High-rate intercity quantum key distribution with a semiconductor single-photon source

Quantum key distribution(QKD)enables the transmission of information that is secure against general attacks by eavesdroppers.The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss.Semiconductor quantum dots(QDs)are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution.Here we report on the first intercity QKD experiment using a bright deterministic single photon source.A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure.Utilising the 79 km long link with 25.49 dB loss(equivalent to 130 km for the direct-connected optical fibre)between the German cities of Hannover and Braunschweig,a record-high secret key bits per pulse of 4.8 x io-5 with an average quantum bit error ratio of~0.65%are demonstrated.An asymptotic maximum tolerable loss of 28.11 dB is found,corresponding to a length of 144 km of standard telecommunication fibre.Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.

3D printed micro-optics for quantum technology:Optimised coupling of single quantum dot emission into a single-mode fibre

Future quantum technology relies crucially on building quantum networks with high fidelity.To achieve this challenging goal,it is of utmost importance to connect individual quantum systems such that their emitted single photons overlap with the highest possible degree of coherence.This requires perfect mode overlap of the emitted light from different emitters,which necessitates the use of single-mode fibres.Here,we present an advanced manufacturing approach to accomplish this task.We combined 3D printed complex micro-optics,such as hemispherical and Weierstrass solid immersion lenses,as well as total internal reflection solid immersion lenses,on top of individual indium arsenide quantum dots with 3D printed optics on single-mode fibres and compared their key features.We observed a systematic increase in the collection efficiency under variations of the lens geometry from roughly 2 for hemispheric solid immersion lenses up to a maximum of greater than 9 for the total internal reflection geometry.Furthermore,the temperature-induced stress was estimated for these particular lens dimensions and results to be approximately 5 meV.Interestingly,the use of solid immersion lenses further increased the localisation accuracy of the emitters to less than 1 nm when acquiring micro-photoluminescence maps.Furthermore,we show that the single-photon character of the source is preserved after device fabrication,reaching a g^((2))(0)value of approximately 0.19 under pulsed optical excitation.The printed lens device can be further joined with an optical fibre and permanently fixed.This integrated system can be cooled by dipping into liquid helium using a Stirling cryocooler or by a closed-cycle helium cryostat without the necessity for optical windows,as all access is through the integrated single-mode fibre.We identify the ideal optical designs and present experiments that demonstrate excellent high-rate single-photon emission.