High-rate intercity quantum key distribution with a semiconductor single-photon source

Quantum key distribution(QKD)enables the transmission of information that is secure against general attacks by eavesdroppers.The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss.Semiconductor quantum dots(QDs)are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution.Here we report on the first intercity QKD experiment using a bright deterministic single photon source.A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure.Utilising the 79 km long link with 25.49 dB loss(equivalent to 130 km for the direct-connected optical fibre)between the German cities of Hannover and Braunschweig,a record-high secret key bits per pulse of 4.8 x io-5 with an average quantum bit error ratio of~0.65%are demonstrated.An asymptotic maximum tolerable loss of 28.11 dB is found,corresponding to a length of 144 km of standard telecommunication fibre.Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.

Influence of Plasmon Excitations in Au Nanoparticles upon Fluorescence and Photostability of Photosynthetic Complexes

Fluorescence spectroscopy is applied to study the influence of plasmon excitations in spherical Au nanoparticles on the optical properties of chlorophyll-containing light-harvesting complexes. The separation between the two nanostructures is controlled via silica layer with varied thickness. We observe strong increase of the emission intensity for a 12- nm-thick spacer and the increase is accompanied with shortening of the fluorescence lifetime, which allows us to separate contributions of absorption and emission rate enhancement. At the same time we find an increase of photobleaching. These findings are interpreted as a result of spectral overlap between plasmon resonance and chlorophyll fluorescence.

Current-induced degradation and lifetime prediction of 310 nm ultraviolet light-emitting diodes

The impact of operation current on the degradation behavior of 310 nm UV LEDs is investigated over 1000 h of stress. It ranges from 50 to 300 mA and corresponds to current densities from 34 to 201 A/cm^2.To separate the impact of current from that of temperature, the junction temperature is kept constant by adjusting the heat sink temperature. Higher current was found to strongly accelerate the optical power reduction during operation. A mathematical model for lifetime prediction is introduced.It indicates that lifetime is inversely proportional to the cube of the current density, suggesting the involvement of Auger recombinati on.

Flip-chip-type high- T_c gradiometer for biomagnetic measurements in unshielded environment

A flip-chip-type gradiometer has been constructed with a 10 mm × 5 mm planar DCSQUID gradiometer fabricated on a SrTiO3 bicrystal substrate and a flux transformer made from a YBCO*/YBCO/CeO2/YSZ multilayer on ?50.8 mm Si wafer. The coupling coefficient between the flux transformer and the planar gradiometer is 0.18. The transformer increases effectively the resolution of the gradiometer. A magnetic field gradient resolution of 73 fT.cm?1Hz?1/2 in the white region and 596 fT.cm?1Hz?1/2 at 1 Hz has been obtained. High quality magnetocardiogram signals have been successfully measured by using this flip-chip-type gradiometer in an unshielded environment.

Asymmetric interfaces and high-TC ferromagnetic phase in La_(0.67)Ca_(0.33)MnO_(3)/SrRuO_(3)superlattices

Interfacial magnetism in functional oxide heterostructures not only exhibits intriguing physical phenomena but also implies great potential for device applications.In these systems,interfacial structural and electronic reconstructions are essential for improving the stability and tunability of the magnetic properties.In this work,we constructed ultra-thinLa_(0.67)Ca_(0.33)MnO_(3)/SrRuO_(3)(SRO)layers into superlattices,which exhibited a robust ferromagnetic phase.The high Curie temperature(TC)reaches 291 K,more than 30 K higher than that of bulk LCMO.We found that the LCMO/SRO superlattices consisted of atomically-sharp and asymmetric heterointerfaces.Such a unique interface structure can trigger a sizable charge transfer as well as a ferroelectric-like polar distortion.These two interfacial effects cooperatively stabilized the high-T_(C)ferromagnetic phase.Our results could pave a promising approach towards effective control of interfacial magnetism and new designs of oxide-based spintronic devices.