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Integrated nanophotonics is an emerging field with high potential for quantum technology applications such as quantum sensing or quantum networks. A desired photonics platform is Si3N4 due to low-p...

Realization of quantum photonic devices requires coupling single quantum emitters to the mode of optical resonators. In this work, a hybrid system consisting of defect centers in few-layer hexagonal boron nitride (hBN) grown by chemical vapor deposition and a fiber-based Fabry–Pérot cavity is presented. The sub 10-nm thickness of hBN and its smooth surface enable efficient integration into the cavity mode. This hybrid platform is operated over a broad spectral range larger than 30 nm and its tuneability is used to explore different coupling regimes. Consequently, very large cavity-assisted signal enhancement up to 50-fold and strongly narrowed linewidths are achieved, which is owing to cavity funneling, a record for hBN-cavity systems. Additionally, an excitation and readout scheme is implemented for resonant excitation that allows to establish cavity-assisted photoluminescence excitation (PLE) spectroscopy. This work marks an important milestone for the deployment of 2D materials coupled to fiber-based cavities in practical quantum technologies.

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.

The negatively-charged NV$^-$-center in diamond has shown great success in nanoscale, high-sensitivity magnetometry. Efficient fluorescence detection is crucial for improving the sensitivity. Furthermore, integrated devices enable practicable sensors. Here, we present a novel architecture which allows us to create NV$^-$-centers a few nanometers below the diamond surface, and at the same time in the mode field maximum of femtosecond-laser-written type-II waveguides. We experimentally verify the coupling efficiency, showcase the detection of magnetic resonance signals through the waveguides and perform first proof-of-principle experiments in magnetic field and temperature sensing. The sensing task can be operated via the waveguide without direct light illumination through the sample, which marks an important step for magnetometry in biological systems which are fragile to light. In the future, our approach will enable the development of two-dimensional sensing arrays facilitating spatially and temporally correlated magnetometry., Comment: 10 pages, 4 figures

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.

We unravel the mystery of mechanically decoupled defect centers in hexagonal boron nitride., Quantum emitters in hexagonal boron nitride were recently reported to hold unusual narrow homogeneous linewidths of tens of megahertz within the Fourier transform limit at room temperature. This unique observation was traced back to decoupling from in-plane phonon modes. Here, we investigate the origins for the mechanical decoupling. New sample preparation improved spectral diffusion, which allowed us to reveal a gap in the electron-phonon spectral density for low phonon frequencies. This sign for mechanical decoupling persists up to room temperature and explains the observed narrow lines at 300 kelvin. We investigate the dipole emission directionality and reveal preferred photon emission through channels between the layers supporting the claim for out-of-plane distorted defect centers. Our work provides insights into the underlying physics for the persistence of Fourier transform limit lines up to room temperature and gives a guide to the community on how to identify the exotic emitters.

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., 7 pages, 4 figures. Nanophotonics (2020)

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

© 2019 American Physical Society. In this Reply, we answer to the comments by Langbein [Phys. Rev. B 100, 047401 (2019)10.1103/PhysRevB.100.047401]. We disagree with the argument that our measured spectral shapes and the extracted linewidths are caused by temporal blinking. We give detailed information on our evaluation process to exclude blinking events. Beyond the question raised in the Comment, we analyze the influence of spectral diffusion. Although spectral diffusion is an ongoing limitation for defect centers in hexagonal boron nitride, we prove that it is not influencing our extracted linewidths.

Integrated nanophotonics is an emerging field with high potential for quantum technology applications such as quantum sensing or quantum networks. A desired photonics platform is Si

Solid state single photon sources with Fourier Transform (FT) limited lines are among the most crucial constituents of photonic quantum technologies and have been accordingly the focus of intensive research over the last several decades. However, so far, solid state systems have only exhibited FT limited lines at cryogenic temperatures due to strong interactions with the thermal bath of lattice phonons. In this work, we report a solid state source that exhibits FT limited lines measured in photo luminescence excitation (sub 100 MHz linewidths) from 3K-300K. The studied source is a color center in the two-dimensional hexagonal boron nitride and we propose that the center's decoupling from phonons is a fundamental consequence of material's low dimensionality. While the center's luminescence lines exhibit spectral diffusion, we identify the likely source of the dffusion and propose to mitigate it via dynamic spectral tuning. The discovery of FT-limited lines at room temperature, which once the spectral diffusion is controlled, will also yield FT-limited emission. Our work motivates a significant advance towards room temperature photonic quantum technologies and a new research direction in the remarkable fundamental properties of two-dimensional materials., 6 pages, 3 figures

The Si-${V}^{\ensuremath{-}}$ center in diamond has emerged as an excellent single-photon source with outstanding properties for photonics and quantum information processing. Here surface-treatment techniques enable researchers to obtain single Si-${V}^{\ensuremath{-}}$ centers with bulklike spectral properties in nanodiamonds. The authors resolve the fine structure of individual Si-${V}^{\ensuremath{-}}$ centers in low-strain nanodiamonds, formulate an analytical strain model for this center, and experimentally find the strain coefficients. They also explore the potential for bottom-up assembly of complex quantum systems, using cantilever nanomanipulation to achieve efficient positioning, rotation, and declustering of nanodiamonds.

We present coupling of the fluorescence of an ensemble of negatively-charged silicon-vacancy center embedded in a single crystal diamond membrane to the optical mode of a fiber based Fabry Perot cavity. The coupled system offers generation of coherent photons at high rates for various quantum optics applications.

The creation of single, negatively charged silicon vacancy ( SiV −) centers in well-defined diamond layers close to the host surface is a crucial step for the development of diamond-based quantum optic devices with many applications in nanophotonics, quantum sensing, or quantum information science. Here, we report on the creation of shallow (10 nm below the surface), single SiV − centers in diamond using low energy Si + ion implantation with subsequent high temperature annealing at 1500 °C. We show transition linewidths down to 99 MHz and narrow inhomogeneous distributions. Furthermore, we achieved a reduction of homogeneous linewidths by a factor of 2 after removing subsurface damage using oxygen plasma processing. These results not only give insights into the formation process of SiV − centers but also indicate a favorable processing method to fabricate shallow single quantum emitters in diamond perfectly suited for coupling to nanostructures on the diamond surface.

We realize a potential platform for an efficient spin-photon interface, namely negatively-charged silicon-vacancy centers in a diamond membrane coupled to the mode of a fully-tunable, fiber-based, optical resonator. We demonstrate that introducing the thin ($\sim 200 \, \text{nm}$), single crystal diamond membrane into the mode of the resonator does not change the cavity properties, which is one of the crucial points for an efficient spin-photon interface. In particular, we observe constantly high Finesse values of up to $3000$ and a linear dispersion in the presence of the membrane. We observe cavity-coupled fluorescence froman ensemble of SiV$^{-}$ centers with an enhancement factor of $\sim 1.9$. Furthermore from our investigations we extract the ensemble absorption and extrapolate an absorption cross section of $(2.9 \, \pm \, 2) \, \cdot \, 10^{-12} \, \text{cm}^{2}$ for a single SiV$^{-}$ center, much higher than previously reported., 8 pages, 4 figures

Color centers in diamond are important quantum emitters for a broad range of applications ranging from quantum sensing to quantum optics. Understanding the internal energy level structure is of fundamental importance for future applications. We experimentally investigate the level structure of an ensemble of few negatively charged silicon-vacancy (SiV$^{-}$) and germanium-vacancy (GeV$^{-}$) centers in bulk diamond at room temperature by photoluminescence (PL) and excitation (PLE) spectroscopy over a broad wavelength range from 460 nm to 650 nm and perform power-dependent saturation measurements. For SiV$^{-}$ our experimental results confirm the presence of a higher energy transition at ~ 2.31 eV. By comparison with detailed theoretical simulations of the imaginary dielectric function we interpret the transition as a dipole-allowed transition from $^{2}E_{g}$-state to $^{2}A_{2u}$-state where the corresponding $a_{2u}$-level lies deeply inside the diamond valence band. Therefore, the transition is broadened by the diamond band. At higher excitation power of 10 mW we indicate signs of a parity-conserving transition at ~2.03 eV supported by saturation measurements. For GeV$^{-}$ we demonstrate that the PLE spectrum is in good agreement with the mirror image of the PL spectrum of the zero-phonon line (ZPL). Experimentally we do not observe a higher lying energy level up to a transition wavelength of 460 nm. The observed PL spectra are identical, independent of excitation wavelength, suggesting a rapid decay to $^{2}E_{u}$ excited state and followed by optical transition to $^{2}E_{g}$ ground state. Our investigations convey important insights for future quantum optics and quantum sensing experiments based on SiV$^{-}$ center and GeV$^{-}$ center in diamond., Comment: 19 pages, 4 figures

We discuss feedback control of the motion of a single neutral atom trapped inside a high-finesse optical cavity. Based on the detection of single photons from a probe beam transmitted through the cavity, the position of the atom in the trap is estimated. Following this information, the trapping potential is switched between a high and a low value in order to counteract the atomic motion. This allowed us to increase the storage time by about one order of magnitude. Here, we describe the technical implementation of the feedback loop and give a detailed analysis of its limitations as deduced from Monte-Carlo simulations. We also discuss different strategies to further improve the performance of the feedback.

Feedback is one of the most powerful techniques for the control of classical systems. An extension into the quantum domain is desirable as it could allow the production of non-trivial quantum states and protection against decoherence. The difficulties associated with quantum, as opposed to classical, feedback arise from the quantum measurement process-in particular the quantum projection noise and the limited measurement rate-as well as from quantum fluctuations perturbing the evolution in a driven open system. Here we demonstrate real-time feedback control of the motion of a single atom trapped in an optical cavity. Individual probe photons carrying information about the atomic position activate a dipole laser that steers the atom on timescales 70 times shorter than the atom's oscillation period in the trap. Depending on the specific implementation, the trapping time is increased by a factor of more than four owing to feedback cooling, which can remove almost all the kinetic energy of the atom in a quarter of an oscillation period. Our results show that the detected photon flux reflects the atomic motion, and thus mark a step towards the exploration of the quantum trajectory of a single atom at the standard quantum limit.

We report on strong as well as weak coupling of self assembled In 0.30 Ga 0.70 As quantum dots (QDs) in high-Q semiconductor micropillar cavities grown by molecular beam epitaxy. Using electron beam lithography and deep plasma etching, micropillars with high Q-factors (about 10000 for 1.6 μm diameter) were realized. The low QD density in the present structures allows us to investigate the interaction of single QD excitons and the vacuum field of the microcavity by temperature tuning of exciton lines into resonance with the cavity mode. We will present an example where QD excitons with different emission energies show strong or weak coupling with the optical mode of the same micropillar at different resonance temperatures. Here, the individual coupling behaviour is described in terms of a different spatial position of the QDs relative to the field maximum of the cavity mode.

We report direct measurement of population dynamics in the excited state manifold of a nitrogen-vacancy (NV) center in diamond. We quantify the phonon-induced mixing rate and demonstrate that it can be completely suppressed at low temperatures. Further, we measure the intersystem crossing (ISC) rate for different excited states and develop a theoretical model that unifies the phonon-induced mixing and ISC mechanisms. We find that our model is in excellent agreement with experiment and that it can be used to predict unknown elements of the NV center's electronic structure. We discuss the model's implications for enhancing the NV center's performance as a room-temperature sensor.

The intersystem crossing (ISC) is an important process in many solid-state atomlike impurities. For example, it allows the electronic spin state of the nitrogen-vacancy (NV) center in diamond to be initialized and read out using optical fields at ambient temperatures. This capability has enabled a wide array of applications in metrology and quantum information science. Here, we develop a microscopic model of the state-selective ISC from the optical excited state manifold of the NV center. By correlating the electron-phonon interactions that mediate the ISC with those that induce population dynamics within the NV center's excited state manifold and those that produce the phonon sidebands of its optical transitions, we quantitatively demonstrate that our model is consistent with recent ISC measurements. Furthermore, our model constrains the unknown energy spacings between the center's spin-singlet and spin-triplet levels. Finally, we discuss prospects to engineer the ISC in order to improve the spin initialization and readout fidelities of NV centers.

We demonstrate quantum interference between indistinguishable photons emitted by two nitrogen-vacancy (NV) centers in distinct diamond samples separated by two meters. Macroscopic solid immersion lenses are used to enhance photon collection efficiency. Quantum interference is verified by measuring a value of the second-order cross-correlation function $g^{(2)}(0) = 0.35 \pm 0.04, 5 pages, 3 figures

Diamond is a unique material with exceptional physical and chemical properties that offers potential for the realization of high-performance devices with novel functionalities. For example diamond's high refractive index, transparency over wide wavelength range, and large Raman gain are of interest for the implementation of novel photonic devices. Recently, atom-like impurities in diamond emerged as an exceptional system for quantum information processing, quantum sensing and quantum networks. For these and other applications, it is essential to develop an integrated nanophotonic platform based on diamond. Here, we report on the realization of such an integrated diamond photonic platform, diamond on insulator (DOI), consisting of a thin single crystal diamond film on top of an insulating silicon dioxide/silicon substrate. Using this approach, we demonstrate diamond ring resonators that operate in a wide wavelength range, including the visible (630nm) and near-infrared (1,550nm). Finally, we demonstrate an integrated, on-chip quantum nanophotonic network, consisting of ring resonators coupled to low loss waveguides with grating couplers, that enables the generation and efficient routing of single photons at room temperature., This paper has been withdrawn to change the outline of the paper

The quantum dynamics of a strongly driven, strongly coupled single-atom-cavity system is studied by evaluating time-dependent second- and third-order correlations of the emitted photons. The coherent energy exchange, first, between the atom and the cavity mode, and second, between the atom-cavity system and the driving laser, is observed. Three-photon detections show an asymmetry in time, a consequence of the breakdown of detailed balance. The results are in good agreement with theory and are a first step towards the control of a quantum trajectory at larger driving strength., 4 pages + 3 pages supplementary information

Single quantum emitters like atoms are well-known as non-classical light sources which can produce photons one by one at given times, with reduced intensity noise. However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example is the predicted ability for a single atom to produce quadrature-squeezed light, with sub-shot-noise amplitude or phase fluctuations. It has long been foreseen, though, that such squeezing would be "at least an order of magnitude more difficult" to observe than the emission of single photons. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms, but despite experimental efforts, single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity, several orders of magnitude larger than for usual macroscopic media. This produces observable quadrature squeezing with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons, the squeezed light stems from the quantum coherence of photon pairs emitted from the system. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters, Main paper (4 pages, 3 figures) + Supplementary information (5 pages, 2 figures). Revised version

We report on a new cavity QED setup with asymmetric mirrors, optimized for high photon flux. Novel photon statistics and real-time feedback experiments are now possible.

A lot of information about a physical system can be gained by characterizing the light field that is emitted. Measurements of second-order intensity correlation functions have become a standard tool to observe the conditional dynamics of various fundamental systems in quantum optics. Here, we present - for the first time - measurements of the time-dependent, third-order intensity correlation function of a genuine quantum system. It consists of a neutral 85Rb atom strongly coupled to a single mode of a high-finesse optical cavity. In order to excite higher-order dressed states of the Jaynes-Cummings ladder we use relatively strong probe powers corresponding to steady state empty-cavity photon numbers as high as 20.

Maxwell realized in a famous thought experiment, which is nowadays known as Maxwell's demon, that the thermodynamic parameters of a system can be altered by performing feedback on the system's individual constituents. Here we report on the implementation of a related scheme to cool the motion of a single neutral atom.

We demonstrate feedback cooling of the motion of a single rubidium atom trapped in a high-finesse optical resonator to a temperature of about 160 \mu K. Time-dependent transmission and intensity-correlation measurements prove the reduction of the atomic position uncertainty. The feedback increases the 1/e storage time into the one second regime, 30 times longer than without feedback. Feedback cooling therefore rivals state-of-the-art laser cooling, but with the advantages that it requires less optical access and exhibits less optical pumping., Comment: 5 pages, 4 figures

Single atoms absorb and emit light from a resonant laser beam photon by photon. We show that a single atom strongly coupled to an optical cavity can absorb and emit resonant photons in pairs. The effect is observed in a photon correlation experiment on the light transmitted through the cavity. We find that the atom-cavity system transforms a random stream of input photons into a correlated stream of output photons, thereby acting as a two-photon gateway. The phenomenon has its origin in the quantum anharmonicity of the energy structure of the atom-cavity system. Future applications could include the controlled interaction of two photons by means of one atom., PRL, 4 figures

Optical nonlinearities typically require macroscopic media, thereby making their implementation at the quantum level an outstanding challenge. Here we demonstrate a nonlinearity for one atom enclosed by two highly reflecting mirrors. We send laser light through the input mirror and record the light from the output mirror of the cavity. For weak laser intensity, we find the vacuum-Rabi resonances. But for higher intensities, we find an additional resonance. It originates from the fact that the cavity can accommodate only an integer number of photons and that this photon number determines the characteristic frequencies of the coupled atom-cavity system. We selectively excite such a frequency by depositing at once two photons into the system and find a transmission which increases with the laser intensity squared. The nonlinearity differs from classical saturation nonlinearities and is direct spectroscopic proof of the quantum nature of the atom-cavity system. It provides a photon-photon interaction by means of one atom, and constitutes a step towards a two-photon gateway or a single-photon transistor., 7 figures, Nature Physics

A single atom strongly coupled to a cavity mode is stored by three-dimensional confinement in blue-detuned cavity modes of different longitudinal and transverse order. The vanishing light intensity at the trap center reduces the light shift of all atomic energy levels. This is exploited to detect a single atom by means of a dispersive measurement with 95% confidence in 10 micros, limited by the photon-detection efficiency. As the atom switches resonant cavity transmission into cavity reflection, the atom can be detected while scattering about one photon.

An experimental sample trace of a trapping event is presented 85Rb atoms have been launched from an atomic fountain along the y-direction. Arming the trigger at 210 ms after launch selects atoms with velocities

We report on strong coupling between a discrete optical mode of a high-Q micropillar cavity and single excitons of self assembled In 0.43 Ga 0.57 As quantum dots and compare the results with previous studies on In0.3Ga0.7As quantum dots.

We report a new type of coupling between quantum dot excitons mediated by the strong single-photon field in a high-finesse micropillar cavity. Coherent exciton coupling is observed for two dots with energy differences of the order of the exciton-photon coupling. The coherent coupling mode is characterized by an anticrossing with a particularly large line splitting of 250 microeV. Because of the different dispersion relations with temperature, the simultaneous photonic coupling of quantum dot excitons can be easily distinguished from cases of sequential strong coupling of two quantum dots.