Strongly Cavity-Enhanced Spontaneous Emission from Silicon-Vacancy Centers in Diamond

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.

Enhancement of dual zero phonon line emissions in nanodiamonds using quasiperiodic photonic structures

Color centers in nanodiamonds (NDs) have been largely explored by coupling to a photonic structured matrix (PSM) to amplify visible range emission features, enhancing their use in quantum technologies. Here, we study the emission enhancement of dual near-infrared zero phonon line (ZPL) emission from silicon-boron (SiB) and silicon-vacancy (SiV-) centers in NDs using a spontaneously emerged low index-contrast quasiperiodic PSM, having micron-scale air pores. An intensity enhancement factor of 6.15 for SiV- and 7.8 for SiB ZPLs is attained for the PSM sample compared to a control sample. We find Purcell enhancement of 2.77 times for the PSM sample using spatial-dependent decay rate measurements, supported by localized field intensity confinement in the sample. Such cavity-like emission enhancement and lifetime reduction are enabled by an in-plane order-disorder scattering in the PSM sample substantiated by pump-dependent emission measurements. The results put forward a facile approach to tailor the near-infrared dual ZPL emission from NDs using nanophotonic structures.

Deterministic Fabrication of a Coupled Cavity-Emitter System in Hexagonal Boron Nitride

Light-matter interactions in optical cavities underpin many applications of integrated quantum photonics. Among various solid-state platforms, hexagonal boron nitride (hBN) is gaining considerable interest as a compelling van der Waals host of quantum emitters. However, progress to date has been limited by an inability to engineer simultaneously an hBN emitter and a narrow-band photonic resonator at a predetermined wavelength. Here, we overcome this problem and demonstrate deterministic fabrication of hBN nanobeam photonic crystal cavities with high quality factors over a broad spectral range (∼400 to 850 nm). We then fabricate a monolithic, coupled cavity-emitter system designed for a blue quantum emitter that has an emission wavelength of 436 nm and is induced deterministically by electron beam irradiation of the cavity hotspot. Our work constitutes a promising path to scalable on-chip quantum photonics and paves the way to quantum networks based on van der Waals materials.

Hybrid Integration of GaP Photonic Crystal Cavities with Silicon-Vacancy Centers in Diamond by Stamp-Transfer

Optically addressable solid-state defects are emerging as some of the most promising qubit platforms for quantum networks. Maximizing photon-defect interaction by nanophotonic cavity coupling is key to network efficiency. We demonstrate fabrication of gallium phosphide 1-D photonic crystal waveguide cavities on a silicon oxide carrier and subsequent integration with implanted silicon-vacancy (SiV) centers in diamond using a stamp-transfer technique. The stamping process avoids diamond etching and allows fine-tuning of the cavities prior to integration. After transfer to diamond, we measure cavity quality factors (Q) of up to 8900 and perform resonant excitation of single SiV centers coupled to these cavities. For a cavity with a Q of 4100, we observe a 3-fold lifetime reduction on-resonance, corresponding to a maximum potential cooperativity of C = 2. These results indicate promise for high photon-defect interaction in a platform which avoids fabrication of the quantum defect host crystal.

Super-Poissonian Light Statistics from Individual Silicon Vacancy Centers Coupled to a Laser-Written Diamond Waveguide

Modifying light fields at the single-photon level is a key challenge for upcoming quantum technologies and can be realized in a scalable manner through integrated quantum photonics. Laser-written diamond photonics offers 3D fabrication capabilities and large mode-field diameters matched to fiber optic technology, though limiting the cooperativity at the single-emitter level. To realize large coupling efficiencies, we combine excitation of single shallow-implanted silicon vacancy centers via high numerical aperture optics with detection assisted by laser-written type-II waveguides. We demonstrate single-emitter extinction measurements with a cooperativity of 0.0050 and a relative beta factor of 13%. The transmission of resonant photons reveals single-photon subtraction from a quasi-coherent field resulting in super-Poissonian light statistics. Our architecture enables light field engineering in an integrated design on the single quantum level although the intrinsic cooperativity is low. Laser-written structures can be fabricated in three dimensions and with a natural connectivity to optical fiber arrays.

Prolonged Orbital Relaxation by Locally Modified Phonon Density of States for the SiV^{-} Center in Nanodiamonds

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. Subsequently measured coherent population trapping shows an extended spin dephasing time compared to the phonon-limited time in a pure bulk diamond. The method works at liquid helium temperatures of few Kelvin and in the low-strain regime.

Purcell Enhancement of a Cavity-Coupled Emitter in Hexagonal Boron Nitride

Integration of solid-state quantum emitters into nanophotonic circuits is a critical step towards fully on-chip quantum photonic-based technologies. Among potential materials platforms, quantum emitters in hexagonal boron nitride (hBN) have emerged as a viable candidate over the last years. While the fundamental physical properties have been intensively studied, only a few works have focused on the emitter integration into photonic resonators. Yet, for a potential quantum photonic material platform, the integration with nanophotonic cavities is an important cornerstone, as it enables the deliberate tuning of the spontaneous emission and the improved readout of distinct transitions for a quantum emitter. In this work, the resonant tuning of a monolithic cavity integrated hBN quantum emitter is demonstrated through gas condensation at cryogenic temperature. In resonance, an emission enhancement and lifetime reduction are observed, with an estimate for the Purcell factor of ≈15.

Spontaneous Light Emission Assisted by Mie Resonances in Diamond Nanoparticles

Spontaneous light emission is known to be affected by the local density of states and enhanced when coupled to a resonant cavity. Here, we report on an experimental study of silicon-vacancy (SiV) color center fluorescence and spontaneous Raman scattering from subwavelength diamond particles supporting low-order Mie resonances in the visible range. For the first time to our knowledge, we have measured the size dependences of the SiV fluorescence emission rate and the Raman scattering intensity from individual diamond particles in the range from 200 to 450 nm. The obtained dependences reveal a sequence of peaks, which we explicitly associate with specific multipole resonances. The results are in agreement with our theoretical analysis and highlight the potential of intrinsic optical resonances for developing nanodiamond-based lasers and single-photon sources.

Single organic molecules for photonic quantum technologies

Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines that are tens of megahertz wide, limited by only the excited-state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the past few decades, their controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single-photon sources and as nonlinear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.

Ag-Diamond Core-Shell Nanostructures Incorporated with Silicon-Vacancy Centers

Silicon-vacancy (SiV) centers in diamond have attracted attention as highly stable fluorophores for sensing and as possible candidates for quantum information science. While prior studies have shown that the formation of hybrid diamond-metal structures can increase the rates of optical absorption and emission, many practical applications require diamond plasmonic structures that are stable in harsh chemical and thermal environments. Here, we demonstrate that Ag nanospheres, produced both in quasi-random arrays by thermal dewetting and in ordered arrays using electron-beam lithography, can be completely encapsulated with a thin diamond coating containing SiV centers, leading to hybrid core-shell nanostructures exhibiting extraordinary chemical and thermal stability as well as enhanced optical properties. Diamond shells with a thickness on the order of 20-100 nm are sufficient to encapsulate and protect the Ag nanostructures with different sizes ranging from 20 nm to hundreds of nanometers, allowing them to withstand heating to temperatures of 1000 °C and immersion in harsh boiling acid for 24 h. Ultrafast photoluminescence lifetime and super-resolution optical imaging experiments were used to study the SiV properties on and off the core-shell structures, which show that the SiV on core-shell structures have higher brightness and faster decay rate. The stability and optical properties of the hybrid Ag-diamond core-shell structures make them attractive candidates for high-efficiency imaging and quantum-based sensing applications.

Detecting initial correlations via correlated spectroscopy in hybrid quantum systems

Generic mesoscopic quantum systems that interact with their environment tend to display appreciable correlations with environment that often play an important role in the physical properties of the system. However, the experimental methods needed to characterize such systems either ignore the role of initial correlations or scale unfavourably with system dimensions. Here, we present a technique that is agnostic to system-environment correlations and can be potentially implemented experimentally. Under a specific set of constraints, we demonstrate the ability to detect and measure specific correlations. We apply the technique to two cases related to Nitrogen Vacancy Centers (NV). Firstly, we use the technique on an NV coupled to a P1 defect centre in the environment to demonstrate the ability to detect dark spins. Secondly, we implement the technique on a hybrid quantum system of NV coupled to an optical cavity with initial correlations. We extract the interaction strength and effective number of interacting NVs from the initial correlations using our technique.

Defect Polaritons from First Principles

Control over the optical properties of defects in solid-state materials is necessary for their application in quantum technologies. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons in an optical cavity. We show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. We also find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single-photon emission from defect centers. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.

Quantum Interfaces to the Nanoscale

Scalable quantum information systems would store, manipulate, and transmit quantum information locally and across a quantum network, but no single qubit technology is currently robust enough to perform all necessary tasks. Defect centers in solid-state materials have emerged as potential intermediaries between other physical manifestations of qubits, such as superconducting qubits and photonic qubits, to leverage their complementary advantages. It remains an open question, however, how to design and to control quantum interfaces to defect centers. Such interfaces would enable quantum information to be moved seamlessly between different physical systems. Understanding and constructing the required interfaces would, therefore, unlock the next big steps in quantum computing, sensing, and communications. In this Perspective, we highlight promising coupling mechanisms, including dipole-, phonon-, and magnon-mediated interactions, and discuss how contributions from nanotechnologists will be paramount in realizing quantum information processors in the near-term.

Nanofabrication of high Q, transferable diamond resonators

Advancement of diamond based photonic circuitry requires robust fabrication protocols of key components - including diamond resonators and cavities. Here, we present 1D (nanobeam) photonic crystal cavities generated from single crystal diamond membranes utilising a metallic tungsten layer as a restraining, conductive and removable hard mask. The use of tungsten instead of a more conventional silicon oxide layer enables good repeatability and reliability of the fabrication procedures. The process yields high quality diamond cavities with quality factors (Q-factors) approaching 1 × 104. Finally, we show that the cavities can be picked up and transferred onto a trenched substrate to realise fully suspended diamond cavities. Our fabrication process demonstrates the capability of diamond membranes as modular components for broader diamond based quantum photonic circuitry.

Hybridized Defects in Solid-State Materials as Artificial Molecules

Two-dimensional materials can be crafted with structural precision approaching the atomic scale, enabling quantum defects-by-design. These defects are frequently described as "artificial atoms" and are emerging optically addressable spin qubits. However, interactions and coupling of such artificial atoms with each other, in the presence of the lattice, warrants further investigation. Here we present the formation of "artificial molecules" in solids, introducing a chemical degree of freedom in control of quantum optoelectronic materials. Specifically, in monolayer hexagonal boron nitride as our model system, we observe configuration- and distance-dependent dissociation curves and hybridization of defect orbitals within the bandgap into bonding and antibonding orbitals, with splitting energies ranging from ∼10 meV to nearly 1 eV. We calculate the energetics of cis and trans out-of-plane defect pairs CH_B-CH_B against an in-plane defect pair C_B-C_B and find that in-plane defect pair interacts more strongly than out-of-plane pairs. We demonstrate an application of this chemical degree of freedom by varying the distance between C_B and V_N of C_BV_N and observe changes in the predicted peak absorption wavelength from the visible to the near-infrared spectral band. We envision leveraging this chemical degree of freedom of defect complexes to precisely control and tune defect properties toward engineering robust quantum memories and quantum emitters for quantum information science.

Tailoring of Typical Color Centers in Diamond for Photonics

On the demand of single-photon entangled light sources and high-sensitivity probes in the fields of quantum information processing, weak magnetic field detection and biosensing, the nitrogen vacancy (NV) color center is very attractive and has been deeply and intensively studied, due to its convenience of spin initialization, operation, and optical readout combined with long coherence time in the ambient environment. Although the application prospect is promising, there are still some problems to be solved before fully exerting its characteristic performance, including enhancement of emission of NV centers in certain charge state (NV^- or NV⁰ ), obtaining indistinguishable photons, and improving of collecting efficiency for the photons. Herein, the research progress in these issues is reviewed and commented on to help researchers grasp the current trends. In addition, the development of emerging color centers, such as germanium vacancy defects, and rare-earth dopants, with great potential for various applications, are also briefly surveyed.

Recent Advances in Single Crystal Diamond Device Fabrication for Photonics, Sensing and Nanomechanics

In the last two decades, the use of diamond as a material for applications in nanophotonics, optomechanics, quantum information, and sensors tremendously increased due to its outstanding mechanical properties, wide optical transparency, and biocompatibility. This has been possible owing to advances in methods for growth of high-quality single crystal diamond (SCD), nanofabrication methods and controlled incorporation of optically active point defects (e.g., nitrogen vacancy centers) in SCD. This paper reviews the recent advances in SCD nano-structuring methods for realization of micro- and nano-structures. Novel fabrication methods are discussed and the different nano-structures realized for a wide range of applications are summarized. Moreover, the methods for color center incorporation in SCD and surface treatment methods to enhance their properties are described. Challenges in the upscaling of SCD nano-structure fabrication, their commercial applications and future prospects are discussed.

Integration of Diamond-Based Quantum Emitters with Nanophotonic Circuits

Nanophotonics provides a promising approach to advance quantum technology by replicating fundamental building blocks of nanoscale quantum optic systems in large numbers with high reproducibility on monolithic chips. While photonic integrated circuit components and single-photon detectors offer attractive performance on silicon chips, the large-scale integration of individually accessible quantum emitters has remained a challenge. Here, we demonstrate simultaneous optical access to several integrated solid-state spin systems with Purcell-enhanced coupling of single photons with high modal purity from lithographically positioned nitrogen vacancy centers into photonic integrated circuits. Photonic crystal cavities embedded in networks of tantalum pentoxide-on-insulator waveguides provide efficient interfaces to quantum emitters that allow us to optically detect magnetic resonances (ODMR) as desired in quantum sensing. Nanophotonic networks that provide configurable optical interfaces to nanoscale quantum emitters via many independent channels will allow for novel functionality in photonic quantum information processors and quantum sensing schemes.

Coupling silicon vacancy centers in a thin diamond membrane to a silica optical microresonator

We report the development of a composite cavity QED system, in which silicon vacancy centers in a diamond membrane as thin as 100 nm couple to optical whispering gallery modes (WGMs) of a silica microsphere with a diameter of order 50 µm. The membrane induces a linewidth broadening of 3 MHz for equatorial and off-resonant WGMs, while the overall linewidth of the composite system remains below 40 MHz. Photoluminescence experiments in the cavity QED setting demonstrate the efficient coupling of optical emissions from silicon vacancy centers into the WGMs. Additional analysis indicates that the composite system can be used to achieve the good cavity limit in cavity QED, enabling an experimental platform for applications such as state transfer between spins and photons.

Photonic crystal cavity-enhanced emission from silicon vacancy centers in polycrystalline diamond achieved without postfabrication fine-tuning

Diamond optical centers have recently emerged as promising single-photon sources for quantum photonics. Particularly, negatively charged silicon vacancy (SiV-) centers show great promise due to their narrow zero-phonon emission line present also at room temperature. However, due to fabrication tolerances it is challenging to prepare directly photonic structures with optical modes spectrally matching the emission of SiV- centers. To reach the spectral overlap, photonic structures must typically undergo complicated post-processing treatment. In this work, suspended photonic crystal cavities made of polycrystalline diamond are engineered and more than 2.5-fold enhancement of the SiV- center zero-phonon line intensity via coupling to the cavity photonic mode is demonstrated. The intrinsic non-homogeneous thickness of the diamond thin layer within the sample is taken as an advantage that enables reaching the spectral overlap between the emission from SiV- centers and the cavity modes without any post-processing. Even with lower optical quality compared to monocrystalline diamond, the fabricated photonic structures show comparable efficiency for intensity enhancement. Therefore, the results of this work may open up a promising route for the application of polycrystalline diamond in photonics.

Coupling Hexagonal Boron Nitride Quantum Emitters to Photonic Crystal Cavities

Quantum photonics technologies require a scalable approach for the integration of nonclassical light sources with photonic resonators to achieve strong light confinement and enhancement of quantum light emission. Point defects from hexagonal boron nitride (hBN) are among the front runners for single photon sources due to their ultra-bright emission; however, the coupling of hBN defects to photonic crystal cavities has so far remained elusive. Here we demonstrate on-chip integration of hBN quantum emitters with photonic crystal cavities from silicon nitride (Si₃N₄) and achieve an experimentally measured quality factor (Q-factor) of 3300 for hBN/Si₃N₄ hybrid cavities. We observed 6-fold photoluminescence enhancement of an hBN single photon emission at room temperature. Our work will be useful for further development of cavity quantum electrodynamic experiments and on-chip integration of two-dimensional (2D) materials.

High-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2

Integrating solid-state quantum emitters with nanophotonic resonators is essential for efficient spin-photon interfacing and optical networking applications. While diamond color centers have proven to be excellent candidates for emerging quantum technologies, their integration with optical resonators remains challenging. Conventional approaches based on etching resonators into diamond often negatively impact color center performance and offer low device yield. Here, we developed an integrated photonics platform based on templated atomic layer deposition of TiO₂ on diamond membranes. Our fabrication method yields high-performance nanophotonic devices while avoiding etching wavelength-scale features into diamond. Moreover, this technique generates highly reproducible optical resonances and can be iterated on individual diamond samples, a unique processing advantage. Our approach is suitable for a broad range of both wavelengths and substrates and can enable high-cooperativity interfacing between cavity photons and coherent defects in diamond or silicon carbide, rare earth ions, or other material systems.

Single-shot ultrafast imaging attaining 70 trillion frames per second

Real-time imaging of countless femtosecond dynamics requires extreme speeds orders of magnitude beyond the limits of electronic sensors. Existing femtosecond imaging modalities either require event repetition or provide single-shot acquisition with no more than 10¹³ frames per second (fps) and 3 × 10² frames. Here, we report compressed ultrafast spectral photography (CUSP), which attains several new records in single-shot multi-dimensional imaging speeds. In active mode, CUSP achieves both 7 × 10¹³ fps and 10³ frames simultaneously by synergizing spectral encoding, pulse splitting, temporal shearing, and compressed sensing-enabling unprecedented quantitative imaging of rapid nonlinear light-matter interaction. In passive mode, CUSP provides four-dimensional (4D) spectral imaging at 0.5 × 10¹² fps, allowing the first single-shot spectrally resolved fluorescence lifetime imaging microscopy (SR-FLIM). As a real-time multi-dimensional imaging technology with the highest speeds and most frames, CUSP is envisioned to play instrumental roles in numerous pivotal scientific studies without the need for event repetition.

Ensemble-Induced Strong Light-Matter Coupling of a Single Quantum Emitter

We discuss a technique to strongly couple a single target quantum emitter to a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic ensemble of emitters. A collective coupling of the latter to both the cavity and the target emitter induces strong photon nonlinearities in addition to polariton formation, in contrast to common schemes for ensemble strong coupling. We demonstrate that strong coupling at the level of a single emitter can be engineered via coherent and dissipative dipolar interactions with the ensemble, and provide realistic parameters for a possible implementation with SiV^{-} defects in diamond. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavity-assisted quantum chemistry.

Developing a time-domain method for simulating statistical behavior of many-emitter systems in the presence of electromagnetic field

In this paper, one of the major shortcomings of the conventional numerical approaches is alleviated by introducing the probabilistic nature of molecular transitions into the framework of classical computational electrodynamics. The main aim is to develop a numerical method which is capable of capturing the statistical attributes caused by the interactions between a group of spontaneous as well as stimulated emitters and the surrounding electromagnetic field. The electromagnetic field is governed by classical Maxwell's equations, while energy is absorbed from and emitted to the (surrounding) field according to the transitions occurring for the emitters, which are governed by time-dependent probability functions. These probabilities are principally consistent with quantum mechanics. In order to validate the proposed method, it is applied to three different test cases: directionality of fluorescent emission in a corrugated single-hole gold nanodisk, spatial and temporal coherence of fluorescent emission in a hybrid photonic-plasmonic crystal, and stimulated emission of a core-shell SPASER (surface plasmon amplification by stimulated emission of radiation). The results are shown to be closely comparable to the experimental results reported in the literature.

Optical coherence of implanted silicon vacancy centers in thin diamond membranes

We report the fabrication and optical characterization of thin diamond membranes implanted with negatively charged silicon vacancy (SiV^-) centers. The variations in the membrane thickness enable the experimental study of optical coherence of SiV^- centers as the membrane thickness is varied from 100 nm to 1100 nm. Photoluminescence excitation spectroscopy at low temperature shows that most of the SiV^- centers in these membranes feature an optical linewidth ranging between 200 and 300 MHz. Furthermore, there is no discernable dependence of the optical linewidth on the membrane thickness for membranes as thin as 100 nm, indicating the feasibility of incorporating SiV^- centers in a varity of diamond nanostructures and still maintaining the excellent optical coherence of these color centers.

Inverse-designed diamond photonics

Diamond hosts optically active color centers with great promise in quantum computation, networking, and sensing. Realization of such applications is contingent upon the integration of color centers into photonic circuits. However, current diamond quantum optics experiments are restricted to single devices and few quantum emitters because fabrication constraints limit device functionalities, thus precluding color center integrated photonic circuits. In this work, we utilize inverse design methods to overcome constraints of cutting-edge diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications. Our design method leverages advanced optimization techniques to search the full parameter space for fabricable device designs. We experimentally demonstrate inverse-designed photonic free-space interfaces as well as their scalable integration with two vastly different devices: classical photonic crystal cavities and inverse-designed waveguide-splitters. The multi-device integration capability and performance of our inverse-designed diamond platform represents a critical advancement toward integrated diamond quantum optical circuits.

Current diamond quantum optics experiments are restricted to single devices and few quantum emitters due to fabrication constraints. Here, the authors utilize inverse design to overcome constraints of diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications.

Efficient Coupling of an Ensemble of Nitrogen Vacancy Center to the Mode of a High-Q, Si3N4 Photonic Crystal Cavity

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-photon loss and well-established fabrication techniques. However, quantum optics applications are not yet established. Here, we investigate an approach toward Si₃N₄-based quantum photonics utilizing a crossed waveguide, pump-probe design. The platform enables efficient, on-chip excitation, strong background suppression, and at the same time, efficient coupling to the mode of a high- Q photonic crystal cavity. The freestanding photonic crystal cavities reach high Q-factors up to 47 × 10³. To test our platform, we positioned an ensemble of negatively charged nitrogen vacancy centers located in a nanodiamond within the interaction zone of the photonic crystal cavity. We quantify the efficiency of the coupling with the β_λ-factor reaching values as large as 0.71. We further demonstrate on-chip excitation of the quantum emitter with strong suppression (∼20 dB) of the background fluorescence. Our results unfold the potential to utilize negatively charged nitrogen vacancy centers in nanodiamonds and Si₃N₄ platforms as an efficient, on-chip spin-photon interface in quantum photonics experiments.

Photon Blockade in Weakly Driven Cavity Quantum Electrodynamics Systems with Many Emitters

We use the scattering matrix formalism to analyze photon blockade in coherently driven cavity quantum electrodynamics systems with a weak drive. By approximating the weak coherent drive by an input single- and two-photon Fock state, we reduce the computational complexity of the transmission and the two-photon correlation function from exponential to polynomial in the number of emitters. This enables us to easily analyze cavity-based systems containing ∼50 quantum emitters with modest computational resources. Using this approach we study the coherence statistics of photon blockade while increasing the number of emitters for resonant and detuned multiemitter cavity quantum electrodynamics systems-we find that increasing the number of emitters worsens photon blockade in resonant systems, and improves it in detuned systems. We also analyze the impact of inhomogeneous broadening in the emitter frequencies on the photon blockade through this system.

Cascaded Cavities Boost the Indistinguishability of Imperfect Quantum Emitters

Recently, Grange et al. [Phys. Rev. Lett. 114, 193601 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.193601] showed the possibility of single-photon generation with a high indistinguishability from a quantum emitter despite strong pure dephasing, by "funneling" emission into a photonic cavity. Here, we show that a cascaded two-cavity system can further improve the photon characteristics and greatly reduce the Q factor requirement to levels achievable with present-day technology. Our approach leverages recent advances in nanocavities with an ultrasmall mode volume and does not require ultrafast excitation of the emitter. These results were obtained by numerical and closed-form analytical models with strong emitter dephasing, representing room-temperature quantum emitters.

Deterministic coupling of site-controlled quantum emitters in monolayer WSe2 to plasmonic nanocavities

Solid-state single-quantum emitters are crucial resources for on-chip photonic quantum technologies and require efficient cavity-emitter coupling to realize quantum networks beyond the single-node level^1,2. Monolayer WSe₂, a transition metal dichalcogenide semiconductor, can host randomly located quantum emitters^3-6, while nanobubbles⁷ as well as lithographically defined arrays of pillars in contact with the transition metal dichalcogenide act as spatially controlled stressors^8,9. The induced strain can then create excitons at defined locations. This ability to create zero-dimensional (0D) excitons anywhere within a 2D material is promising for the development of scalable quantum technologies, but so far lacks mature cavity integration and suffers from low emitter quantum yields. Here we demonstrate a deterministic approach to achieve Purcell enhancement at lithographically defined locations using the sharp corners of a metal nanocube for both electric field enhancement and to deform a 2D material. This nanoplasmonic platform allows the study of the same quantum emitter before and after coupling. For a 3 × 4 array of quantum emitters we show Purcell factors of up to 551 (average of 181), single-photon emission rates of up to 42 MHz and a narrow exciton linewidth as low as 55 μeV. Furthermore, the use of flux-grown WSe₂ increases the 0D exciton lifetimes to up to 14 ns and the cavity-enhanced quantum yields from an initial value of 1% to up to 65% (average 44%).

Phonon-Assisted Photoluminescence Up-Conversion of Silicon-Vacancy Centers in Diamond

Phonon-assisted anti-Stokes photoluminescence (ASPL) up-conversion lies at the heart of optical refrigeration in solids. The thermal energy contained in the lattice vibrations is taken away by the emitted anti-Stokes photons' ASPL process, resulting in laser cooling of solids. To date, net laser cooling of solids is limited in rare-earth (RE)-doped crystals, glasses, and direct band gap semiconductors. Searching more solid materials with efficient phonon-assisted photoluminescence up-conversion is important to enrich optical refrigeration research. Here, we demonstrate the phonon-assisted PL up-conversion process from the silicon vacancy (SiV) center in diamond for the first time by studying ASPL spectra for the dependence of temperature, laser power, and excitation energy. Although net cooling has not been observed, our results show that net laser cooling might be eventually achieved in diamond by improving the external quantum efficiency to higher than 95%. Our work provides a promising route to investigate the laser cooling effect in diamond.

Photon-mediated interactions between quantum emitters in a diamond nanocavity

Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes.

Cavity-Enhanced Raman Emission from a Single Color Center in a Solid

We demonstrate cavity-enhanced Raman emission from a single atomic defect in a solid. Our platform is a single silicon-vacancy center in diamond coupled with a monolithic diamond photonic crystal cavity. The cavity enables an unprecedented frequency tuning range of the Raman emission (100 GHz) that significantly exceeds the spectral inhomogeneity of silicon-vacancy centers in diamond nanostructures. We also show that the cavity selectively suppresses the phonon-induced spontaneous emission that degrades the efficiency of Raman photon generation. Our results pave the way towards photon-mediated many-body interactions between solid-state quantum emitters in a nanophotonic platform.