Time-Bin Superposition Methods for DPS-QKD

Key generation efficiency, and security, in differential phase-shift quantum key distribution (DPS-QKD) improve with an increase in the number of optical delays or time-bin superpositions. We demonstrate the implementation of superposition states using time-bins, with two different approaches. In Type-A, we use an optical pulse and create superposition states with optical splitters and path delays. Similar superposition states are created, in Type-B, by applying direct phase modulation within a single weak coherent pulse. We establish the equivalence between both the approaches, and note that higher-order superposition states of Type-B are easier to generate for DPS-QKD. We set up DPS-QKD, over 105 km of single mode optical fiber, with a quantum bit error rate of less than 15% at a secure key rate of 2 kbps. With temporal guard bands, the QBER reduced to less than 10%, but with a 20% reduction in the key rate.