Interference of multi-mode weak coherent states for twin-field quantum communication applications

Subject of Research. We present the results of analytical research and experimental implementation of quantum key distribution protocol based on multi-mode weak coherent states with untrusted detection node. Such states are based on interference of phase-coded sidebands in case, where legitimate users are sending these states to the untrusted detection node that could be controlled by an eavesdropper. Method. The method of initial states generation is applied. Their propagation via fiber-optic lines and interference result is shown. A classical approximation is used for description. The experiment is carried out with the power measurement system connected on detection node side. We present the experimental scheme and show that in the classical regime the interference pattern is obtained at the fiber-optic 2x2 beam splitter with 50:50 ratio depending on the phase difference of the radiofrequency modulating signals (4.8 GHz) applied to LiNbO3 phase modulators, which modulate an optical carrier (1550 nm) in the blocks of legitimate users. Main Results. Experimental results are in accordance with analytical ones. Harmonical dependence of the optical power at the sidebands is obtained as an interference result. In this case, visibility of interference pattern is up to 97.4% and is good enough. Thus, application of these results in terms of quantum optics and experimentation in quantum single-photon regime might be a subject of future research. Practical Relevance. Practical application of research results lies in the development of quantum key distribution protocols and optical schemes with special attention to eavesdropping of quantum states and attacking the detection nodes. Application of the proposed multi-mode coherent states enables the legitimate users to extract information, while an eavesdropper does not obtain any information about encoded bits due to ambiguity of detection events.