Optical transmitter for time-bin encoding Quantum Key Distribution

Abstract: We introduce an electro-optical arrangement that is able to produce time-bin encoded symbols with the decoy state method over a standard optical fiber in the C-band telecom window. The device consists of a specifically designed pulse pattern generator for pulse production, a field-programmable gate array that controls timing and synchronization. The electrical pulse output drive a sequence of intensity modulators acting on a continuous laser that deliver bursts of weak optical pulse pairs of discrete intensity values. Such transmitter allows for the generation of all the quantum states needed to implement a discrete variable Quantum Key Distribution protocol over a single-mode fiber channel. Symbols are structured in bursts; the minimum relative delay between pulses is 1.25 ns, and the maximum symbol rate within a burst is 200 MHz. We test the transmitter on simulated optical channels of 7dB and 14dB loss, obtaining maximum extractable secure key rates of 3.0 kb/s and 0.57 kb/s respectively. Time bin state parameters such as symbol rate, pulse separation and intensity ratio between signal and decoy states can be easily accessed and changed, allowing the transmitter to adapt to different experimental conditions and contributing to standardization of QKD implementations.