Joscha Hanel

The generation and manipulation of single photons at telecommunication wavelengths play a vital role for the quantum networks based on optical fiber infrastructures. Extensive protocols have previously been proposed and implemented by using the photons from weak coherent laser pulses for long-distance communication. One such robust and mature solution for stable communication is time-bin states for practical fiber networks. Semiconductor quantum dots (QDs) are one of the genuine quantum light sources with deterministic single-photon emissions and have recently achieved a significant milestone in terms of high-brightness single-photon emission at the telecommunication wavelengths, assisted by the micro- and nanophotonic technology. This poster presents the first demonstration of the time-bin encoded quantum key distribution (QKD) using the telecom single photons from a QD. In the experiment, three time-bin states are randomly prepared with the active modulation on each single photon in a 16-bit repeating sequence, after the photons have passed through a Sagnac interferometer incorporating Mach-Zehnder interferometer. An average quantum bit error ratio (QBER) of less than 1% is achieved for the Z-basis of the system is achieved at a quantum channel distance of 0 km, resulting in a secure key rate (SKR) of over 10 kbits/s in the finite-key regime. The stable preparation, transmission, and readout of the single-photon time-bin qubits is validated by implementing time-dependent QBER/SKR measurements over 6 hours for the cases of variable optical fiber channels ranging from 0 to 120 km. A maximum tolerable transmission distance of 127 km is identified as the system error ratio approaches the security threshold, resulting in a SKR of approximately 15 bits/s, which is still realistic for practical applications. The improvement of system performance in terms of the light source, optical setups, and instruments has been comprehensively analyzed. Unlike previous research activities concerning QKD with polarized single photons, the successful verification of the QKD with time-bin encoded single-photon qubits in this study demonstrates the feasibility of realizing robust and scalable quantum communication network based on solid-state single-photon technology.