. 10, 216–226 (2016)

[ii]. Lee, J., Mak, K. & Shan, J. Electrical control of the valley Hall effect in bilayer MoS2 transistors. Nature Nanotech 11, 421–425 (2016)

[iii]. Zeng, H., Dai, J., Yao, W., Xiao, X., Cui, X., Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech 7, 490–493 (2012).

[iv]. Palacios-Berraquero, C., Kara, D. M., Montblanch, A. R.P., Barbone, M., Latawiec, P., Yoon, D., Ott, A. K., Loncar, M., Ferrari, A. C., Atatüre, M., Large-scale quantum-emitter arrays in atomically thin semiconductors. Nat Commun 8, 15093 (2017).

[v]. Yu, L., Deng, M., Zhang, J. L., Borghardt, S., Kardynal, B., Vučković, J., and Heinz, T. F., Site-Controlled Quantum Emitters in Monolayer MoSe2, Nano Lett. 21, 6, 2376–2381 (2021)

[vi]. Peng, L., Chan, H., Choo, P., Odom, T. W., Sankaranarayanan, S. K. R. S., Ma, X., Creation of Single-Photon Emitters in WSe2 Monolayers Using Nanometer-Sized Gold Tips, Nano Lett. 20, 8, 5866–5872 (2020)

Quantum effects in 2D materials can manifest at very different length scales. Charge density waves, magnetic ordering, interlayer excitons are now studied also to understand their atomistic origin. Moreover, starting from exciting properties of low-twist angle graphene, twisted transition metal dichalcogenides are now explored, whereby the future of moiré superlattices is also dependent on reliable twist angle control. Here we show advances in sample preparation as well as that the electron probe in our low-voltage-tuneable spherical and chromatic aberration-corrected transmission electron microscope is a powerful tool to measure and, knock-on-damage-threshold-assisted, introduce structural and chemical variations in free-standing 2D materials on the atomic scale. Together with quantum-mechanical calculations their new properties can be understood
Such we report on electron-beam-induced defects and their electronic properties and follow the migration paths and associated property changes in a variety of single and few-layered free-standing structures of transition metal di-chalcogenides (TMDs) and transition metal phosphorus tri-chalcogenides (TMPTs). In addition, we investigate the twist-angle-dependent moiré pattern formation in bilayers of TMDs by theoretical prediction-followed TEM experiments. From the comparison of monolayer, bi-layer and 2° twisted bilayer experimental images we determine twist-angle-induced inhomogeneous stacking-related localized strain in the layers as well as the twist-angle-induced changes of the interlayer excitons located in the low-loss range of the EELS spectrum. On the more fundamental base, we show that differentiating between the bond nature of two metal atoms is now possible when confined in the narrow space of a single walled carbon nanotube and that we begin to image lattice vibration in TEM.

Silicon-vacancy defects have been identified as a promising optical transition for quantum communications, quantum control, and quantum information processing. In the work presented here, we demonstrate a voltage-controlled mechanism by which the photoluminescent (PL) emission from silicon-vacancy (Si-V) defects in diamond can be modulated. In particular, we can selectively produce emission from the negatively charged state of this defect (i.e., Si-V^–), which exhibits narrow (Γ = 4 nm) emission at 738 nm at low laser power. This approach uses high voltage (2-5kV) nanosecond pulses applied across top and bottom electrodes on a 0.5 mm thick diamond substrate. In the absence of high voltage pulses, we observe no emission at 738nm. This feature increases monotonically with peak pulse voltage, pulse repetition rate (i.e., frequency), and incident laser intensity. We observe saturation of the PL intensity for pulse voltages above 3.2 kV and frequency above 100 Hz. Based on electrostatic simulations, we estimated the local electric field intensity near the tip of the Cu electrode to be 1.5 x 10⁵ V/cm at these voltages. However, as a function of laser power, we observe a linear dependence of PL intensity without saturation. These saturating and non-saturating behaviors provide important insight into the voltage-induce charging mechanisms and kinetics associated with this process.