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New Journal of Physics

An integrated fiber trap for single-ion photonics

Hiroki Takahashi, Alex Wilson, Andrew Riley-Watson, Fedja Oručević, Nicolas Seymour-Smith, Matthias Keller, Wolfgang Lange

Published May 8, 2013• 45 citations
Publisher

Abstract

We report on a novel single-photon source using a single calcium ion trapped between the end facets of two optical fibers. The optical fibers act as photonic channels, and in addition their metallic jackets provide a trapping electric field for the ion. Our system successfully combines a stable single-atom emitter with fiber optics, demonstrating remarkable compactness and scalability. In consequence, it is very well suited for use in quantum networks, where single ions interface with single photons traveling through optical fibers. We have demonstrated the non-classical character of the photons generated by this efficient source in continuous as well as pulsed mode.

DOI
10.1088/1367-2630/15/5/053011

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We report on a novel single-photon source using a single calcium ion trapped between the end facets of two optical fibers. The optical fibers act as photonic channels, and in addition their metallic jackets provide a trapping electric field for the ion. Our system successfully combines a stable single-atom emitter with fiber optics, demonstrating remarkable compactness and scalability. In consequence, it is very well suited for use in quantum networks, where single ions interface with single photons traveling through optical fibers. We have demonstrated the non-classical character of the photons generated by this efficient source in continuous as well as pulsed mode.
Single atoms or atom-like emitters are the purest source of single photons, they are intrinsically incapable of multi-photon emission. To demonstrate this degree of photon number-state purity we have realized a single-photon source using a single ion trapped at the common focus of high numerical aperture lenses. Our trapped-ion source produces single-photon pulses with without any background subtraction. After subtracting detector dark counts the residual is less than 3 × 10 −4 (95% confidence interval). The multi-photon component of the source light field is low enough that we measure violation of a quantum non-Gaussian state witness, by this characterization the source output is indistinguishable from ideal attenuated single photons. In combination with efforts to enhance collection efficiency from single emitters, our results suggest that single trapped ions are not only ideal stationary qubits for quantum information processing, but promising sources of light for scalable optical quantum networks.
We have realized a compact system to efficiently couple the fluorescent light emitted by a single trapped ion to two opposing optical fibers. The fibers are tightly integrated in the center electrodes of a miniature endcap trap. They capture light from the ion with a numerical aperture of 0.34 each, corresponding to 6% of the solid angle in total. The high collection efficiency and high signal-to-background ratio make the setup an ideal quantum light source. We have observed strong antibunching of the photons emitted from the two fibers. The system has a range of applications from single-ion state detection in quantum information processing to strong coupling cavity-QED with ions.
Miniaturization of quantum optical devices down to μm-dimensions and integration into fibre optical networks is a major prerequisite for future implementations of quantum information communication and processing applications. Also scalability, long-term stability and room-temperature operation are important properties of such devices. Lately there have been major improvements in down-sizing logical structures and functionalizing optical fibers. Here we present an alignment free, μm-scale single photon source consisting of a single quantum emitter on an optical fiber operating at room temperature. It easily integrates into fiber optic networks for quantum cryptography or quantum metrology applications. Near-field coupling of a single nitrogen-vacancy center is achieved in a bottom-up approach by placing a pre-selected nanodiamond directly on the fiber facet. Its high photon collection efficiency is equivalent to a far-field collection via an objective with a numerical aperture of 0.82. Furthermore, simultaneous excitation and recollection through the fiber is possible introducing a fiber-connected single emitter sensor that allows near-field probing with quantum mechanical properties.
The controlled emission and absorption of single photons is an important enabling technology in the fields of quantum communication, cryptography and computing. We have realised a novel single photon source, based on a miniature ‘endcap’ ion trap with integrated optical fibres. To minimise distortion of the trapping field the fibres are tightly integrated and recessed within the two hollow cylindrical rf electrodes of the trap. This allows us to bring the fibres to within approximately 300 µm of the trapped ion. With the fibres in place we are able to collect the ion’s fluorescence using no further optics. In this thesis the ion trap is fully characterised, and the quantum nature of the light demonstrated in the results of a variant on the Hanbury Brown-Twiss photon-correlation experiment. The scheme will ultimately be extended to implement a coherent ion-photon interface through strong coupling cavity-QED. Towards this end, an ultra-high-finesse cavity has been designed and fabricated by laser-machining and coating surfaces with a range of radii of curvature on the end facets of the fibres. To improve the stability and precision manipulation of the cavity in-vacuum, an entirely new trap has been designed and built. Finally, the current status of the fibre cavity and the outlook for the experiment are presented.
Abstract Trapped ions in micro-cavities constitute a key platform for advancing quantum information processing and quantum networking. By providing an efficient light–matter interface within a compact architecture, they serve as highly efficient quantum nodes with strong potential for a scalable quantum network. However, in such systems, ion trapping stability is often compromised by surface charging effects, and nearby dielectric materials are known to cause a dramatic increase in the ion heating rate by several orders of magnitude. These challenges significantly hinder the practical implementation of ion trap systems integrated with micro-cavities. To overcome these limitations, we present the design and fabrication of metal-shielded fiber-cavity mirrors, enabling the stable realization of ion trap systems integrated with fiber cavities. Using this method, we constructed a needle ion trap integrated with a fiber Fabry–Pérot cavity and successfully achieved stable trapping of a single ion within the cavity. The measured ion heating rate was reduced by more than an order of magnitude compared with unshielded configurations. This work establishes a key technique toward fully integrated ion–photon interfaces for scalable quantum networks.