Deterministic solid state quantum light sources are considered key building blocks for future communication networks. While several proof-of-principle experiments of quantum communication using such sources have been realized, most of them required large setups—often involving liquid helium infrastructure or bulky closed-cycle cryotechnology. In this work, we report on the first quantum key distribution (QKD) testbed using a compact benchtop quantum dot single-photon source operating at telecom wavelengths. The plug&play device emits single-photon pulses at O-band wavelengths (1321 nm) and is based on a directly fiber-pigtailed deterministically fabricated quantum dot device integrated into a compact Stirling cryocooler. The Stirling is housed in a 19 in. rack module including all accessories required for stand-alone operation. Implemented in a simple QKD testbed emulating the BB84 protocol with polarization coding, we achieve an multiphoton suppression of $g^{(2)}(0)=0.10\pm 0.01$ and a raw key rate of up to $(4.72\hspace{0.17em}\pm \hspace{0.17em}0.13)$ kHz using an external pump laser. In this setting, we further evaluate the performance of our source in terms of the quantum bit error ratios, secure key rates, and tolerable losses expected in full implementations of QKD while accounting for finite key size effects. Furthermore, we investigate the optimal settings for a two-dimensional temporal acceptance window applied on the receiver side, resulting in predicted tolerable losses up to 23.19 dB. Not least, we compare our results with previous proof-of-concept QKD experiments using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.

We present an effective method for direct fiber coupling of a quantum dot (QD) that is deterministically incorporated into a cylindrical mesa. For precise positioning of the fiber with respect to the QD-mesa, we use a scanning procedure relying on interference of light reflected back from the fiber end-face and the top surface of the mesa, applicable for both single-mode and multi-mode fibers. The central part of the fiber end-face is etched to control the required distance between the top surface of the mesa and the fiber core. Emission around 1260 nm from a fiber-coupled InGaAs/GaAs QD is demonstrated and its stability is proven over multiple cooling cycles. Moreover, a single photon character of emission from such system for a line emitting above 1200 nm is proven experimentally by photon autocorrelation measurements with an obtained value of the second order correlation function at zero time-delay well below 0.5.

Solid‐state quantum light sources based on semiconductor quantum dots (QDs) are increasingly employed in photonic quantum information applications. Especially when moving towards real‐world scenarios outside shielded lab environments, the efficient and robust coupling of nanophotonic devices to single‐mode optical fibers offers substantial advantage by enabling “plug‐and‐play” operation. In this work we present a fiber‐pigtailed cavity‐enhanced source of flying qubits emitting single indistinguishable photons at clock‐rates exceeding 1 GHz. This is achieved by employing a fully deterministic technique for fiber‐pigtailing optimized QD‐devices based on hybrid circular Bragg grating (hCBG) micro‐cavities. The fabricated fiber‐pigtailed hCBGs feature emission lifetimes of ps, corresponding to a Purcell factor of ∼9, a suppression of multi‐photon emission events with g ⁽²⁾ (0) < 1 %, a photon‐indistinguishability % and a measured single‐photon coupling efficiency of 53 % in a high numerical aperture single‐mode fiber, corresponding to 1.2 Megaclicks per second at the single‐photon detectors under 80 MHz excitation clock‐rates. Furthermore, we show that high multi‐photon suppression and indistinguishability prevail for excitation clock‐rates exceeding 1 GHz. Our results show that Purcell‐enhanced fiber‐pigtailed quantum light sources based on hCBG cavities are a prime candidate for applications of quantum information science.

We report on a new type of long-period-grating generated in a helical core fiber by bending. The grating arises from bend-induced modulation of an equivalent refractive index in the helical core with a period equal to the helix pitch. We experimentally demonstrate that such grating induces multiple resonant couplings between the fundamental modes guided in the central core and the helical side-core. We have also shown that by varying a direction of bending, one can generate the phase shifted grating. The experimental results are supported by numerical simulations based on the coupled mode equations.

We propose an improvement of the interferometric method used up to now to measure the chromatic dispersion in single mode optical fibers, which enables dispersion measurements in higher-order modes over a wide spectral range. To selectively excite a specific mode, a spatial light modulator was used in the reflective configuration to generate an appropriate phase distribution across an input supercontinuum beam. We demonstrate the feasibility of the proposed approach using chromatic dispersion measurements of the six lowest order spatial modes supported by an optical fiber in the spectral range from 450 to 1600 nm. Moreover, we present the results of numerical simulations that confirm sufficient selectivity of higher-order mode excitation. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We experimentally demonstrate the possibility of quasi-adiabatic conversion of L P 11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the value of the effective indices, the L P 11 modes are selectively converted to right- and left-handed circularly polarized vortex modes H E 21 with a total angular momentum of ± 2 and to quasi- T E 01 / T M 01 modes with a total angular momentum of 0. Since the proposed conversion method has a purely topological origin, it is broadband in nature, in contrast to the methods based on resonant effects.