We study the possible application of the decoy state method on a basic two way quantum key distribution (QKD) scheme to extend its distance. Noting the obvious advantage of such a QKD scheme in allowing for single as well as double photon contributions, we derive relevant lower-bounds on the corresponding gains in a practical decoy state implementation using two intensities for decoy states. We work with two different approaches in this vein and compare these with an ideal infinite decoy state case as well as the simulation of the original.

We extend study of the Jaynes-Cummings model involving a pair of identical two-level atoms (or qubits) interacting with a single mode quantized field. We investigate the effects of replacing the radiation field mode with a composite spin, comprising N qubits, or spin-1/2 particles. This model is relevant for physical implementations in superconducting circuit QED, ion trap and molecular systems. For the case of the composite spin prepared in a spin coherent state, we demonstrate the similarities of this set-up to the qubits-field model in terms of the time evolution, attractor states and in particular the collapse and revival of the entanglement between the two qubits. We extend our analysis by taking into account an effect due to qubit imperfections. We consider a difference (or 'mismatch') in the dipole interaction strengths of the two qubits, for both the field mode and composite spin cases. To address decoherence due to this mismatch, we then average over this coupling strength difference with distributions of varying width. We demonstrate in both the field mode and the composite spin scenarios that increasing the width of the 'error' distribution increases suppression of the coherent dynamics of the coupled system, including the collapse and revival of the entanglement between the qubits.

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Vernier and Fano resonances are promising approaches for enhancing the sensitivity of an alloptical sensor. A theoretical analysis was performed to integrate a Fano-like resonance shape with a Vernier resonance by considering the presence of partial reflective end facets at a double microring resonator waveguide. The system was developed based on scattering matrix and optical transfer function. The double and all-pass racetrack microring configurations were compared with and without the end facet at the waveguide to analyze the dynamic change of the output resonance spectrum. The spectrum were analyzed based on the free spectral range and resonance pattern. The resonator systems were applied to refractive index-based sensing protocol, which was operated by a shift of resonance wavelength with a change of refractive index. The sensitivity was optimized by varying the configuration parameters such as the radius of the ring, the distance between end facet, and the coupling coefficients. Integrating Vernier spectrum with Fano resonance improved the sensitivity for all-pass racetrack configuration by 5.16 % and the sensitivity for double racetrack configuration by 6.31 %. The recorded limit of detection (LOD) of double racetrack was 3.30 × 10 -5 .