Silicon Vacancy Research Articles

Enhancement of silicon vacancy fluorescence intensity in silicon carbide using a dielectric cavity.

Over the past decades, spin qubits in silicon carbide (SiC) have emerged as promising platforms for a wide range of quantum technologies. The fluorescence intensity holds significant importance in the performance of quantum photonics, quantum information process, and sensitivity of quantum sensing. In this work, a dual-layer Au/SiO2 dielectric cavity is employed to enhance the fluorescence intensity of a shallow silicon vacancy ensemble in 4H-SiC. Experimental results demonstrate an effective fourfold augmentation in fluorescence counts at saturating laser power, corroborating our theoretical predictions. Based on this, we further investigate the influence of dielectric cavities on the contrast and linewidth of optically detected magnetic resonance (ODMR). There is a 1.6-fold improvement in magnetic field sensitivity. In spin echo experiments, coherence times remain constant regardless of the thickness of dielectric cavities. These experiments pave the way for broader applications of dielectric cavities in SiC-based quantum technologies.

Entanglement of nanophotonic quantum memory nodes in a telecom network

A key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected by fibre optical infrastructure1–3. Here we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centres in nanophotonic diamond cavities integrated with a telecommunication fibre network. Remote entanglement is generated by the cavity-enhanced interactions between the electron spin qubits of the SiVs and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits to telecommunication frequencies (1,350 nm), we demonstrate the entanglement of two nuclear spin memories through 40 km spools of low-loss fibre and a 35-km long fibre loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.

Effect of Negative Thermal Expansion on the Zero-Phonon Line of Implanted Silicon Vacancy Color Centers in Diamond

Effect of Negative Thermal Expansion on the Zero-Phonon Line of Implanted Silicon Vacancy Color Centers in Diamond

Annealing process and temperature effects on silicon-vacancy and germanium-vacancy centers in CVD grown polycrystalline diamond

The annealing treatment plays a crucial role in tailoring the properties of synthetic diamond materials, especially those doped with various elements in order to form specific color centers like nitrogen-vacancy (NV), silicon-vacancy (Si-V), germanium-vacancy (Ge-V), etc. This study delves into the annealing of 175 μm-thick Ge-doped polycrystalline diamond (PCD) films grown by microwave plasma-assisted chemical vapor deposition (MPCVD). Large-area PCD plate was cut into smaller equivalent 5 × 5 mm2 pieces, which were separately subjected to annealing in microwave plasma in H2 atmosphere, to annealing in vacuum or to annealing under high-pressure high-temperature conditions (HPHT, 5.9 GPa, 2000 °C). The structure, phase composition and photoluminescence (PL) of samples before and after various annealing processes were investigated. All applied types of annealing enhance both the Si-V and Ge-V lines in PL at room temperature. Increasing annealing temperature leads to gradual decrease of full widths at half-maxima (FWHM) of diamond Raman peak (1332.5 cm−1), as well as Si-V (738 nm) and Ge-V (602 nm) PL peaks. In addition, the limitations for each type of annealing are established. The obtained results are crucial for the design of CVD-grown Ge-doped and Si-doped PCD materials that can be used for applications in photonics such as single photon sources, biomarkers, as well as for the fabrication of optical diamond thermometers.

Limitations in design and applications of ultra-small mode volume photonic crystals

Ultra-small mode volume nanophotonic crystal cavities have been proposed as powerful tools for increasing coupling rates in cavity quantum electrodynamics systems. However, their adoption in quantum information applications remains elusive. In this work, we investigate possible reasons why, and analyze the impact of different low mode volume resonator design choices on their utility in quantum optics experiments. We analyze band structure features and loss rates of low mode volume bowtie cavities in diamond and demonstrate independent design control over cavity-emitter coupling strength and loss rates. Further, using silicon vacancy centers in diamond as exemplary emitters, we investigate the influence of placement imprecision. We find that the benefit on photon collection efficiency and indistinguishability is limited, while the fabrication complexity of ultra-small cavity designs increases substantially compared to conventional photonic crystals. We conclude that ultra-small mode volume designs are primarily of interest for dispersive spin-photon interactions, which are of great interest for future quantum networks.

Experimental Generation of Spin-Photon Entanglement in Silicon Carbide.

A solid-state approach for quantum networks is advantageous, as it allows the integration of nanophotonics to enhance the photon emission and the utilization of weakly coupled nuclear spins for long-lived storage. Silicon carbide, specifically point defects within it, shows great promise in this regard due to the easy of availability and well-established nanofabrication techniques. Despite of remarkable progresses made, achieving spin-photon entanglement remains a crucial aspect to be realized. In this Letter, we experimentally generate entanglement between a silicon vacancy defect in silicon carbide and a scattered single photon in the zero-phonon line. The spin state is measured by detecting photons scattered in the phonon sideband. The photonic qubit is encoded in the time-bin degree of freedom and measured using an unbalanced Mach-Zehnder interferometer. Photonic correlations not only reveal the quality of the entanglement but also verify the deterministic nature of the entanglement creation process. By harnessing two pairs of such spin-photon entanglement, it becomes straightforward to entangle remote quantum nodes at long distance.

Silicon Vacancies Diamond/Silk/PVA Hierarchical Physical Unclonable Functions for Multi-Level Encryption.

Physical unclonable functions (PUFs) have emerged as a promising encryption technology, utilizing intrinsic physical identifiers that offer enhanced security and tamper resistance. Multi-level PUFs boost system complexity, thereby improving system reliability and fault tolerance. However, crosstalk-free multi-level PUFs remain a persistent challenge. In this study, a hierarchical PUF system that harnesses the spontaneous phase separation of silk fibroin /PVA blend and the random distribution of silicon-vacancy diamonds within the blend is presented. The thermodynamic instability of phase separation and inherent unpredictability of diamond dispersion gives rise to intricate random patterns at two distinct scales, enabling time-efficient hierarchical authentication for cryptographic keys. These patterns are complementary yet independent, inherently resistant to replication and damage thus affording robust security and reliability to the proposed system. Furthermore, customized authentication algorithms are constructed: visual PUFs authentication utilizes neural network combined structural similarity index measure, while spectral PUFs authentication employs Hamming distance and cross-correlation bit operation. This hierarchical PUF system attains a high recognition rate without interscale crosstalk. Additionally, the coding capacity is exponentially enhanced using M-ary encoding to reinforce multi-level encryption. Hierarchical PUFs hold significant potential for immediate application, offering unprecedented data protection and cryptographic key authentication capabilities.

Spectral stability of V2 centres in sub-micron 4H-SiC membranes

Colour centres in silicon carbide emerge as a promising semiconductor quantum technology platform with excellent spin-optical coherences. However, recent efforts towards maximising the photonic efficiency via integration into nanophotonic structures proved to be challenging due to reduced spectral stabilities. Here, we provide a large-scale systematic investigation on silicon vacancy centres in thin silicon carbide membranes with thicknesses down to 0.25 μm. Our membrane fabrication process involves a combination of chemical mechanical polishing, reactive ion etching, and subsequent annealing. This leads to highly reproducible membranes with roughness values of 3–4 Å, as well as negligible surface fluorescence. We find that silicon vacancy centres show close-to lifetime limited optical linewidths with almost no signs of spectral wandering down to membrane thicknesses of ~0.7 μm. For silicon vacancy centres in thinner membranes down to 0.25 μm, we observe spectral wandering, however, optical linewidths remain below 200 MHz, which is compatible with spin-selective excitation schemes. Our work clearly shows that silicon vacancy centres can be integrated into sub-micron silicon carbide membranes, which opens the avenue towards obtaining the necessary improvements in photon extraction efficiency based on nanophotonic structuring.

Unraveling sources of emission heterogeneity in Silicon Vacancy color centers with cryo-cathodoluminescence microscopy

Diamond color centers have proven to be versatile quantum emitters and exquisite sensors of stress, temperature, electric and magnetic fields, and biochemical processes. Among color centers, the silicon-vacancy (SiV[Formula: see text]) defect exhibits high brightness, minimal phonon coupling, narrow optical linewidths, and high degrees of photon indistinguishability. Yet the creation of reliable and scalable SiV[Formula: see text]-based color centers has been hampered by heterogeneous emission, theorized to originate from surface imperfections, crystal lattice strain, defect symmetry, or other lattice impurities. Here, we advance high-resolution cryo-electron microscopy combined with cathodoluminescence spectroscopy and 4D scanning transmission electron microscopy (STEM) to elucidate the structural sources of heterogeneity in SiV[Formula: see text] emission from nanodiamond with sub-nanometer-scale resolution. Our diamond nanoparticles are grown directly on TEM membranes from molecular-level seedings, representing the natural formation conditions of color centers in diamond. We show that individual subcrystallites within a single nanodiamond exhibit distinct zero-phonon line (ZPL) energies and differences in brightness that can vary by 0.1 meV in energy and over 70% in brightness. These changes are correlated with the atomic-scale lattice structure. We find that ZPL blue-shifts result from tensile strain, while ZPL red shifts are due to compressive strain. We also find that distinct crystallites host distinct densities of SiV[Formula: see text] emitters and that grain boundaries impact SiV[Formula: see text] emission significantly. Finally, we interrogate nanodiamonds as small as 40 nm in diameter and show that these diamonds exhibit no spatial change to their ZPL energy. Our work provides a foundation for atomic-scale structure-emission correlation, e.g., of single atomic defects in a range of quantum and two-dimensional materials.

Real Structure, Magnetism and Chemical Bonding of SmSi3‐x

New metastable SmSi3‐x (x = 0‐0.05) is obtained by high‐pressure high‐temperature synthesis (9.5 GPa, 870‐1270 K). Powder diffraction data refinements reveal that the crystal structure of SmSi3 is isotypic to that of YbSi3 (space group I4/mmm, a = 7.23634(5) Å, c = 11.0854(1) Å). In the crystal structure, two types of Si2 dumbbells agglomerate into layers, which embed the samarium atoms. At ambient pressure, SmSi3 decomposes exothermally upon heating into Si and SmSi2‐x. Single‐crystal structure refinements of a specimen SmSi3‐x (x=0.05) reveal considerable electron density, which is not accounted for by the YbSi3 model. The additional maxima can be assigned to disorder which affects the samarium positions and induces silicon vacancies. Scanning transmission electron microscopy experiments evidence that the disorder can be attributed to extended defects. Magnetic measurements on SmSi3‐x reveal van Vleck paramagnetic behavior and antiferromagnetic ordering at low temperatures. Computations within the local spin density approximation (LSDA and LSDA+U) on the crystal structure of SmSi3 reproduce the anti­ferro­magnetic coupling as the favored long‐range order. Quantum chemical analysis of the chemical bonding in SmSi3 reveals two‐center two‐electron bonds within the Si2 dumbbells plus a total of a little less than four electrons in lone pairs at each silicon atom.

Plasmonic Diamond Membranes for Ultrafast Silicon Vacancy Emission.

Silicon vacancy centers (SiVs) in diamond have emerged as a promising platform for quantum sciences due to their excellent photostability, minimal spectral diffusion, and substantial zero-phonon line emission. However, enhancing their slow nanosecond excited-state lifetime by coupling to optical cavities remains an outstanding challenge, as current demonstrations are limited to ∼10-fold. Here, we couple negatively charged SiVs to sub-diffraction-limited plasmonic cavities and achieve an instrument-limited ≤8 ps lifetime, corresponding to a 135-fold spontaneous emission rate enhancement and a 19-fold photoluminescence enhancement. Nanoparticles are printed on ultrathin diamond membranes on gold films which create arrays of plasmonic nanogap cavities with ultrasmall volumes. SiVs implanted at 5 and 10 nm depths are examined to elucidate surface effects on their lifetime and brightness. The interplay between cavity, implantation depth, and ultrathin diamond membranes provides insights into generating ultrafast, bright SiV emission for next-generation diamond devices.

Photo-Induced Charge State Dynamics of the Neutral and Negatively Charged Silicon Vacancy Centers in Room-Temperature Diamond.

The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. Comparatively little is known, however, about the dynamics governing SiV charge state interconversion mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. Here, multi-color confocal microscopy and density functional theory are used to examine photo-induced SiV recombination - from neutral, to single-, to double-negatively charged - over a broad spectral window in chemical-vapor-deposition (CVD) diamond under ambient conditions. For the SiV0 to SiV- transition, a linear growth of the photo-recombination rate with laser power at all observed wavelengths is found, a hallmark of single photon dynamics. Laser excitation of SiV‒, on the other hand, yields only fractional recombination into SiV2‒, a finding that is interpreted in terms of a photo-activated electron tunneling process from proximal nitrogen atoms.

Qudit-Based Spectroscopy for Measurement and Control of Nuclear-Spin Qubits in Silicon Carbide.

Nuclear spins with hyperfine coupling to single electron spins are highly valuable quantum bits. Here we probe and characterize the particularly rich nuclear-spin environment around single silicon vacancy color centers (V2) in 4H-SiC. By using the electron spin-3/2 qudit as a four level sensor, we identify several sets of ^{29}Si and ^{13}C nuclear spins through their hyperfine interaction. We extract the major components of their hyperfine coupling via optical detected nuclear magnetic resonance, and assign them to shells in the crystal via the density function theory simulations. We utilize the ground-state level anticrossing of the electron spin for dynamic nuclear polarization and achieve a nuclear-spin polarization of up to 98±6%. We show that this scheme can be used to detect the nuclear magnetic resonance signal of individual spins and demonstrate their coherent control. Our work provides a detailed set of parameters and first steps for future use of SiC as a multiqubit memory and quantum computing platform.

Fast optoelectronic charge state conversion of silicon vacancies in diamond.

Group IV vacancy color centers in diamond are promising spin-photon interfaces with strong potential for applications in photonic quantum technologies. Reliable methods for controlling and stabilizing their charge state are urgently needed for scaling to multiqubit devices. Here, we manipulate the charge state of silicon vacancy (SiV) ensembles by combining luminescence and photocurrent spectroscopy. We controllably convert the charge state between the optically active SiV- and dark SiV2- with megahertz rates and >90% contrast by judiciously choosing the local potential applied to in-plane surface electrodes and the laser excitation wavelength. We observe intense SiV- photoluminescence under hole capture, measure the intrinsic conversion time from the dark SiV2- to the bright SiV- to be 36.4(67) ms, and demonstrate how it can be enhanced by a factor of 105 via optical pumping. Moreover, we obtain previously unknown information on the defects that contribute to photoconductivity, indicating the presence of substitutional nitrogen and divacancies.

Selective Generation of V2 Silicon Vacancy Centers in 4H-Silicon Carbide.

The deterministic generation of individual color centers with defined orientations or types in solid-state systems is paramount for advancements in quantum technologies. Silicon vacancies in 4H-silicon carbide (4H-SiC) can be formed in V1 and V2 types. However, silicon vacancies are typically generated randomly between V1 and V2 types with similar probabilities. Here, we show that the preferred V2 centers can be selectively generated by focused ion beam (FIB) implantation on the m-plane in 4H-SiC. When implantation is on the m-plane (a-plane), the generation probability ratio between V1 and V2 centers increase exponentially (remains constant) with decreasing FIB fluences. With a fluence of 10 ions/spot, the probability to generate V2 centers is seven times higher than V1 centers. Our results represent a critical step toward the deterministic creation of specific defect types.

Heisenberg-limited spin squeezing in a hybrid system with silicon-vacancy centers.

In this paper, we investigate the spin squeezing in a hybrid quantum system consisting of a Silicon-Vacancy (SiV) center ensemble coupled to a diamond acoustic waveguide via the strain interaction. Two sets of non-overlapping driving fields, each contains two time-dependent microwave fields, are applied to this hybrid system. By modulating these fields, the one-axis twist (OAT) interaction and two-axis two-spin (TATS) interaction can be independently realized. In the latter case the squeezing parameter scales to spin number as ξ R2∼1.61N -0.64 with the consideration of dissipation, which is very close to the Heisenberg limit. Furthermore, this hybrid system allows for the study of spin squeezing generated by the simultaneous presence of OAT and TATS interactions, which reveals sensitivity to the parity of the number of spins Ntot, whether it is even or odd. Our scheme enriches the approach for generating Heisenberg-limited spin squeezing in spin-phonon hybrid systems and offers the possibility for future applications in quantum information processing.

Creation of color centers in diamond by recoil implantation through dielectric films

The need of near-surface color centers in diamond for quantum technologies motivates the controlled doping of specific extrinsic impurities into the crystal lattice. Recent experiments have shown that this can be achieved by momentum transfer from a surface precursor via ion implantation, an approach known as “recoil implantation.” Here, we extend this technique to incorporate dielectric precursors for creating nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers in diamond. Specifically, we demonstrate that gallium focused-ion-beam exposure to a thin layer of silicon nitride or silicon dioxide on the diamond surface results in the introduction of both extrinsic impurities and carbon vacancies. These defects subsequently give rise to near-surface NV and SiV centers with desirable properties after annealing.

Telecom Networking with a Diamond Quantum Memory

Practical quantum networks require interfacing quantum memories with existing channels and systems that operate in the telecom band. Here we demonstrate low-noise, bidirectional quantum frequency conversion that enables a solid-state quantum memory to directly interface with telecom-band systems. In particular, we demonstrate conversion of visible-band single photons emitted from a silicon-vacancy (SiV) center in diamond to the telecom O band, maintaining low noise (g2(0)<0.1) and high indistinguishability (V=89±8%). We further demonstrate the utility of this system for quantum networking by converting telecom-band time-bin pulses, sent across a lossy and noisy 50-km deployed fiber link, to the visible band and entangling them with a diamond quantum memory with fidelity F≥87±2.5%. These results demonstrate the viability of SiV quantum memories integrated with telecom-band systems for scalable quantum networking applications. Published by the American Physical Society 2024

Residual optical absorption from native defects in CdSiP2 crystals

CdSiP2 crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (VSi−) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP2 crystal is near 80 K converts VSi− acceptors to their neutral and doubly ionized charge states (VSi0 and VSi2−, respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed.

First-principles calculation of intrinsic point defects and doping performance of MoSi<sub>2</sub>N<sub>4</sub>

MoSi<sub>2</sub>N<sub>4</sub> is an emergent two-dimensional (2D) material, which has received much attention because of its excellent performance over semiconductors, including excellent environmental stability and high carrier mobility. However, the formation of intrinsic defects in semiconductors is often inevitable and can significantly affect device performance. By using density functional theory (DFT), we analyze the properties and effects of intrinsic point defects in MoSi<sub>2</sub>N<sub>4</sub>. We first confirm the consistency of our results with current experimental data. After that, the formation energy values of twelve native defects reveal that the antisite defect of molybdenum substituting for silicon (Mo<sub>Si</sub>) defect dominates in all intrinsic defects. Under the constraint of overall charge neutrality, self-consistent Fermi level calculations reveal that MoSi<sub>2</sub>N<sub>4</sub> with only intrinsic defects exhibits intrinsic characteristics, highlighting its potential as a semiconductor device material. However, this intrinsic nature contradicts the p-type characteristics observed in two-dimensional MoSi<sub>2</sub>N<sub>4</sub>. In the subsequent defect concentrations, we find that both n-type and p-type behavior can be easily realized by doping appropriate impurities without being compensated by native defects. This suggests that the p-type characteristics of MoSi<sub>2</sub>N<sub>4</sub> during growth may result from p-type impurities introduced under non-equilibrium growth conditions or silicon vacancy defects. Our findings not only demonstrate the potential applications of MoSi<sub>2</sub>N<sub>4</sub> in semiconductor devices but also provide valuable guidance for future studying the defect mechanisms of this material.