Theoretical study of a submarine to submarine quantum key distribution systems

Due to the absorption of water, communication between two parties submersed below the water is normally performed with acoustic waves. However, with the need for higher data rates, the use of RF or optical frequencies is needed. Currently, optical wavelengths have been demonstrated for classical communication over short distances. For these short distances, if a large amount of data needs to be transmitted securely, it is not feasible for both parties to return to the surface to communicate. Additionally, it can be assumed that a third party (Eve) is located in the channel trying to gather information. The solution is to use quantum key distribution (QKD) to generate the secure key, allowing the parties to continuously encrypt and transmit the data. It is assumed the BB84 protocol using pairs of polarization entangled photons generated from a spontaneous parametric down conversion (SPDC) source of Type-II. By using entangled photons, Eve is not able to gain information without being detected. In this work, horizontal oceanic channel is studied for various distances ranging from 10 m to 100 m, depth ranging from 100 m to 200 m, and surface chlorophyll-a concentrations at a wavelength of 532 nm. The secure key rates are calculated, assuming that a low-density parity check (LDPC) error correction code is used for information reconciliation. The maximum secure key rate and optimal number of average entangled photons transmitted are then studied for the various channels.