A feasible route toward implementing long-distance quantum key distribution (QKD) systems relies on probabilistic schemes for entanglement distribution and swapping as proposed in the work of Duan, Lukin, Cirac, and Zoller (DLCZ) [Nature (London) 414, 413 (2001)]. Here, we calculate the conditional throughput and fidelity of entanglement for DLCZ quantum repeaters by accounting for the DLCZ self-purification property in the presence of multiple excitations in the ensemble memories as well as loss and other sources of inefficiency in the channel and measurement modules. We then use our results to find the generation rate of secure key bits for QKD systems that rely on DLCZ quantum repeaters. We compare the key generation rate per logical memory employed in the two cases with and without a repeater node. We find the crossover distance beyond which the repeater system outperforms the nonrepeater one. That provides us with the optimum internode distancing in quantum repeater systems. We also find the optimal excitation probability at which the QKD rate peaks. Such an optimum probability, in most regimes of interest, is insensitive to the total distance.

A comprehensive study is done for optimizing antireflection coatings at mid-infrared wavelengths around 3.4 μm for applications in semiconductor lasers. Different materials are tested for single and double layer coatings. The coatings are characterized by comparing the change in the threshold currents before and after coating the lasers. The change in the differential slope efficiency was used to estimate the loss in the coatings. The lowest value achieved for a single layer coating was 5% using aluminum oxide while a minimum value of reflectivity of 7 × 10−4 was achieved with a double layer coating of zinc selenide and silicon dioxide. The coatings' quality was tested using scanning electron microscopy and tape test. The study provides a comprehensive list of materials that can be used for antireflection coating applications in the mid-infrared region.

No abstract

No abstract

No abstract