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Adiabatic optical fiber coupling for diamond quantum nanophotonic devices

K. Aoyama, M. Kamata, S. Hachuda, M. Kunii, T. Tamanuki, S. Ishida, H. Matsukiyo, Natthajuks Pholsen, Masao Nishioka, S. Ji, H. Otsuki, Kimura Kosuke, Takenaka Kouta, Onoda Shinobu, T. Baba, S. Iwamoto, H. Kosaka

Published Oct 10, 2025• 0 citations
OpenAlex

Abstract

Efficient and stable interfaces between quantum devices and optical fibers are a key requirement for realizing large-scale quantum networks and distributed quantum computing. Adiabatic coupling between a diamond waveguide and a tapered fiber provides a promising route to broadband, efficient, and robust optical coupling [*]. In this work, we present the design, fabrication, and characterization of such interface. First, we performed FDTD simulations to optimize the diamond nanobeam geometry. These simulations predict a maximum coupling efficiency of 98% with an optimized design. Next, tapered optical fibers with angles <3° and tips <150 nm were fabricated using an optimized hydrofluoric acid etching process. Finally, we experimentally evaluated the coupling efficiency by measuring photoluminescence from nitrogen-vacancy centers in a diamond nanobeam through the fiber, yielding an efficiency of 14%. These results establish a clear design path for a practical interface. The gap between simulation and experiment highlights the need to improve physical contact and nanobeam fabrication. Future work will target these optimizations to realize a more efficient and robust packaged device, a critical component for quantum networks.

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•Jul 2, 2019•0 citations
Efficient and stable interfaces between quantum devices and optical fibers are a key requirement for realizing large-scale quantum networks and distributed quantum computing. Adiabatic coupling between a diamond waveguide and a tapered fiber provides a promising route to broadband, efficient, and robust optical coupling [*]. In this work, we present the design, fabrication, and characterization of such interface. First, we performed FDTD simulations to optimize the diamond nanobeam geometry. These simulations predict a maximum coupling efficiency of 98% with an optimized design. Next, tapered optical fibers with angles <3° and tips <150 nm were fabricated using an optimized hydrofluoric acid etching process. Finally, we experimentally evaluated the coupling efficiency by measuring photoluminescence from nitrogen-vacancy centers in a diamond nanobeam through the fiber, yielding an efficiency of 14%. These results establish a clear design path for a practical interface. The gap between simulation and experiment highlights the need to improve physical contact and nanobeam fabrication. Future work will target these optimizations to realize a more efficient and robust packaged device, a critical component for quantum networks.
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. A compact way to efficiently couple a solid-state qubit, the nitrogen vacancy center in diamond, with a single-mode optical fiber is shown. The ability to efficiently couple stationary quantum memories to a single spatial mode in a silica fiber is highly desired for various applications in quantum information processing. Now, Rishi Patel and co-workers, working in the group of Dirk Englund at the Massachusetts Institute of Technology have proposed and experimentally demonstrated an approach involving adiabatic power transfer between a tapered silica fiber and a single-mode micro-waveguide made from high-quality diamond. This integrated diamond–silica waveguide system enables optical collection from nitrogen vacancy centers with overall efficiencies in the range 16%–37%—a roughly four-fold improvement over previous approaches that employed diamond point emitters rather than waveguiding structures. Numerical optimization analysis suggests that near-unity efficiencies are theoretically feasible.
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.
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.
Tapered optical fibers are promising one-dimensional nanophotonic waveguides that can provide efficient coupling between their fundamental mode and quantum nanoemitters placed inside them. Here, we present numerical studies on the coupling of single nitrogen-vacancy (NV) centers (single point dipoles) in nanodiamonds with tapered fibers. Our results lead to two important conclusions: (1) A maximum coupling efficiency of 53.4% can be realized for the two fiber ends when the NV bare dipole is located at the center of the tapered fiber. (2) NV centers even in 100-nm-sized nanodiamonds where bulk-like optical properties were reported show a coupling efficiency of 22% at the taper surface, with the coupling efficiency monotonically decreasing as the nanodiamond size increases. These results will be helpful in guiding the development of hybrid quantum devices for applications in quantum information science.
Quantum networks are a rapidly developing field of quantum information processing that have the potential to enable long-range entanglement between future quantum computers as well as the implementation of secure communication through quantum key distribution. Two key components of such networks are quantum memories to store entangled photon pairs for use in the quantum repeater protocol and quantum light-matter interfaces to efficiently interconvert between stationary qubits and flying qubits in the form of photons. Rare-earth ion-doped crystals are a promising solid-state platform that show promise for both of these applications due to their long optical and spin coherence times. Due to their relatively weak optical transitions, rare-earth ions have been coupled with nanophotonic resonators to enhance their transition strengths, with past work in the Faraon group utilizing focused ion beam milled photonic crystal resonators with 45-degree angled couplers to couple light in and out. Such resonators have the disadvantage of requiring manual alignment to fabricate, and the couplers are also relatively inefficient which limits the performance of such devices. It is therefore desirable to move towards silicon photonics, where mature techniques such as electron-beam lithography can allow for scalable fabrication of nanophotonic cavities together with high coupling efficiencies. In this thesis, we demonstrate significant progress towards the usage of acid-etched tapered optical fibers as an efficient interface for coupling light into tapered silicon nanophotonic waveguides. We show comprehensive simulations of the taper geometries required to achieve adiabatic coupling with theoretical efficiencies of more than 99%, and design a silicon photonic crystal mirror to be used in the measurement of the fiber-waveguide coupling efficiency. We then optimize the hydrofluoric acid fiber etching process and demonstrate the ability to make tapered fibers which are 200 microns long with a taper half-angle of 2 degrees and a tip diameter of 50 nm. Using these tapered fibers to couple light into tapered silicon waveguides fabricated using electron-beam lithography shows a moderately high coupling efficiency of 11.4% with the potential for improvement. This method of tapered fiber coupling shows promise to be integrated into silicon nanophotonic resonators on rare-earth ion doped crystals and allow for highly efficient quantum memories and quantum light-matter interfaces in the solid-state.