| PhD Viva

Name of the Speaker: Mr. Gautam Shaw (EE15D047)
Guide: Prof. Anil Prabhakar
Online meeting link: https://meet.google.com/nhn-vsuo-bme
Date/Time: 2nd January 2024 (Tuesday), 10:00 AM
Title: Developing Differential Phase-Shift Quantum Key Distribution System

Abstract

Rapid advancements in building scalable quantum computers pose a significant threat to data security over existing communication networks. As a result, developing a Quantum key distribution (QKD) system is a pressing need, as it offers the possibility of unconditional security in communication. QKD enables secure key exchange between two authenticated users, Alice and Bob, by relying on two aspects of quantum mechanics, Heisenberg’s uncertainty principle and the no-cloning theorem.

In this thesis, we focus on the implementation of a differential phase-shift quantum key distribution (DPS-QKD) protocol, in which a key is encoded in the form of a differential phase. DPS-QKD, as proposed by Inoue et al, is simple to implement with existing optical fiber networks and robust against slowly varying environmental fluctuations. We present two different experimental approaches to generate time-bin superposition states, and realize a 4-state DPS-QKD experiment over a 105 km quantum channel. In Type-A, we use an optical pulse and create superposition states with optical splitters and path delays. In Type-B, similar superposition states are created, by applying direct phase modulation within a single weak coherent pulse. We have established an equivalence between both approaches and note that higher-order superposition states of Type-B are easier to generate for DPS-QKD. In the thesis, we have shown that the use of one temporal multiplexed single photon detector along with temporal filtering can reduce the quantum bit error rate (QBER) of our DPS-QKD implementation but at a reduced secure key rate.

To develop quantum sources, we have built a fiber-based mode-locked laser (MLL) using a saturation absorber mirror (SAM) and Erbium-doped fiber, capable of generating sub-picosecond coherent pulses at C band. We used these short pulses to characterize the gated single photon detector (SPD). We have also shown that the sub-picosecond pulses from MLL, coupled with a continuous wave laser beam, while passing through a highly non-linear fiber (HNLF), can generate a correlated frequency comb.

The last part of this thesis consists of various schemes and implementations of true random number generators (TRNGs), essential for raw key generation in Alice.