Polarization control algorithm for QKD systems

The crucial task for polarization-encoding fiber QKD is to compensate polarization drift occurring in a quantum channel. To solve this problem, the receiver usually uses a polarization controller. For proper operation, this device must be efficiently managed in real-time. In this work, a gradient-descent-based algorithm is proposed to solve this problem. The algorithm was implemented and tested on a QRate commercial QKD fiber system, that utilizes BB84-protocol. Low and stable QBER has been obtained during a day of continuous operation.

QKD key generation control protocol

Polarization-encoding fiber QKD requires compensation of polarization distortion caused by birefringence in optical fiber. Solving this task inevitably requires losing some effectiveness in terms of the final key rate. In this work, a time-division multiplexing protocol for polarisation calibration is suggested. This protocol was implemented in a QRate commercial QKD fiber system, utilizing BB84-protocol. Parameters of the protocol were optimized to maximize the secret key rate.

Polarization-Encoded Fully-Phase Encryption Using Transport-of-Intensity Equation

In this study, we propose a novel method to encrypt fully-phase information combining the concepts of the transport of intensity equation and spatially variant polarization encoding. The transport of intensity equation is a non-iterative and non-interferometric phase-retrieval method which recovers the phase information from defocused intensities. Spatially variant polarization encoding employs defocused intensity measurements. The proposed cryptosystem uses a two-step optical experimentation process—primarily, a simple set-up for defocused intensities recording for phase retrieval and then a set-up for encoding. Strong security, convenient intensity-based measurements, and noise-free decryption are the main features of the proposed method. The simulation results have been presented in support of the proposed idea. However, the TIE section of the cryptosystem, as of now, has been experimentally demonstrated for micro-lens.

Time-Spectral based Polarization-Encoding for Spatial-Temporal Super-Resolved NSOM Readout

Detection of evanescent waves through Near-field Scanning Optical Microscopy (NSOM) has been simulated in the past, using Finite Elements Method (FEM) and 2D advanced simulations of a silicon Schottky diode, shaped as a truncated trapezoid photodetector, and sharing a subwavelength pin hole aperture. Towards enhanced resolution and next applications, the study of polarization’s influence was added to the scanning. The detector has been horizontally shifted across a vertically oriented Gaussian beam while several E-field modes, are projected on the top of the device. Both electrical and electro-optical simulations have been conducted. These results are promising towards the fabrication of a new generation of photodetector devices which can serve for Time-Spectral based Polarization-Encoding for Spatial-Temporal Super-Resolved NSOM Readout, as developed in the study.