Hyper-entanglement signals in quantum optical circuits

Secure free-space data links can be established using conventional laser communication technology or, if necessary, they can be further enhanced with quantum encryption. Security features of these systems are based on the protocols that make use of the inherent properties of laser light. In this case, encryption does not rely on complex mathematical algorithms that add overhead to the communication stream, but instead it is based on physical-layer processes in the laser sources and other modulating components. One promising approach is based on polarization entanglement between correlated photon pairs to achieve data encryption in quantum communication systems. The foundation of security lies in the response of photons to polarization measurements. Additional degrees of freedom can be added to each "single-particle" state by using hyper-entanglement. The situation can be visualized when several carrier waves are assigned specific frequencies in the 100 GHz International Telecommunication Union (ITU) grid. The two technologies that can be eventually integrated to achieve this task include hyperspectral quantum circuits and the entangled pair source and detection systems. This results in frequency/polarization hyper-entanglement, which can be processed with additional wavelength-division multiplexing (WDM) components to achieve efficient separation of the signals It is important to understand that most of the previous work is theoretical and assumes ideal properties of all optical parts. In reality, many non-ideal features of the quantum circuits and their components can change the way the quantum states are processed, and this constitutes the main focus of our paper.