The fully integrated 800 μm. diameter avalanche photodiode optical receiver is implemented in 0.35 μm BiCMOS technology without any process modifications. The integrated receiver reaches sensitivities of-33 dBm at 1 Gbit/s and-29.3 dBm at 2 Gbit/s. The reached sensitivities are well within the state-of-the-art of integrated avalanche photodiode receivers and can even be compared to a hybrid avalanche photodiode receiver comprised of high-performing commercial components. The performance of the designed receiver was verified in visible light communication experiments. The receiver could reach up to 16.5 m at 2 Gbit/s and 27 m at 1 Gbit/s of error free transmission distance using a 675-nm point laser source as transmitter. The common indoor illuminance levels up to 500 lux could be tolerated when pointed directly towards the receiver. High sensitivity and high speed make this integrated receiver suitable for future optical wireless communication systems, where due to its integrated nature the manufacturing cost can be lowered, and at the same time the design is compact in size and easy to assemble and scale. Furthermore, no optics is used in front of the receiver due to its large area resulting in a wide field of view.

A lens-less receiver with a monolithically integrated avalanche photodiode (APD) in 0.35 µm BiCMOS technology has been developed for establishing an indoor 2 Gb/s optical wireless communication (OWC) over a distance of 6.5 m with a receiving angle of 22°. Immunity toward background light was demonstrated up to 6000 lux. Four additional PIN photodiodes with highly sensitive differential nonlinear transimpedance amplifiers (TIA) were implemented on the receiver chip for centering the highly collimated transmitter beam position.

A coherent transmission methodology for a continuous-variable quantum key distribution (CV-QKD) system based on quantum-heterodyne measurement through a coherent intradyne receiver is experimentally demonstrated in the framework of 5G mobile fronthaul links. Continuous optical carrier synchronization is obtained through training information, which is multiplexing to the quantum signal as pilot tone in both, frequency and polarization. Spectral tailoring by means of optical carrier suppression and single-sideband modulation is adopted to simultaneously mitigate crosstalk into the quantum channel and self-interference for the pilot tone, thus allowing for a high signalto-noise ratio for this training signal. Frequency offset correction and optical phase estimation for the free-running local oscillator of the receiver is accurately performed and guarantees low-noise quantum signal reception at high symbol rates of 250 MHz and 500 MHz with additional Nyquist pulse shaping. A low excess noise in the order of 0.1% to 0.5% of shot-noise units is obtained for fiber-based transmission over a fronthaul link reach of 13.2 km. Moreover, co-existence with 11 carrier-grade classical signals is experimentally investigated. Joint signal transmission in the C-band of both, quantum signal and classical signals, is successfully demonstrated. Secure-key rates of 18 and 10 Mb/s are obtained under strict security assumptions, where Eve has control of the receiver noise, for a dark and a lit fiber link, respectively. Moreover, rates of 85 and 72 Mb/s are resulting for a trusted receiver scenario. These secure-key rates are well addressing the requirements for time-shared CV-QKD system in densified 5G radio access networks with cloud-based processing.

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We demonstrate high-rate CV-QKD supporting a secure-key rate of 22Mb/s through spectral tailoring and optimal use of quantum receiver bandwidth. Co-existence with 11 adjacent carrier-grade C-band channels spaced by only 20nm is accomplished at >10Mb/s.

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The integration of quantum communication functions often requires dedicated opto-electronic components that do not bode well with the technology roadmaps of telecom systems. We investigate the capability of commercial coherent transceiver subsystems to support quantum random number generation next to classical data transmission, and demonstrate how the quantum entropy source based on vacuum fluctuations can be potentially converted into a true random number generator for this purpose. We discuss two possible implementations, building on a receiver-and a transmitter-centric architecture. In the first scheme, balanced homodyne broadband detection in a coherent intradyne receiver is exploited to measure the vacuum state at the input of a 90-degree hybrid. In our proof-ofprinciple demonstration, a clearance of >2 dB between optical and electrical noise is obtained over a wide bandwidth of more than 11 GHz. In the second scheme, we propose and evaluate the re-use of monitoring photodiodes of a polarization-multiplexed inphase/quadrature modulator for the same purpose. Time-interleaved random number generation is demonstrated for 10 Gbaud polarizationmultiplexed quadrature phase shift keyed data transmission. The availability of detailed models will allow to calculate the extractable entropy and we accordingly show randomness extraction for our two proof-of-principle experiments, employing a two-universal strong extractor.

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