Integration of solid-state quantum emitters such as NV center in diamond with tapered optical fiber is demanded by number of applications ranging from sensing and imaging to quantum communications and computations. Nevertheless, utilization of a single NV center coupled with an optical fiber meets significant challenge of fiber fluorescence that can considerably mask emission of the quantum object. In this paper, we analyze main sources of such fluorescence for the case of NV center coupled to tapered single mode optical fiber and discuss possible ways of improving signal to noise ratio in this case.
Integration of solid-state quantum emitters such as NV center in diamond with tapered optical fiber is demanded by number of applications ranging from sensing and imaging to quantum communications and computations. Nevertheless, utilization of a single NV center coupled with an optical fiber meets significant challenge of fiber fluorescence that can considerably mask emission of the quantum object. In this paper, we analyze main sources of such fluorescence for the case of NV center coupled to tapered single mode optical fiber and discuss possible ways of improving signal to noise ratio in this case.
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.
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)
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.
Optical fibers are crucial for guiding light in various sensing applications. Especially for quantum sensors such as the nitrogen-vacancy (NV) center in diamond, they enable light control and device miniaturization. However, fluorescence and scattering within the fiber, often referred to as fiber background, autofluorescence, or autoluminescence, can overlap spectrally with the NV centers' fluorescence, degrading the signal-to-noise ratio and thus limiting sensor sensitivity. Here, we investigate the optical spectra of standard optical fibers, considering material dependencies, physical influences, and their fluorescence scaling with excitation power and wavelength. Our results identify spectral components and fiber types with minimal unwanted background signals, guiding the selection of optimal fibers for NV-based quantum sensing.
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.