ABSTRACT
The field of quantum information processing offers secure communication protected by the laws of quantum mechanics and is on the verge of finding wider application for the information transfer of sensitive data. To improve cost-efficiency, extensive research is being carried out on the various components required for high data throughput using quantum key distribution (QKD). Aiming for an application-oriented solution, we report the realization of a multichannel QKD system for plug-and-play high-bandwidth secure communication at telecom wavelengths. We designed a rack-sized multichannel superconducting nanowire single photon detector (SNSPD) system, as well as a highly parallelized time-correlated single photon counting (TCSPC) unit. Our system is linked to an FPGA-controlled QKD evaluation setup for continuous operation, allowing us to achieve high secret key rates using a coherent-one-way protocol.
ABSTRACT
We analytically derive a set of formulas for the transmission loss in vacuum between antennas that send and receive single-mode Gaussian beams. We relate our results to standard far-field link budget parameters.
ABSTRACT
Optical data links through the atmosphere suffer from turbulence-induced signal scintillation. In a coaxially-symmetric bidirectional link scenario, the variations of the axial intensities at both ends are correlated. This relation can be used as an inherent feedback mechanism, with negligible delay, to enhance the capacity of the transmission system. By experiment, we show the correlation coefficient of both received signals can reach values close to one over long atmospheric distances, provided the receiver apertures are smaller than specific intensity speckle structures, while the correlation reduces gradually with larger apertures. This allows transmission capacity to be optimized with adaptive transceiver systems that take into account the degree of correlation.
ABSTRACT
According to the point-source point-receiver (PSPR) reciprocity, the received field remains equal when the positions of a point source and point receiver are interchanged. We extend the PSPR scenario to a finite receiver that spatially averages scintillation over its aperture. By use of weak-fluctuation theory, an analytical expression for the correlation coefficient between the received powers at both link ends is provided. The effects of turbulence profile, receiver aperture size, and central obscuration on the correlation are assessed. Because correlation is obtained to the detriment of antenna gain and aperture averaging, the net benefit of the channel reciprocity is highly scenario dependent.
ABSTRACT
The Rytov theory is applied to find the wave structure function of a laser beam transmitted from one satellite to another and propagating through the turbulent atmosphere. The phase-screen approximation is used. Taking into account refractive-index anisotropy, outer scale, and atmospheric mean-refraction defocusing, we provide expressions of the wave structure function for a spherical wave. The width and time of coherence at the receiver are evaluated. Expression for the beam spread is found using the extended Huygens-Fresnel principle, and beam wander is assessed. Beam wander occurs only for very narrow beams. Links involving low-Earth-orbit and geosynchronous satellites are studied as examples. Finally, conditions where optical tracking is perturbed by the atmosphere are examined.
