Numerical simulations of nanodiamond nitrogen-vacancy centers coupled with tapered optical fibers as hybrid quantum nanophotonic devices

Numerical analysis of the ultra-wide tunability of nanofiber Bragg cavities

Nanofiber Bragg cavities (NFBCs) are solid-state microcavities fabricated in optical tapered fiber. They can be tuned to a resonance wavelength of more than 20 nm by applying mechanical tension. This property is important for matching the resonance wavelength of an NFBC with the emission wavelength of single-photon emitters. However, the mechanism of the ultra-wide tunability and the limitation of the tuning range have not yet been clarified. It is important to comprehensively analyze both the deformation of the cavity structure in an NFBC and the change in the optical properties due to the deformation. Here, we present an analysis of the ultra-wide tunability of an NFBC and the limitation of the tuning range using three dimensional (3D) finite element method (FEM) and 3D finite-difference time-domain (FDTD) optical simulations. When we applied a tensile force of 200 μN to the NFBC, a stress of 5.18 GPa was concentrated at the groove in the grating. The grating period was extended from 300 to 313.2 nm, while the diameter slightly shrank from 300 to 297.1 nm in the direction of the grooves and from 300 to 298 nm in the direction orthogonal to the grooves. This deformation shifted the resonance peak by 21.5 nm. These simulations indicated that both the elongation of the grating period and the small shrinkage of the diameter contributed to the ultra-wide tunability of the NFBC. We also calculated the dependence of the stress at the groove, the resonance wavelength, and the quality Q factor while changing the total elongation of the NFBC. The dependence of the stress on the elongation was 1.68 × 10^-2 GPa/μm. The dependence of the resonance wavelength was 0.07 nm/μm, which almost agrees with the experimental result. When the NFBC, assumed to have the total length of 32 mm, was stretched by 380 μm with the tensile force of 250 μN, the Q factor for the polarization mode parallel to the groove changed from 535 to 443, which corresponded to a change in Purcell factor from 5.3 to 4.9. This slight reduction seems acceptable for the application as single photon sources. Furthermore, assuming a rupture strain of the nanofiber of 10 GPa, it was estimated that the resonance peak could be shifted by up to about 42 nm.

Nanodiamond-Based Optical-Fiber Quantum Probe for Magnetic Field and Biological Sensing

Owing to the unique electronic spin properties, nitrogen-vacancy (NV) centers hosted in diamond have emerged as a powerful quantum tool for detecting various physical parameters and biological species. In this work, an optical-fiber quantum probe, configured by chemically modifying nanodiamonds on the surface of a cone fiber tip, is developed. Based on the continuous-wave optically detected magnetic resonance method and lock-in amplification technique, it is found that the sensing performance of probes can be engineered by varying the nanodiamond dispersion concentration and modification duration during the chemical modification process. Combined with a pair of magnetic flux concentrators, the magnetic field detection sensitivity has reached 0.57 nT/Hz^1/2@1 Hz, a new record among the fiber magnetometers based on nanodiamonds. Taking Gd³⁺ as the demo, the capability of probes in paramagnetic species detection is also demonstrated experimentally. Our work provides a new approach to develop NV centers as quantum probes featuring high integration, multifunction, high sensitivity, etc.

Simulating topological phases with atom arrays in an optical waveguide

In this paper, we employ the atomic arrays in one-dimensional optical waveguides to simulate topological phases, where the waveguide is modeled as a one-dimensional infinitely long coupled cavity array. Under the Markov approximation, the coherent and dissipative coupling between atoms is established by eliminating waveguide modes. When the detuning between atoms and cavity fields lies in the band gap, the dynamics of the system is completely dominated by the coherent interaction. Under this condition, we designed three atomic arrays with different geometries and show that the topologically trivial and non-trivial phases of atomic arrays can be simulated. Furthermore, by introducing periodic atomic driving, the topological phase transition can be induced by adjusting the driving parameters. Finally, we investigate the effect of next-nearest neighbor interactions on topological state transfer and find that the next-nearest neighbor interactions break the degenerated bandgap state and establish a topological state transfer channel.

Hybrid device of hexagonal boron nitride nanoflakes with defect centres and a nano-fibre Bragg cavity

Solid-state quantum emitters coupled with a single mode fibre are of interest for photonic and quantum applications. In this context, nanofibre Bragg cavities (NFBCs), which are microcavities fabricated in an optical nanofibre, are promising devices because they can efficiently couple photons emitted from the quantum emitters to the single mode fibre. Recently, we have realized a hybrid device of an NFBC and a single colloidal CdSe/ZnS quantum dot. However, colloidal quantum dots exhibit inherent photo-bleaching. Thus, it is desired to couple an NFBC with hexagonal boron nitride (hBN) as stable quantum emitters. In this work, we realize a hybrid system of an NFBC and ensemble defect centres in hBN nanoflakes. In this experiment, we fabricate NFBCs with a quality factor of 807 and a resonant wavelength at around 573 nm, which matches well with the fluorescent wavelength of the hBN, using helium-focused ion beam (FIB) system. We also develop a manipulation system to place hBN nanoflakes on a cavity region of the NFBCs and realize a hybrid device with an NFBC. By exciting the nanoflakes via an objective lens and collecting the fluorescence through the NFBC, we observe a sharp emission peak at the resonant wavelength of the NFBC.

Preferential coupling of diamond NV centres in step-index fibres

Diamonds containing the negatively charged nitrogen-vacancy centre are a promising system for room-temperature magnetometry. The combination of nano- and micro-diamond particles with optical fibres provides an option for deploying nitrogen-vacancy magnetometers in harsh and challenging environments. Here we numerically explore the coupling efficiency from nitrogen-vacancy centres within a diamond doped at the core/clad interface across a range of commercially available fibre types so as to inform the design process for a diamond in fibre magnetometers. We determine coupling efficiencies from nitrogen-vacancy centres to the guided modes of a step-index fibre and predict the optically detected magnetic resonance (ODMR) generated by a ensemble of four nitrogen-vacancy centres in this hybrid fibre system. Our results show that the coupling efficiency is enhanced with a high refractive index difference between the fibre core and cladding and depends on the radial position of the nitrogen-vacancy centres in the fibre core. Our ODMR simulations show that due to the preferential coupling of the nitrogen-vacancy emission to the fibre guided modes, certain magnetometry features such as ODMR contrast can be enhanced and lead to improved sensitivity in such diamond-fibre systems, relative to conventional diamond only ensemble geometries.

High-efficiency coupling of single quantum emitters into hole-tailored nanofibers

We propose a scheme to enhance the coupling efficiency of photons from a single quantum emitter into a hole-tailored nanofiber. The single quantum emitter is positioned inside a circular hole etched along the radial axis of the nanofiber. The coupling efficiency can be effectively enhanced and is twice as high as the case in which only an intact nanofiber without the hole is used. The effective enhancement independent of a cavity can avoid the selection of a single emitter for the specific wavelength, which means a broad operating wavelength range. Numerical simulations are performed to optimize the coupling efficiency by setting appropriate diameters of the nanofiber and the hole. The simulation results show that the coupling efficiency can reach 62.8% when the single quantum emitter with azimuthal polarization (x direction) is at a position 200 nm from the middle of the hole along the hole-axial direction. The diameters of the nanofiber and the hole are 800 nm and 400 nm, respectively, while the wavelength of the single quantum emitter is 852 nm. Hole-tailored nanofibers have a simple configuration and are easy to fabricate and integrate with other micro/nanophotonic structures; this fiber structure has wide application prospects in quantum information processing and quantum precision measurement.

Direct optical excitation of an NV center via a nanofiber Bragg-cavity: a theoretical simulation

Direct optical excitation of a nitrogen-vacancy (NV) center in nanodiamond by light via a nanofiber is of interest for all-fiber-integrated quantum applications. However, the background light induced by the excitation light via the nanofiber is problematic as it has the same optical wavelength as the emission light from the NV center. In this paper, we propose using a nanofiber Bragg cavity to address this problem. We numerically simulate and estimate the electric field of a nanodiamond induced by excitation light applied from an objective lens on a confocal microscope system, a nanofiber, and nanofiber Bragg-cavities (NFBCs). We estimate that by using a nanofiber, the optical excitation intensity can be decreased by roughly a factor of 10 compared to using an objective lens, while for an NFBC with a grating number of 240 (120 for one side) on a nanofiber the optical excitation intensity can be significantly decreased by roughly a factor of 100. This reduction of optical excitation intensity will make it possible to distinguish the fluorescence of the NV center from the background light.

Fabrication of a nanofiber Bragg cavity with high quality factor using a focused helium ion beam

Nanofiber Bragg cavities (NFBCs) are solid-state microcavities fabricated in an optical tapered fiber. NFBCs are promising candidates as a platform for photonic quantum information devices due to their small mode volume, ultra-high coupling efficiencies, and ultra-wide tunability. However, the quality (Q) factor has been limited to be approximately 250, which may be due to limitations in the fabrication process. Here we report high Q NFBCs fabricated using a focused helium ion beam. Whenan NFBC with grooves of 640 periods is fabricated, the Q factor is over 4170, which is more than 16 times larger than that previously fabricated using a focused gallium ion beam.

Non-contact detection of nanoscale structures using optical nanofiber

The detection of nanoscale structure/material property in a wide observation area is becoming very important in various application fields. However, it is difficult to utilize current optical technologies. Toward the realization of novel alternative, we have investigated a new optical sensing method using an optical nanofiber. When the nanofiber vertically approached a glass prism with a partial gold film, the material differences between the glass and the gold were detected as a transmittance difference of 6% with a vertical resolution of 9.6 nm. The nanofiber was also scanned 100 nm above an artificial small protruding object with a width of 240 nm. The object was detected with a horizontal resolution of 630 nm, which was less than the wavelength of the probe light.

Tailoring a nanofiber for enhanced photon emission and coupling efficiency from single quantum emitters

We present a novel approach to enhance the spontaneous emission rate of single quantum emitters in an optical nanofiber-based cavity by introducing a narrow air-filled groove into the cavity. Our results show that the Purcell factor for single quantum emitters inside the groove of the nanofiber-based cavity can be at least six times greater than for such an emitter on the fiber surface when using an optimized cavity mode and groove width. Moreover, the coupling efficiency of single quantum emitters into the guided mode of this nanofiber-based cavity can reach up to ∼80% with only 35 cavity-grating periods. This new system has the potential to act as an all-fiber platform to realize efficient coupling of photons from single emitters into an optical fiber for quantum information applications.

Fiber-Coupled Diamond Micro-Waveguides toward an Efficient Quantum Interface for Spin Defect Centers

We report the direct integration and efficient coupling of nitrogen vacancy (NV) color centers in diamond nanophotonic structures into a fiber-based photonic architecture at cryogenic temperatures. NV centers are embedded in diamond micro-waveguides (μWGs), which are coupled to fiber tapers. Fiber tapers have low-loss connection to single-mode optical fibers and hence enable efficient integration of NV centers into optical fiber networks. We numerically optimize the parameters of the μWG-fiber-taper devices designed particularly for use in cryogenic experiments, resulting in 35.6% coupling efficiency, and experimentally demonstrate cooling of these devices to the liquid helium temperature of 4.2 K without loss of the fiber transmission. We observe sharp zero-phonon lines in the fluorescence of NV centers through the pigtailed fibers at 100 K. The optimized devices with high photon coupling efficiency and the demonstration of cooling to cryogenic temperatures are an important step to realize fiber-based quantum nanophotonic interfaces using diamond spin defect centers.

Preparing entangled states between two NV centers via the damping of nanomechanical resonators

We propose an efficient scheme for preparing entangled states between two separated nitrogen-vacancy (NV) centers in a spin-mechanical system via a dissipative quantum dynamical process. The proposal actively exploits the nanomechanical resonator (NAMR) damping to drive the NV centers to the target state through a quantum reservoir engineering approach. The distinct features of the present work are that we turn the detrimental source of noise into a resource and only need high-frequency low-Q mechanical resonators, which make our scheme more simple and feasible in experimental implementation. This protocol may have interesting applications in quantum information processing with spin-mechanical systems.

Efficient Single-Photon Coupling from a Nitrogen-Vacancy Center Embedded in a Diamond Nanowire Utilizing an Optical Nanofiber

Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.

Wiring up pre-characterized single-photon emitters by laser lithography

Future quantum optical chips will likely be hybrid in nature and include many single-photon emitters, waveguides, filters, as well as single-photon detectors. Here, we introduce a scalable optical localization-selection-lithography procedure for wiring up a large number of single-photon emitters via polymeric photonic wire bonds in three dimensions. First, we localize and characterize nitrogen vacancies in nanodiamonds inside a solid photoresist exhibiting low background fluorescence. Next, without intermediate steps and using the same optical instrument, we perform aligned three-dimensional laser lithography. As a proof of concept, we design, fabricate, and characterize three-dimensional functional waveguide elements on an optical chip. Each element consists of one single-photon emitter centered in a crossed-arc waveguide configuration, allowing for integrated optical excitation and efficient background suppression at the same time.

Detailed numerical analysis of photon emission from a single light emitter coupled with a nanofiber Bragg cavity

Coupling of a single dipole with a nanofiber Bragg cavity (NFBC) approximating an actually fabricated structure was numerically analyzed using three dimensional finite-difference time-domain simulations for different dipole positions. For the given model structure, the Purcell factor and coupling efficiency reached to 19.1 and 82%, respectively, when the dipole is placed outside the surface of the fiber. Interestingly, these values are very close to the highest values of 20.2 and 84% obtained for the case when the dipole was located inside the fiber at the center. The analysis performed in this study will be useful in improving the performance of single-photon emitter-related quantum devices using NFBCs.

Efficient photon coupling from a diamond nitrogen vacancy center by integration with silica fiber

A central goal in quantum information science is to efficiently interface photons with single optical modes for quantum networking and distributed quantum computing. Here, we introduce and experimentally demonstrate a compact and efficient method for the low-loss coupling of a solid-state qubit, the nitrogen vacancy (NV) center in diamond, with a single-mode optical fiber. In this approach, single-mode tapered diamond waveguides containing exactly one high quality NV memory are selected and integrated on tapered silica fibers. Numerical optimization of an adiabatic coupler indicates that near-unity-efficiency photon transfer is possible between the two modes. Experimentally, we find an overall collection efficiency between 16% and 37% and estimate a single photon count rate at saturation above 700 kHz. This integrated system enables robust, alignment-free, and efficient interfacing of single-mode optical fibers with single photon emitters and quantum memories in solids.

Ultrathin fiber-taper coupling with nitrogen vacancy centers in nanodiamonds at cryogenic temperatures

We demonstrate cooling of ultrathin fiber tapers coupled with nitrogen vacancy (NV) centers in nanodiamonds to cryogenic temperatures. Nanodiamonds containing multiple NV centers are deposited on the subwavelength 480-nm-diameter nanofiber region of fiber tapers. The fiber tapers are successfully cooled to 9 K using our home-built mounting holder and an optimized cooling speed. The fluorescence from the nanodiamond NV centers is efficiently channeled into a single guided mode and shows characteristic sharp zero-phonon lines (ZPLs) of both neutral and negatively charged NV centers. The present nanofiber/nanodiamond hybrid systems at cryogenic temperatures can be used as NV-based quantum information devices and for highly sensitive nanoscale magnetometry in a cryogenic environment.

Highly efficient coupling of nanolight emitters to a ultra-wide tunable nanofibre cavity

Solid-state microcavities combining ultra-small mode volume, wide-range resonance frequency tuning, as well as lossless coupling to a single mode fibre are integral tools for nanophotonics and quantum networks. We developed an integrated system providing all of these three indispensable properties. It consists of a nanofibre Bragg cavity (NFBC) with the mode volume of under 1 μm(3) and repeatable tuning capability over more than 20 nm at visible wavelengths. In order to demonstrate quantum light-matter interaction, we establish coupling of quantum dots to our tunable NFBC and achieve an emission enhancement by a factor of 2.7.