Abstract:We propose and experimentally demonstrate that, contrary to what was thought up to now, an efficient BB84 operation is feasible using the double phase modulator (PM-PM) configuration in frequency-coded quantum key distribution systems without dispersion compensation. This is achieved by exploiting the chromatic dispersion provided by the fiber linking Alice and Bob. Thus, we refer to this system as dispersion supported or as the DS BB84 PM-PM configuration.

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As one of the first proposed topologically protected states, the quantum spin Hall effect in graphene relies critically on the existence of a spin-dependent gap at the K/K points of the Brillouin zone. Using a tight-binding formulation based on the method of invariants, we identify the origin of such an intrinsic gap as the threecenter interaction between the π-orbitals caused by spin-orbit interactions. This methodology incorporates all symmetry compliant interactions previously neglected and has wider applications for comparisons between first-principle calculations and the tight-binding method. It also identifies a correction to the Haldane model and its generalization, which incorporates the spin degrees of freedom and reproduces all the salient features required for the quantum spin Hall effect in graphene.

We present the first experimental data showing a restriction of the radiative recombination in strainbalanced quantum well solar cells. This arises due to the combined effects of quantum confinement and strain splitting the valence band in the quantum wells into heavy and light hole sub-bands. Under increasing compressive strain, the light hole sub-band moves further in energy from the conduction band, suppressing the conduction-band-to-light-hole recombination which couples primarily to photons emitted in the plane of the quantum wells. As a solar cell fundamentally need only emit in the angular range of absorption, the strainbalanced quantum well solar cell operates more closely to this optimal regime than a bulk solar cell.