The Implementation of the Frequency-Time Encoded Decoy-State Protocol with the Slow-Light Effect for Quantum Memories

Abstract: Quantum key distribution (QKD) is a secure encryption key generation process to be used by two users in the presence of an eavesdropper. The no-cloning theorem allows the sender "Alice" to securely send qubits with single photons to the receiver "Bob". However, due to real-life imperfections, it is not always possible to have a single-photon source with its properties matching to the quantum memories based on the rare-earth ions. Besides, it might be resource demanding to build and use such a source on our quantum memories. In order to overcome this problem, one can use a special protocol called the decoy state protocol. In the decoy state protocol, it is possible to have a secure communication channel while having a multi-photon source that can send two different states with different photon number distributions. In this project, the decoy state protocol has been implemented on our current setup to be used in the determination of the efficiencies of the quantum memories. The performance of the quantum memories developed in this group is polarization dependent. Besides, maintaining and detecting the polarization of the qubits is another challenge. Thus, the encoding type of the protocol has been selected to be Frequency-Time (FT), in which the security of the protocol is maintained by the time-frequency uncertainty. Moreover, in this project, a new technique has been introduced, which allows Bob to detect the frequencies of few-photon pulses in the single photon regime. This new technique is based on the slow-light effect, which can be achieved by using special materials, where the speed of light is reduced by 4 to 5 orders of magnitude compared to its speed in vacuum. The speed of light in this special material is frequency dependent, thus, photons with different frequencies will be distinguishable, since they will be separated in time domain. It has been determined that this method gives promising results for the measurements in the field of the QKD. Additionally, this thesis contains some discussions about possible developmental steps which can be used to improve the implemented protocol.