Mastering the generation, propagation and detection of electro-magnetic waves has enableduda technological breakthrough that has changed our entire society. World-wide communication through the telephone and the internet has become an integral part of our daily-life,udwhich is expected to grow even further with the emergence of the internet of things. Whileudsecure communication was of concern mostly for governmental and financial institutions,uddigital security has now caught the attention of the general public. The weaknesses of cur-udrent encryption protocols, such as the existence of back-doors or the predicted breakdownudof popular algorithms such as RSA, reveal the need for alternative encryption schemesudensuring unconditional security on all types of devices.udQuantum Key Distribution (QKD) has emerged as a powerful option to ensure a privateudcommunication between two users. Based on the laws of quantum mechanics, this class ofudprotocols offers the possibility to detect the presence of a third party trying to intercept theudkey during its distribution, and even to quantify the amount of leaked information. Whileudmost research projects focus on long distance applications, little attention has been devotedudto short distance schemes such as wireless payment, network access and authentication,udwhich could highly benefit from QKD-enhanced security.udThis thesis focuses on the development of a miniature QKD sender add-on that couldudbe embedded either in mobile devices or in existing optical communication platforms,udthus allowing for a secure key exchange with a shared dedicated receiver over a free-udspace link. The proposed optics architecture (35 × 20 × 8 mm 3 ) is optimised for BB84-likeudprotocols and uses an array of four Vertical-Cavity Surface-Emitting Lasers with highlyudsimilar properties to generate 40 ps long near-infrared faint coherent pulses at 100 MHzudrepetition rate. Under strong modulation, the polarisation of the pulses is not well definedudand enables an external control of each diode’s emission by a wire-grid polariser. Theudfour beams are spatially overlapped in a polarisation-insensitive femtosecond laser writtenudwaveguide array, and combined with a red beacon laser using an external beamsplitter toudensure a stable, synchronised optical link with the receiver.udThe complete module is compatible with current smartphone technology, allowing toudrun the classical post-processing over WLAN in the future. First tests with a free-spaceudreceiver indicate an average error ratio of 3.3 % and an asymptotic secure key rate ofud54 kHz under static alignment. For the first time, a secure key exchange between a mobileudplatform held by a user and a receiver equipped with a dynamic alignment system couldudbe demonstrated with an error ratio of 4.1 % and a secure key rate of 31 Hz. The furtherudoptimisation of the experimental parameters and the implementation of a decoy protocoludwill enhance the key generation rate as well as the general security of the system. Theudresults of this thesis pave the way towards unprecedented security in wireless opticaludnetworks, as examplified for the communication between a mobile device and a dedicatedudreceiver.
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