Fast laser carving for a three-state BB84 protocol implementation with time-bin encoding and decoy states

Quantum Key Distribution (QKD) can provide random and secure keys for information-theoretically secure cryptographic protocols, thereby ensuring the privacy of both current and future communications. This master’s thesis presents the implementation of a simplified three-state BB84 protocol with time-bin encoding and the one-decoy method, whose security against collective attacks was proven by Rusca et al. in 2018. The optical transmitter setup consists of a continuous wave laser with a wavelength at 1550 nm, modulated by a phase randomizer and a primary intensity modulator used for fast laser carving. This intensity modulator generates 400 ps-wide pulses with an intra-symbol pulse separation of 1 ns in the classical optical regime. A second intensity modulator selects among three intensity levels for the Z and X basis states and their corresponding decoy state counterparts. These pulses are then attenuated to the quantum level to match the respective photon number occupancy of the states, with a targeted mean photon number of 0.1 for signal states. The receiver’s side includes a beam splitter for passive basis selection, two single-photon detectors, and an unbalanced interferometer. With this setup, a quantum bit error rate (QBER) below 2 % was achieved for attenuations up to 19 dB. The quantum visibility of the setup was found to be 94 %. A QKD exchange was performed, and using a corrected estimate for the decoy mean photon number, a secret key rate of 1.7 kHz was extracted. The presented results demonstrate the successful implementation of the setup and the potential of fast laser carving in QKD experiments.