An international team of researchers from the Austrian Academy of Sciences and the University of Vienna have established a quantum encrypted connection between Malta and Sicily via a conventional submarine telecommunications cable using entangled light particles. The record distance of 192 kilometres is being viewed as a further step in the development of a secure quantum internet. 

They explained that at a communication station on the coast of Malta, they first generated pairs of light particles that were entangled with each other via their random direction of oscillation,(polarisation).

The quantum key obtained in this way is secure: if an eavesdropper attempts to intercept such encrypted communication, the quantum physical state of the light particles would immediately change due to physical laws, and the "hacker" would immediately be caught.

"This total distance of 192 kilometres is a new record for the method of producing quantum keys through polarisation-entangled light particles over longer distances on Earth. In this test setup, the transmission speed reached four bits per second" the researchers said.  

The experiment at the bottom of the sea provided evidence of the unexpectedly high stability of the method and thus of the great potential it holds for the creation of a quantum internet of the future, by employing a conventional telecommunications infrastructure, they added.

The connection was maintained over the long distance without further stabilization over a period of more than six hours.

“Many of today's fiber-optic connections are laid on the ocean floor. Thanks to this experiment, it is now certain that these are also very suitable for the transfer of quantum states,” the team added. 

The team published its results  in the scientific journal Nature Quantum Information.