Several emission features mark semiconductor quantum dots as promising non-classical light sources for prospective quantum implementations. For long-distance transmission [1] and Si-based on-chip processing[2, 3], the possibility to match the telecom C-band [4] stands out, while source brightness and high single-photon purity are key features in virtually any quantum implementation [5, 6]. Here we present an InAs/InGaAs/GaAs quantum dot emitting in the telecom C-band coupled to a circular Bragg grating. The Purcell enhancement of the emission enables a simultaneously high brightness with a ﬁber-coupled single-photon count rate of 13 . 9 MHz for an excitation repetition rate of 228 MHz (ﬁrst-lens collection eﬃciency ∼ 17 % for NA = 0 . 6), while maintaining a low multi-photon contribution of g (2) (0) = 0 . 0052. Moreover, the compatibility with temperatures of up to 40 K attainable with compact cryo coolers, further underlines the suitability for out-of-the-lab implementations.

High emission rate from a Purcell-enhanced, triggered source of pure single photons in the telecom C-band


 Photonic quantum technology is essentially based on the exchange of individual photons as information carriers. Therefore, the development of practical single-photon sources that emit single photons on-demand is a crucial contribution to advance this emerging technology and to promoting its first real-world applications. In the last two decades, a large number of quantum light sources based on solid state emitters have been developed on a laboratory scale. Corresponding structures today have almost ideal optical and quantum-optical properties. For practical applications, however, one crucial factor is usually missing, namely direct on-chip fiber coupling, which is essential, for example, for the direct integration of such quantum devices into fiber-based quantum networks. In fact, the development of fiber-coupled quantum light sources is still in its infancy, with very promising advances having been made in recent years. Against this background, this review article presents the current status of the de-velopment of fiber-coupled quantum light sources based on solid state quantum emitters and discusses challenges, technological solutions and future prospects. Among other things, the numerical optimiza-tion of the fiber coupling efficiency, coupling methods, and important realizations of such quantum devices are presented and compared. Overall, this article provides an important overview of the state of the art and the performance parameters of fiber-coupled quantum light sources that have been achieved so far. It is aimed equally at experts in the scientific field and at students and newcomers who want to get an overview of the current developments.

Fiber-coupled quantum light sources based on solid-state quantum emitters

A key resource in quantum-secured communication protocols are single photon emitters. For long-haul optical networks, it is imperative to use photons at wavelengths compatible with telecom single mode fibers. We demonstrate high purity single photon emission at 1.31 μm using deterministically positioned InP photonic waveguide nanowires containing single InAsP quantum dot-in-a-rod structures. At excitation rates that saturate the emission, we obtain a single photon collection efficiency at first lens of 27.6% and a probability of multiphoton emission of g(2)(0) = 0.021. We have also evaluated the performance of the source as a function of temperature. Multiphoton emission probability increases with temperature with values of 0.11, 0.34, and 0.57 at 77, 220 and 300 K, respectively, which is attributed to an overlap of temperature-broadened excitonic emission lines. These results are a promising step toward scalably fabricating telecom single photon emitters that operate under relaxed cooling requirements.

Position-Controlled Telecom Single Photon Emitters Operating at Elevated Temperatures

In this work, we measure polarization-resolved photoluminescence spectra from excitonic complexes in tens of single InAs/GaAs quantum dots (QDs) at the telecom O-band with strain-coupled bilayer structure. QDs often show fine-structure splitting (FSS) ~100 μeV in uniform anisotropy and valence-band mixing of heavy holes (HH) and light holes (LH); the biaxial strain also induces LH excitons with small FSS (especially XX, <5 μeV, 70% of QDs); delocalized LH reduces the Coulomb interaction between holes Vhh and enhances population on LH excitons XX, XX11, X11+ and XX21+.

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Light Hole Excitons in Strain-Coupled Bilayer Quantum Dots with Small Fine-Structure Splitting

An 120° hybrid for bimodal interferometers in the 220 nm silicon-on-insulator technology is presented. Three output signals enable an unambiguous phase detection over a 360°-range as well as a constant sensitivity. The length of the hybrid is only 190 μm with a simulated excess loss of 0.16 dB. Measurements combined with digital signal processing verify the functionality of a fabricated 120° hybrid. 

120° Hybrid for Bimodal Interferometers

The well-established silicon-on-insulator platform is very promising for large-scale integrated photonic and quantum photonic technologies due to the mature manufacturing technology and integration density. Here, we present an efficient and stable fiber-to-chip coupling, which enables the injection of single photons from telecom quantum dots into a silicon-on-insulator photonic chip. Two additional fibers further couple the chip to single-photon detectors. The approach chosen to achieve steady fiber-chip coupling is based on the use of grating couplers steadily packaged with angled single-mode fibers. Using this technique, coupling efficiencies between the fiber and the SOI chip as high as 69.1% per grating coupler (including the taper losses) are reached. The effective interface between the quantum light generated by quantum dots and the silicon components is verified via the measurement of the second-order correlation function using a Hanbury–Brown and Twiss setup. With g(2)(0)=0.051±0.001, it clearly proves the single-photon nature of the injected QD photons. This demonstrates the reliability of the interfacing method and opens the route to employ telecom quantum dots as non-classical light sources with high complexity silicon photonic functionalities.

Achieving stable fiber coupling of quantum dot telecom C-band single-photons to an SOI photonic device

An improved phase detection scheme for Mach-Zehnder and bimodal interferometers is presented. By using a 90° hybrid, always two outputs operate at a highly sensitive point and the phase-shift-unambiguousness is extended to a range of 2π. The phase detection is independent of mode attenuations and input power fluctuations.

Improved Phase Detection in On-Chip Refractometers

A broadband integrated notch filter, using the silicon nitride (Si 3 N 4 ) platform, is presented. It achieves an extinction ratio (ER) of 60 dB and a full width at half maximum (FWHM) of 10 nm at the central wavelength (CW) 785 nm. The main filter components are Bragg gratings (BGs). For separating the occurring reflections from the input waveguide, two identical BGs are combined with a directional coupler (DC) acting as optical circulator. The 3-dB bandwidth of the passband (790 nm to 1200 nm) is more than 400 nm.

Design of a Broadband Integrated Notch Filter in Silicon Nitride

A switch-fabric for the channel routing is presented which combines the wavelength-separation-ability of arrayed waveguide gratings (AWGs) with the tunability of generalized Mach-Zehnder interferometers (GMZIs). The resulting tunable filter allows to maximize and shift a desired spectral transmission-window, whereby the window-bandwidth is defined by waveguide-length-offsets.

Enhanced Generalized Mach-Zehnder Interferometer for Tunable Channel Routing

Fluorescence-based assays are widespread in life sciences and used for instance in diagnostics, cell sorting, or to resolve cellular and tissue structures. Optofluidic engineering can help improve such assays, especially when sensitivity or throughput are important. Towards optofluidic integration, we combine a silicon nitride photonic chip including grating couplers for exciting and detecting fluorescence signals out of plane with a soft lithography based microfluidic chip. This work investigates the structures of grating couplers for wavelengths of 405 nm and 532 nm by simulations and shows as a proof of principle different measurements with fluorescein-concentrations (10 μM up to 10 mM) in deionized (DI) water using an excitation-wavelength of 405 nm. By exciting through a grating coupler and recording the fluorescence emission through a glass fiber, a limit of detection (LOD) of 73 μM is measured. In addition, the fluorescence detection through grating couplers in microchannels is demonstrated. This is considered as a first step towards a fully integrated optofluidic chip interface for high density fluorescence assay multiplexing.

Christian Schweikert

Papers

Achieving stable fiber coupling of quantum dot telecom C-band single-photons to an SOI photonic device

Improved Phase Detection in On-Chip Refractometers

Design of a Broadband Integrated Notch Filter in Silicon Nitride

Enhanced Generalized Mach-Zehnder Interferometer for Tunable Channel Routing

Grating Couplers for Chip-Integrated Optofluidic Fluorescence Quantification