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The ability to emit narrow exciton lines, preferably with a clearly defined polarization, is one of the key conditions for the use of nanostructures based on III-VI monochalcogenides and other layered crystals in quantum technology to create non-classical light. Currently, the main method of their formation is exfoliation followed by strain and defect engineering. A factor limiting the use of epitaxy is the presence of different phases in the grown films. In this work, we show that control over their formation makes it possible to create structures with the desired properties. We propose Ga$_2$Se$_3$/GaSe nanostructures by van der Waals epitaxy with a high VI/III flux ratio as a source of narrow exciton lines. Actually, these nanostructures are a combination of allotropes: GaSe and Ga$_2$Se$_3$, consisting of the same atoms in different arrangements. The energy position of the narrow lines is determined by the quantum confinement in Ga$_2$Se$_3$ inclusions of different sizes in the GaSe matrix, similar to quantum dots, and their linear polarization is due to the ordering of Ga vacancies in a certain crystalline direction in Ga$_2$Se$_3$. Such nanostructures exhibit single-photon emission with second-order correlation function $g^{(2)}(0)\sim$0.1 at 10 K that makes them promising for quantum technologies.

Ultracompact chip-integrated single-photon sources of collimated beams with polarizationencoded states are crucial for integrated quantum technologies. However, most of currently available single-photon sources rely on external bulky optical components to shape the polarization and phase front of emitted photon beams. Efficient integration of quantum emitters with beam shaping and polarization encoding functionalities remains so far elusive. Here, we present ultracompact single-photon sources of linearly polarized vortex beams based on chip-integrated quantum emitter-coupled metasurfaces, which are meticulously designed by fully exploiting the potential of nanobrick arrayed metasurfaces. We first demonstrate on-chip single-photon generation of high-purity linearly polarized vortex beams with prescribed topological charges of -1, 0, and +1. We further realize multiplexing of single-photon emission channels with orthogonal linear polarizations carrying different topological charges and demonstrate their entanglement. Our work illustrates the potential and feasibility of ultracompact quantum emitter-coupled metasurfaces as a new quantum optics platform for realizing chip-integrated high-dimensional single-photon sources., Comment: 17 pages and 4 figures in the main text; 22 pages and 14 figures in supplementary information

Channelling single-photon emission in multiple well-defined directions and simultaneously controlling its polarization characteristics is highly desirable for numerous quantum technology applications. We show that this can be achieved by using quantum emitters (QEs) nonradiatively coupled to surface plasmon polaritons (SPPs), which are scattered into outgoing free-propagating waves by appropriately designed metasurfaces. The QE-coupled metasurface design is based on the scattering holography approach with radially diverging SPPs as reference waves. Using holographic metasurfaces fabricated around nanodiamonds with single Ge vacancy centers, we experimentally demonstrate on-chip integrated efficient generation of two well-collimated single-photon beams propagating along different 15-degree off-normal directions with orthogonal linear polarizations.

Impurity-vacancy centers in diamond offer a new class of robust photon sources with versatile quantum properties. While individual color centers commonly act as single-photon sources, their ensembles have been theoretically predicted to have tunable photon-emission statistics. Importantly, the particular type of excitation affects the emission properties of a color center ensemble within a diamond crystal. While optical excitation favors non-synchronized excitation of color centers within an ensemble, electron-beam excitation can synchronize the emitters and thereby provides a control of the second-order correlation function $g_2(0)$. In this letter, we demonstrate experimentally that the photon stream from an ensemble of color centers can exhibit $g_2(0)$ both above and below unity. Such a photon source based on an ensemble of few color centers in a diamond crystal provides a highly tunable platform for informational technologies operating at room temperature.

Synthesized micro- and nanotubes composed of transition metal dichalcogenides (TMDCs) such as MoS$_2$ are promising for many applications in nanophotonics, because they combine the abilities to emit strong exciton luminescence and to act as whispering gallery microcavities even at room temperature. In addition to tubes in the form of hollow cylinders, there is an insufficiently-studied class of twisted tubes, the flattened cross section of which rotates along the tube axis. As shown by theoretical analysis, in such nanotubes the interaction of electromagnetic waves excited at opposite sides of the cross section can cause splitting of the whispering gallery modes. By studying micro-photoluminescence spectra measured along individual MoS$_2$ tubes, it has been established that the splitting value, which controls the energies of the split modes, depends exponentially on the aspect ratio of the cross section, which varies in "breathing" tubes, while the relative intensity of the modes in a pair is determined by the angle of rotation of the cross section. These results open up the possibility of creating multifunctional tubular TMDC nanodevices that provide resonant amplification of self-emitting light at adjustable frequencies.

Optically active color centers in nanodiamonds offer unique opportunities for generating and manipulating quantum states of light. These mechanically, chemically, and optically robust emitters can be produced in mass quantities, deterministically manipulated, and integrated with a variety of quantum device geometries and photonic material platforms. Nanodiamonds with deeply sub-wavelength sizes coupled to nanophotonic structures feature a giant enhancement of light-matter interaction, promising high bitrates in quantum photonic systems. We review the recent advances in controlled techniques for synthesizing, selecting, and manipulating nanodiamond-based color centers for their integration with quantum nanophotonic devices., Comment: 27 pages, 7 figures

Transition metal dichalcogenides (TMDs) is a promising platform for new generation optoelectronics and nanophotonics due to their unique optical properties. However, in contrast to direct bandgap TMDs monolayers, bulk samples have an indirect bandgap that restricts their application as light emitters. On the other hand, the high refractive index of these materials seems ideal for creating high-quality nanophotonic resonators with a strong Purcell effect. In this work, we fabricate Whispering-gallery mode (WGM) resonators from bulk (i.e., indirect bandgap) MoSe$_2$ using resistless scanning probe lithography and study their optical properties. Micro-photoluminescence($\mu$-PL) investigation revealed WGM spectra of resonators with an enhancement factor of 100 compared to pristine flake. Scattering experiments and modeling also confirm the WGM nature of spectra observed. Temperature dependence of PL revealed two components of photoluminescence. The first one quenches with decreasing temperature, the second one does not and becomes dominant. Therefore, this suggests that resonators amplify both direct and temperature-activated indirect PL. Thus, here we demonstrated the novel approach to fabricating nanophotonic resonators from bulk TMDs and obtaining PL from indirect bandgap materials. We believe that the suggested approach and structures have great prospects in nanophotonics., Comment: Main text: 18 pages, 5 figures. Supplementary: 3 pages, 2 figures

Coherent quantum systems are a key resource for emerging quantum technology. Solid-state spin systems are of particular importance for compact and scalable devices. However, interaction with the solid-state host degrades the coherence properties. The negatively-charged silicon vacancy center in diamond is such an example. While spectral properties are outstanding, with optical coherence protected by the defects symmetry, the spin coherence is susceptible to rapid orbital relaxation limiting the spin dephasing time. A prolongation of the orbital relaxation time is therefore of utmost urgency and has been tackled by operating at very low temperatures or by introducing large strain. However, both methods have significant drawbacks, the former requires use of dilution refrigerators and the latter affects intrinsic symmetries. Here, a novel method is presented to prolong the orbital relaxation with a locally modified phonon density of states in the relevant frequency range, by restricting the diamond host to below 100 nm. The method works at liquid Helium temperatures of few Kelvin and in the low-strain regime.

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. We study time-resolved micro-photoluminescence in MoS$_2$ monolayers and bilayers, both unstrained and compressively strained, in a wide temperature range (10-300 K) to distinguish between exciton states optically allowed and forbidden, both in spin and momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times were found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. Our results unambiguously show that, in an unstrained monolayer, the spin-allowed state is the lowest for a series of A excitons (1.9 eV) with the dark state being < 2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in bilayers (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS$_2$.

Coherent exchange of single photons is at the heart of applied Quantum Optics. The negatively-charged silicon vacancy center in diamond is among most promising sources for coherent single photons. Its large Debye-Waller factor, short lifetime and extraordinary spectral stability is unique in the field of solid-state single photon sources. However, the excitation and detection of individual centers requires high numerical aperture optics which, combined with the need for cryogenic temperatures, puts technical overhead on experimental realizations. Here, we investigate a hybrid quantum photonics platform based on silicon-vacancy center in nanodiamonds and metallic bullseye antenna to realize a coherent single-photon interface that operates efficiently down to low numerical aperture optics with an inherent resistance to misalignment.

Spin-based quantum photonics promise to realize distributed quantum computing and quantum networks. The performance depends on efficient entanglement distribution, where the efficiency can be boosted by means of cavity quantum electrodynamics. The central challenge is the development of compact devices with large spin-photon coupling rates and high operation bandwidth. Photonic crystal cavities comprise strong field confinement but put high demands on accurate positioning of an atomic system in the mode field maximum. Color center in diamond, and in particular the negatively-charged Silicon-Vacancy center, emerged as a promising atom-like systems. Large spectral stability and access to long-lived, nuclear spin memories enabled elementary demonstrations of quantum network nodes including memory-enhanced quantum communication. In a hybrid approach, we deterministically place SiV$^-$-containing nanodiamonds inside one hole of a one-dimensional, free-standing, Si$_3$N$_4$-based photonic crystal cavity and coherently couple individual optical transitions to the cavity mode. We optimize the light-matter coupling by utilizing two-mode composition, waveguiding, Purcell-enhancement and cavity resonance tuning. The resulting photon flux is increased by more than a factor of 14 as compared to free-space. The corresponding lifetime shortening to below 460 ps puts the potential operation bandwidth beyond GHz rates. Our results mark an important step to realize quantum network nodes based on hybrid quantum photonics with SiV$^-$- center in nanodiamonds.

Motivated by the success of group IV colour centres in nanodiamonds (NDs) for hybrid technology requiring a single photon source, we study single germanium-vacancy (GeV$^-$) centres in NDs at room temperature with size rangingfrom 10 to 50 nm and with remarkable spectral properties. We characterize their zero-phonon line (ZPL), study their internal population dynamics and compare their emission properties in the framework of a three level model with intensity dependent de-shelving. Furthermore, we characterize their lifetime, polarization and brightness. We find amaximum photon emission count rate of 1.6 MHz at saturation. We also report a polarization visibility of 92% from the fluorescence light, which potentially makes GeV$^-$ centres good candidates for quantum key distribution (QKD)requiring polarized single photons. We show that the GeV$^-$ in NDs presented in this work have a comparable spectral stability compared to their bulk counterpart which is needed for future applications using NDs., Comment: 7 pages, 5 figures

Hybrid quantum photonics combines classical photonics with quantum emitters in a postprocessing step. It facilitates to link ideal quantum light sources to optimized photonic platforms. Optical cavities enable to harness the Purcell-effect boosting the device efficiency. Here, we postprocess a free-standing, crossed-waveguide photonic crystal cavity based on Si$_3$N$_4$ with SiV$^-$ center in nanodiamonds. We develop a routine that holds the capability to optimize all degrees of freedom of the evanescent coupling term utilizing AFM nanomanipulation. After a few optimization cycles we resolve the fine-structure of individual SiV$^-$ centers and achieve a Purcell enhancement of more than 4 on individual optical transitions, meaning that four out of five spontaneously emitted photons are channeled into the photonic device. Our work opens up new avenues to construct efficient quantum photonic devices., Comment: 7 pages, 4 figures. Nanophotonics (2020)

Photons for quantum technologies have been identified early on as a very good candidate for carrying quantum information encoded onto them, either by polarization encoding, time encoding or spatial encoding. Quantum cryptography, quantum communications, quantum networks in general and quantum computing are some of the applications targeted by what is now called quantum photonics. Nevertheless, it was pretty clear at an early stage that bulk optics for handling quantum states of light with photons would not be able to deliver what is needed for these technologies. More recently, single photons, entangled photons and quantum optics in general have been coupled to more integrated approaches coming from classical optics in order to meet the requirements of scalability, reliablility and efficiency for quantum technologies. In this article, we develop our recent advances in two different nanophotonic platforms for quantum photonics using elongated optical fibers and integrated glass waveguides made by the so-called ion-exchange technique. We also present our latest results on quantum nanoemitters that we plan to couple and incorporate with our photonics platforms. These nanoemitters are of two kinds: nanocrystals made of perovskites as well as silicon-vacancy defect centers in nanodiamonds. Some of their properties are developed in this work. We will then give the general steps necessary in order to couple these nanoemitters efficiently with our platforms in the near future., Comment: 9 pages

Optical coupling enables intermediate- and long-range interactions between distant quantum emitters. Such interaction may be the basic element in bottom-up approaches of coupled spin systems or for integrated quantum photonics and quantum plasmonics. Here, we prepare nanodiamonds carrying single, negatively-charged silicon-vacancy centers for evanescent optical coupling with access to all degrees of freedom by means of atomic force nanomanipulation. The color centers feature excellent optical properties, comparable to silicon-vacancy centers in bulk diamond, resulting in a resolvable fine structure splitting, a linewidth close to the Fourier-Transform limit under resonant excitation and a good polarization contrast. We determine the orbital relaxation time $T_{1}$ of the orbitally split ground states and show that all optical properties are conserved during translational nanomanipulation. Furthermore, we demonstrate the rotation of the nanodiamonds. In contrast to the translational operation, the rotation leads to a change in polarization contrast. We utilize the change in polarization contrast before and after nanomanipulation to determine the rotation angle. Finally, we evaluate the likelihood for indistinguishable, single photon emission of silicon-vacancy centers located in different nanodiamonds. Our work enables ideal evanescent, optical coupling of distant nanodiamonds containing silicon-vacancy centers with applications in the realization of quantum networks, quantum repeaters or complex quantum systems., Comment: 15 pages, 4 figures

We have characterized thermal susceptibilities of the spectral band at 740 nm of silicon-vacancy (SiV) centers in Si- and Si,P-doped nanodiamonds over a temperature range from 295 K to 350 K, which is of interest for thermometry in biological systems. Si-doped crystals reveal linear dependence of the SiV zero-phonon line position, width and relative amplitude with susceptibilities of 0.0126(4) nm/K, 0.062(2) nm/K and $-0.037(2)$ K$^{-1}$, respectively. Si,P-doped nanodiamonds show significantly smaller (up to 35 % for the width) susceptibilities and prove control of SiV properties with additional chemical doping. It is argued that a significant contribution to the heating of the nanodiamonds induced by laser light can be intrinsic due to a high concentration and low luminescence quantum yield of SiV centers., Comment: 3 figure, 4 pages

Striving for nanometer-sized solid-state single-photon sources, we investigate atom-like quantum emitters based on single germanium vacancy (GeV) centers isolated in crystalline nanodiamonds (NDs). Cryogenic characterization indicated symmetry-protected and bright (> 10^6 counts/s with off-resonance excitation) zero-phonon optical transitions with up to 6-fold enhancement in energy splitting of their ground states as compared to that found for GeV centers in bulk diamonds (i.e., up to 870 GHz in highly strained NDs vs 150 GHz in bulk). Utilizing lithographic alignment techniques, we demonstrate an integrated nanophotonic platform for deterministic interfacing plasmonic waveguides with isolated GeV centers in NDs that enables 10-fold enhancement of single-photon decay rates along with the emission direction control by judiciously designing and positioning a Bragg reflector. This approach allows one to realize the unidirectional emission from single-photon dipolar sources introducing a novel method that is alternative to the propagation-direction-dependent techniques based on chiral interactions or topological protection. The developed plasmon-based nanophotonic platform opens thereby new perspectives for quantum nanophotonics in general and for realizing entanglement between single photons and spin qubits, in particular.

Nanoscale thermometry is paramount to study primary processes of heat transfer in solids and is a subject of hot debate in cell biology. Here we report ultrafast temperature sensing using all-optical thermometry exploiting synthetic nanodiamonds with silicon-vacancy (SiV) centres embedded at a high concentration. Using multi-parametric analysis of photoluminescence (PL) of these centres, we have achieved an intrinsic noise floor of about 10 mK Hz$^{-1/2}$, which is a thousand-fold increase in the readout speed in comparison to the current record values demonstrated with all-optical methods of comparable spatial-resolution and precision. Our thermometers are smaller than 250-nm across but can detect a 0.4$^\circ$C change of temperature in a measurement taking only 0.001 second. The exceptional sensitivity and simplicity of these thermometers enable a wide range of applications such as temperature monitoring and mapping within intracellular regions and in state-of-the-art solid-state electronic nanodevices.

Ultra-small, low-strain, artificially produced diamonds with an internal, active color center have substantial potential for quantum information processing and biomedical applications. Thus, it is of great importance to be able to artificially produce such diamonds. Here, we report on the high-pressure, high-temperature synthesis of such nanodiamonds about 10 nm in size and containing an optically active, single silicon-vacancy color center. Using special sample preparation technique, we were able to prepare samples containing single nanodiamonds on the surface. By correlating atomic-force microscope images and confocal optical images we verified presents of optically active color centers in single nanocrystals, and using second-order correlation measurements proved single-photon emission statistics of this nanodiamonds. This color centers have non-blinking, spectrally narrow emission with narrow distribution of spectral width and positions of zero-phonon line thus proving high quality of the nanodiamonds produced

We report on the isolation of single SiV$^-$ centers in nanodiamonds. We observe the fine-structure of single SiV$^-$ center with improved inhomogeneous ensemble linewidth below the excited state splitting, stable optical transitions, good polarization contrast and excellent spectral stability under resonant excitation. Based on our experimental results we elaborate an analytical strain model where we extract the ratio between strain coefficients of excited and ground states as well the intrinsic zero-strain spin-orbit splittings. The observed strain values are as low as best values in low-strain bulk diamond. We achieve our results by means of H-plasma treatment of the diamond surface and in combination with resonant and off-resonant excitation. Our work paves the way for indistinguishable, single photon emission. Furthermore, we demonstrate controlled nano-manipulation via atomic force microscope cantilever of 1D- and 2D-alignments with a so-far unreached accuracy of about 10nm, as well as new tools including dipole rotation and cluster decomposition. Combined, our results show the potential to utilize SiV$^-$ centers in nanodiamonds for the controlled interfacing via optical coupling of individually well-isolated atoms for bottom-up assemblies of complex quantum systems.

Monolithic integration of quantum emitters in nanoscale plasmonic circuitry requires low-loss plasmonic configurations capable of confining light well below the diffraction limit. We demonstrate on-chip remote excitation of nanodiamond-embedded single quantum emitters by plasmonic modes of dielectric ridges atop colloidal silver crystals. The nanodiamonds are produced to incorporate single germanium-vacancy (GeV) centers, providing bright, spectrally narrow and stable single-photon sources suitable for highly integrated circuits. Using electron-beam lithography with hydrogen silsesquioxane (HSQ) resist, dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) are fabricated on single crystalline silver plates so as to contain those of spin-casted nanodiamonds that are found to feature appropriate single GeV centers. The low-loss plasmonic configuration enabled the 532 nm pump laser light to propagate on-chip in the DLSPPW and reach to an embedded nanodiamond where a single GeV center is incorporated. The remote GeV emitter is thereby excited and coupled to spatially confined DLSPPW modes with an outstanding figure-of-merit of 180 due to a ~6-fold Purcell enhancement, ~56% coupling efficiency and ~33 {\mu}m transmission length, revealing the potential of our approach for on-chip realization of nanoscale functional quantum devices.

Nanodiamonds containing color centers open up many applications in quantum information processing, metrology, and quantum sensing. In particular, silicon vacancy (SiV) centers are prominent candidates as quantum emitters due to their beneficial optical qualities. Here we characterize nanodiamonds produced by a high-pressure high-temperature method without catalyst metals, focusing on two samples with clear SiV signatures. Different growth temperatures and relative content of silicon in the initial compound between the samples altered their nanodiamond size distributions and abundance of SiV centers. Our results show that nanodiamond growth can be controlled and optimized for different applications.

We demonstrate an all-optical thermometer based on an ensemble of silicon-vacancy centers (SiVs) in diamond by utilizing a temperature dependent shift of the SiV optical zero-phonon line transition frequency, $\Delta\lambda/\Delta T= 6.8\,\mathrm{GHz/K}$. Using SiVs in bulk diamond, we achieve $70\,\mathrm{mK}$ precision at room temperature with a sensitivity of $360\,\mathrm{mK/\sqrt{Hz}}$. Finally, we use SiVs in $200\,\mathrm{nm}$ nanodiamonds as local temperature probes with $521\,\mathrm{ mK/\sqrt{Hz}}$ sensitivity. These results open up new possibilities for nanoscale thermometry in biology, chemistry, and physics, paving the way for control of complex nanoscale systems., Comment: 5 pages, 3 figures

Silicon vacancy (SiV) centers in diamonds have emerged as a very promising candidate for quantum emitter due to their narrow emission line resulting in their indistinguishability. While many different quantum emitters have already been used for excitation of various propagating plasmonic modes, the corresponding exploitation of SiV centers remained so far uncharted territory. Here, we report on excitation of surface plasmon modes supported by silver nanowires using SiV centers in nanodiamonds. The coupling of SiV center fluorescence to surface plasmons is observed, when a nanodiamond situated close to a nanowire is illuminated by the pump, as radiated emission from the distal nanowire end. The effect of coupling is also seen as a change in the SiV center lifetime. Finally, we discuss possible avenues for strengthening the SiV center coupling to surface plasmon modes.

Nanodiamonds (NDs) hosting optically active defects are an important technical material for applications in quantum sensing, biological imaging, and quantum optics. The negatively charged silicon vacancy (SiV) defect is known to fluoresce in molecular sized NDs (1 to 6 nm) and its spectral properties depend on the quality of the surrounding host lattice. This defect is therefore a good probe to investigate the material properties of small NDs. Here we report unprecedented narrow optical transitions for SiV colour centers hosted in nanodiamonds produced using a novel high-pressure high-temperature (HPHT) technique. The SiV zero-phonon lines were measured to have an inhomogeneous distribution of 1.05 nm at 5 K across a sample of numerous NDs. Individual spectral lines as narrow as 354 MHz were measured for SiV centres in nanodiamonds smaller than 200 nm, which is four times narrower than the best SiV line previously reported for nanodiamonds. Correcting for apparent spectral diffusion yielded a homogeneous linewith of about 200 MHz, which is close to the width limit imposed by the radiative lifetime. These results demonstrate that the direct HPHT synthesis technique is capable of producing nanodiamonds with high crystal lattice quality, which are therefore a valuable technical material.