Phase auto-compensating high-dimensional quantum cryptography in elliptical-core few-mode fibres

ABSTRACT

High-dimensional quantum cryptography through optical fibres with several spatial modes requires an efficient quantum key distribution (QKD). However, optical modes acquire different phases and lags due to modal dispersion and random fluctuations, and a modal crosstalk appears under propagation. At present, special optical fibres for spatial multiplexing are being proposed in order to reduce notably the modal crosstalk, however, arbitrary relative phases and lags between modes are always present, which prevents getting an efficient phase encoding QKD. In this work, we take advantage of elliptical-core few-mode optical fibres presenting a very low modal crosstalk and propose an exact phase auto-compensating method by making photons travel several times the path between Alice and Bob (rounds) and by using appropriate modal inversions in each round trip. In order to make clear the proposed phase auto-compensating method, we study in detail a four-dimensional BB84 QKD case with single photon states excited in both polarization and spatial LP modes.

KEYWORDS:

• Spatial multiplexing
• quantum communications
• quantum cryptography
• elliptical-core few-mode fibres

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Authors wish to acknowledge the financial support of this work by Consellería de Cultura, Educación e Ordenación Universitaria, Xunta de Galicia with funds of the European Social Fund, and the partial financial support by Ministerio de Economía y Competitividad, Central Government of Spain, Contract number AYA2016-78773-C2-2-P (MINECO/FEDER, UE), and the partial support of Xunta de Galicia, Consellería de Educación, Universidade e FP [grant (GRC) no. ED431C 2018/11].