Various optical quantum key distribution protocols have been proposed and examined to implement a global quantum communication system both in fiber-based network and in free-space systems. Employing a satellite as a quantum relay is a possible solution to increase the secure covered distance, which is available for sharing secure keys between two remote ground stations. In this paper, combining with the idea of measurement device independent quantum key distribution, a modified free-space quantum communication protocol is proposed to build a QKD network with untrusted nodes, where the parties on the ground send quantum signals to the untrusted satellite relay that perform a joint measurement on the incoming signals. Our results demonstrate that though the modified protocol suffers higher link attentuation than the entanglement-based quantum communication protocol owing to higher effects of atmospheric turbulence for uplink, our modified protocol provides secure quantum communications with untrusted relays. Furthermore, the complex and non-space-qualified sources remain on the ground in our scheme , which makes our scheme more practical for bridging a seamless connection form ground to space.
Theory and experimental results show that the state of the circular polarization of laser changes little in the atmospheric channel, and circular polarization modulation technology can improve the performance of free-space optical communication systems. This article starts from the principle of circular polarization modulation technology, introduces several common circular polarization transmission system structure, and then briefly describe its application and research direction in the future.
In this paper, by combining measurement-device-independent quantum key distribution (MDI-QKD) scheme with entangled photon sources, we present a modified MDI-QKD scheme with pairs of vector vortex(VV) beams, which shows a structure of hybrid entangled entanglement corresponding to intrasystem entanglement and intersystem entanglement. The former entanglement, which is entangled between polarization and orbit angular momentum within each VV beam, is adopted to overcome the polarization misalignment associated with random rotations in quantum key distribution. The latter entanglement, which is entangled between the two VV beams, is used to perform entangled-based MDI-QKD protocol with pair of VV beams to inherit the merit of long distance. The numerical simulations show that our modified scheme can tolerate 97dB with practical detectors. Furthermore, our modified protocol only needs to insert q-plates in practical experiment.
PM2.5 and PM10 are very important indexes reflecting the air quality in atmospheric environment monitoring. Based on Mie angular scattering theory, a system measuring particulates diameter and concentration of PM2.5 and PM10 was designed, which used optical fiber as transmission medium. Theoretical deduction and Matlab program simulation were presented to illustrate the basic principle of Mie angular scattering. The influences of laser wavelength, detection angle and other parameters on the measurement accuracy were discussed in detail. It had been found that the measurement results had multi-valued problem when the measurement angle range was too small. However, this problem was eliminated obviously when the angle range is greater than 25 degrees.
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