The interaction of photons with individual quantum systems is a very fundamental process in physics. Thereby, the emission rate as well as the angular emission pattern of a quantum emitter are not only a function of intrinsic properties of the emitter itself, but are also strongly modified by its surrounding. For instance, by restricting the optical modes which are allowed at the position of the dipole, the emission rate can be strongly modified and the emitted photons can be directed into specific optical modes. This effect can be demonstrated by the interaction of a single optically active quantum emitter with the strongly confined optical mode of a single-mode dielectric waveguide. Efficient coupling of the emitter to the dielectric structure can be achieved by placing the quantum emitter inside the evanescent field of the guided mode. This evanescent field coupling mechanism is discussed and demonstrated experimentally. A single nitrogen-vacancy center (NV-center), hosted in a nanodiamond is deterministically coupled to a tapered optical fiber (TOF) via the evanescent field of its guided mode (coupling efficiencies exceeding 30% are predicted). By employing an AFM-based nanomanipulation technique, the diamond nanocrystal is placed on the nanofiber waist of the TOF. Beforehand, the diamond nanocrystal has been characterized to guarantee that it hosts only one fluorescing NV-center. While the diamond nanocrystal is optically exited, single photon fluorescence of the NV-center is detected at both outputs of the tapered optical fiber. This verifies the evanescent coupling of the emitter to the guided mode. In order to quantify the coupling, the comparison of the emission rate into free space with the rate into the fiber yields that 10.0(5) of the emitted photons are coupled into the tapered optical fiber. In the determination of this value, the orientation of the emitting dipoles and the emission pattern, which are modified by the TOF, have been considered. The NV-center features a broad emission spectrum which can be used to investigate the wavelength-dependence of the coupling. Comparing the spectra of the emission into the fiber mode with the emission into free space modes roughly resembles the expected wavelength dependency of the coupling efficiency. The evanescent coupling and the deterministic positioning of preselected fluorescing diamond nanocrystals, which has been demonstrated with the TOF, can be applied to other waveguide structures as well. Dielectric single-mode waveguides made of Ta2O5 on a SiO2 substrate promise similar coupling efficiencies to tapered optical fibers (above 30%). With the design of the on-chip wave-guiding structure being flexible, the combination with other optical on-chip elements is feasible, rendering it a promising platform for on-chip photonic experiments. Test structures of this waveguide design are realized using lithographic processes and are characterized. These waveguides are equipped with inverted taper structures to allow efficient off-chip coupling with butt-coupling to standard single-mode fibers. The evanescent coupling of a single quantum emitter to a singe optical mode can be used to efficiently collect emission of the quantum emitter. This can help building a compact single photon source and is beneficial for the optical read-out of the quantum emitter's internal degree of freedom, which can be either used as probe (sensing) or as information-storage. Utilizing the high coupling efficiency, for instance, the non-linearities of the quantum system can be exploited to build a single photon transistor. The evanescent coupling is very broadband (about hundred nanometers), allowing to efficiently collect emission from broadband emitters like the NV-center, but it can also be used for multi-wavelength manipulation schemes.
The interaction of photons with individual quantum systems is a very fundamental process in physics. Thereby, the emission rate as well as the angular emission pattern of a quantum emitter are not only a function of intrinsic properties of the emitter itself, but are also strongly modified by its surrounding. For instance, by restricting the optical modes which are allowed at the position of the dipole, the emission rate can be strongly modified and the emitted photons can be directed into specific optical modes. This effect can be demonstrated by the interaction of a single optically active quantum emitter with the strongly confined optical mode of a single-mode dielectric waveguide. Efficient coupling of the emitter to the dielectric structure can be achieved by placing the quantum emitter inside the evanescent field of the guided mode. This evanescent field coupling mechanism is discussed and demonstrated experimentally. A single nitrogen-vacancy center (NV-center), hosted in a nanodiamond is deterministically coupled to a tapered optical fiber (TOF) via the evanescent field of its guided mode (coupling efficiencies exceeding 30% are predicted). By employing an AFM-based nanomanipulation technique, the diamond nanocrystal is placed on the nanofiber waist of the TOF. Beforehand, the diamond nanocrystal has been characterized to guarantee that it hosts only one fluorescing NV-center. While the diamond nanocrystal is optically exited, single photon fluorescence of the NV-center is detected at both outputs of the tapered optical fiber. This verifies the evanescent coupling of the emitter to the guided mode. In order to quantify the coupling, the comparison of the emission rate into free space with the rate into the fiber yields that 10.0(5) of the emitted photons are coupled into the tapered optical fiber. In the determination of this value, the orientation of the emitting dipoles and the emission pattern, which are modified by the TOF, have been considered. The NV-center features a broad emission spectrum which can be used to investigate the wavelength-dependence of the coupling. Comparing the spectra of the emission into the fiber mode with the emission into free space modes roughly resembles the expected wavelength dependency of the coupling efficiency. The evanescent coupling and the deterministic positioning of preselected fluorescing diamond nanocrystals, which has been demonstrated with the TOF, can be applied to other waveguide structures as well. Dielectric single-mode waveguides made of Ta2O5 on a SiO2 substrate promise similar coupling efficiencies to tapered optical fibers (above 30%). With the design of the on-chip wave-guiding structure being flexible, the combination with other optical on-chip elements is feasible, rendering it a promising platform for on-chip photonic experiments. Test structures of this waveguide design are realized using lithographic processes and are characterized. These waveguides are equipped with inverted taper structures to allow efficient off-chip coupling with butt-coupling to standard single-mode fibers. The evanescent coupling of a single quantum emitter to a singe optical mode can be used to efficiently collect emission of the quantum emitter. This can help building a compact single photon source and is beneficial for the optical read-out of the quantum emitter's internal degree of freedom, which can be either used as probe (sensing) or as information-storage. Utilizing the high coupling efficiency, for instance, the non-linearities of the quantum system can be exploited to build a single photon transistor. The evanescent coupling is very broadband (about hundred nanometers), allowing to efficiently collect emission from broadband emitters like the NV-center, but it can also be used for multi-wavelength manipulation schemes.
A diamond nano-crystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelength-diameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nano-manipulation technique, we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NV-center to the guided mode of the TOF. By comparing photon count rates of the fiber-guided and the free-space modes and with the help of numerical finite-difference time domain simulations, we determine a lower and upper bound for the coupling efficiency of (9.5 ± 0.6)% and (10.4 ± 0.7)%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.
The coupling of a quantum emitter to the modes of a silica toroid is presented. A fiber taper is used to manipulate and transfer a preselected diamond nanocrystal onto the toroid. Optical coupling of few nitrogen vacancy (NV) color centers contained inside the nanocrystal to the resonator modes is demonstrated by detecting the fluorescence via a tapered optical fiber coupler. A clear antibunching in the photon correlation measurement is observed indicating emission from only six NV centers residing inside the nanocrystal. The latter is confirmed by a photoluminescence spectrum at liquid helium temperature resolving individual zero phonon lines.
The development of integrated quantum technologies relies on efficient single-photon sources and deterministic quantum operations. Solid-state quantum emitters coupled to nanophotonic structures present a promising solution. This work investigates color centers in nanodiamonds,particularly Germanium Vacancy (GeV) [1], Silicon Vacancy (SiV), and Magnesium Vacancy (MgV) centers, a novel, understudied emitter [2], at both room and cryogenic temperatures. These nanodiamonds are chosen for their stability and superior coherence properties at cryogenic conditions. To collect their emission we have successfully deposited a single GeV- nanodiamond on tapered optical nanofibers by gently contacting a diluted droplet to the nanofiber [3], demonstrating an average coupling of 15% into a conventional optical fiber through the confined evanescent field of the nanofiber mode and single photon emission. Building on this achievement, we aim to study the system at cryogenic temperatures, which should enable the demonstration of efficient and indistinguishable single photons coupling to conventional optical fibers.Current collection efficiencies are intrinsically limited to 30% with the nanofiber[4], and improving light-matter coupling efficiency remains a key goal for high-performance single-photon sources and more advanced applications.To achieve this, we are investigating a newly developed technique [5]: the deterministic fabrication of dielectric and plasmonic nanostructures directly on the nanofiber surface using an “electron beam induced deposition” (EBID). The precision and versatility of this technique openthe way to different applications: Purcell enhancement of the quantum emitter emission using metallic nanoantennas and efficient light-matter coupling using dielectric structures. Our first calculation shown that optimizing the composition, geometry, and placement of these nanostructures a simple pattern of carbon pillars around the emitter should increase coupling efficiency up to 69% while maintaining high transmission in the nanofiber. We also demonstratethe fabrication of those nanostructures on top of the nanofiber.This work advances integrated nanophotonic devices for single-photon sources.[1] M. Nahra et al.,Single germanium vacancy centers in nanodia-monds with bulk-like spectral stability, AVS Quantum Science3(1), (2021).[2] E. Corte et al, Magnesium-Vacancy Optical Centers in Diamond, ACS Photonics, 10(1), p101-110, (2023)[3] Pierini et al. Highly Photostable Perovskite Nanocubes : TowardIntegrated Single Photon Sources Based on Tapered Nanofibers.ACS Photonics, 7(8), p.2265-2272, (2020).[4] Fujiwara et al., Highly efficient coupling of photons from na-noemitters into single-mode optical fibers, Nano Letters, 11(10),4362–4365, (2011).[5] A.Balena et al, Deterministic Bottom-Up Fabrication of Plasmonic Nanostructures on OpticalNanofibers via Blurred Electron Beam Deposition (soon to be published in Advance Material).
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.
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.
NV center in diamond is attracting a lot of attention in quantum information processing community [1]. Been spin system in clean and well-controlled environment of diamond it shows outstanding performance as quantum memory even at room temperature, spin control with single shot optical readout and possibility to build up quantum registers even on single NV center. Moreover, NV centers could be used as high-resolution sensitive elements of detectors of magnetic or electric field, temperature, tension, force or rotation. For all of these applications collection of the light emitted by NV center is crucial point. There were number of approaches suggested to address this issue, proposing use of surface plasmoms [2], manufacturing structures in diamond [3] etc. One of the key feature of any practically important interface is compatibility with the fiber technology. Several groups attacking this problem using various approaches. One of them is placing of nanodiamonds in the holes of photonic crystal fiber [4], another is utilization of AFM to pick and place nanodiamond on the tapered fiber[5]. We have developed a novel technique of placing a nanodiamond with single NV center on the tapered fiber by controlled transfer of a nanodiamond from one “donor” tapered fiber to the “target” clean tapered fiber. We verify our ability to transfer only single color centers by means of measurement of second order correlation function. With this technique, we were able to double collection efficiency of confocal microscope. The majority of the factors limiting the collection of photons via optical fiber are technical and may be removed allowing order of magnitude improved in collection. We also discuss number of extensions of this technique to all fiber excitation and integration with nanostructures. References: [1] Marcus W. Doherty, Neil B. Manson, Paul Delaney, Fedor Jelezko, Jörg Wrachtrup, Lloyd C.L. Hollenberg , " The nitrogen-vacancy colour centre in diamond," Physics Reports, vol. 528, no. 1, p. 1–45, 2013. [2] A.V. Akimov, A. Mukherjee, C.L. Yu, D.E. Chang, A.S. Zibrov, P.R. Hemmer, H. Park and M.D. Lukin, "Generation of single optical plasmons in metallic nanowires coupled to quantum dots," Nature, vol. 450, p. 402–406, 2007. [3] Michael J. Burek , Yiwen Chu, Madelaine S.Z. Liddy, Parth Patel, Jake Rochman , Srujan Meesala, Wooyoung Hong, Qimin Quan, Mikhail D. Lukin and Marko Loncar High quality-factor optical nanocavities in bulk single-crystal diamond, Nature communications 6718 (2014) [4] Tim Schroder, Andreas W. Schell, Gunter Kewes, Thomas Aichele, and Oliver Benson Fiber-Integrated Diamond-Based Single Photon Source, Nano Lett. 2011, 11, 198-202 [5]Lars Liebermeister, et. al. “Tapered fiber coupling of single photons emitted by a deterministically positioned single nitrogen vacancy center”, Appl. Phys. Lett. 104, 031101 (2014)