KIP-Veröffentlichungen

A key bottleneck for practical Quantum Key Distribution (QKD) over optical fibre is the achievable secret key rate, which is ultimately limited by receiver noise, detection efficiency and timing resolution. One route towards higher rates is multiplexing, motivating scalable receiver implementations based on integrated photonics. This thesis experimentally characterises and optimises a cryogenic, time-bin QKD receiver based on a Si3N4 photonic integrated circuit (PIC) with waveguide-integrated Superconducting Nanowire Single-Photon Detectors (SNSPDs). The receiver integrates passive on-chip routing, wavelength-division multiplexing (WDM) demultiplexing and a shared Delay Line Interferometer (DLI) for time-bin analysis. A pre-existing indium phosphide (InP) transmitter generating time-bin encoded decoy-state Weak Coherent Pulses (WCPs) is used solely as a photon source to evaluate receiver performance.

Receiver performance is quantified through measurements of System Detection Efficiency (SDE), dark count rate (DCR), timing jitter and maximum usable count rate (CR) as functions of wavelength and detector bias. From these data, operating points are identified that provide high efficiency while maintaining low noise and stable operation. In addition, X-basis wavelength sweeps verify high interferometric visibility of the on-chip DLI. Finally, proof-of-principle sender–receiver measurements are analysed to extract Z- basis gain and Quantum Bit Error Rate (QBER), and a forward model relates detector timing jitter to inter-bin leakage under the employed time gating.