Quantum key distribution with polarized qubits has not yet been realized over the aerial fiber, due to rapid polarization changes. Here, we report our recent work towards quantum communication through an aerial fiber channel. We designed a fast polarization feedback module featuring high efficiency, fast speed, and good stability. With this module, we implemented long-distance quantum key distribution over different types of aerial fiber links based on polarization encoding. Our work takes a significant step towards the application of quantum communications in complex environments.

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We propose a novel porous-core photonic crystal fiber (PCF) consisting of asymmetrical rectangular air holes in the core and six-ring hexagonal lattice circular air holes in the cladding for achieving low-loss polarization terahertz transmission in a wide frequency range. By assuming TOPAS as the host material, the finite element method (FEM) is used to investigate its properties. The near-zero flattened dispersion of −0.01±0.02 ps/THz/cm is achieved over a frequency range of 1.0-2.0 THz, as well as a high birefringence of 7.1×10 −2 which can be useful for polarization-maintaining applications. Also, critical parameters such as mode field distribution, effective material loss, confinement loss, and effective mode area are discussed in detail. Further, fabrication possibilities are discussed briefly by comparing recent work on similar waveguide structures.

A single-mode hollow-core anti-resonant (HC-AR) waveguide designed for low-loss terahertz (THz) wave propagation is fabricated by three-dimensional (3D) printing. Compared to similar structures reported recently, the rotating-nested semi-elliptical tubes (SETs) in the HC-AR THz waveguide cladding suppress multiple high-order modes (LP11, LP21, and LP02 modes) at the same time giving rise to enhanced single-mode transmission and low losses. Three HC-AR THz waveguides with different wall thicknesses are produced using two photosensitive resins and analyzed by THz time-domain spectroscopy (THz-TDS). The experimental results show that the electric field distributions at the output end of these waveguides have a Gaussian-like distribution reflecting that of the single mode. The smallest transmission losses determined by the ‘cut-back’ method are 0.03 cm− 1 at 0.31 THz for sample A, 0.02 cm− 1 at 0.4 THz for sample B, and 0.01 cm− 1 at 0.23 THz for sample C. The consistent experimental and simulated results reveal that the HC-AR THz waveguide has many advantages over current ones by achieving low losses and single-mode operation simultaneously.

A high-birefringence and low-loss terahertz (THz) hollow-core anti-resonant fiber (THz HC-ARF) is designed and analyzed numerically by the finite element method (FEM). The THz HC-ARF is composed of an elliptical tube as the core for high birefringence guidance and a pair of symmetrical slabs arranged vertically as the cladding to attain low loss. Numerical analysis indicates that the birefringence reaches 10−2 in the transmission window between 0.21 and 0.35 THz. The highest birefringence is 4.61 × 10−2 at 0.21 THz with a loss of 0.15 cm-1. To verify the theoretical results, the THz HC-ARF is produced by three-dimensional (3D) printing, and the transmission characteristics are determined by THz time-domain spectroscopy (THz-TDS). High birefringence in the range of 2.17 × 10−2 to 3.72 × 10−2 and low loss in the range of 0.12 to 0.18 cm-1 are demonstrated experimentally in the 0.2 to 0.27 THz transmission window. The highest birefringence is 3.72 × 10−2 at 0.22 THz and the corresponding loss is 0.18 cm-1. The THz HC-ARF shows the highest birefringence besides relatively low loss compared to similar THz HC-ARFs reported recently.