Multichannel Hybrid Quantum Cryptography for Submarine Optical Communications

TL;DR

This work tackles the problem of extending quantum key distribution to submarine-scale distances by proposing a multichannel hybrid QKD framework that leverages autocompensating, high-dimensional DM-CV-QKD deployed along continental shelf segments. The core method uses product coherent states across multiple spatial modes with an autocompensating, plug-and-play architecture to minimize stabilization hardware, while enabling higher SKR through high dimensionality. Security is strengthened by distributing keys over multiple submarine channels and combining them via XOR or similar operations, with additional physical layers of security (both passive and active) applied to buried non-quantum links to force an adversary to compromise several isolated pathways simultaneously. Collectively, the approach offers practical paths to secure long-distance submarine communications, balancing hardware simplicity, scalability across $M$ channels, and layered defenses against sophisticated eavesdropping.

Abstract

We present a multichannel hybrid quantum cryptography approach intended for submarine quantum optical communications between Alice and Bob separated a distance beyond the current QKD possibilities, each located on a coastline. It is based on the difficult of a simultaneous access to $M$ optical submarine channels. The optical lines from the coastline and ideally to the end of the continental platform are governed by the quantum properties of the light under an autocompensating high-dimensional discrete-modulation continuous variable QKD protocol. The hybrid approach consists of combining several secret keys of the $M$ channels and introducing extra layers of security, passive and/or active, on the non quantum optical lines located beyond the continental platform.

Figures (8)

• Figure 1: Sketch of a submarine optical communication line. It is shown A and B, and arbitrary points with quantum lines A-A$_{i}$ and B$_{i}$-B. Moreover, points $A'_{i}$ and $B'_{i}$ are considered to implement possible additional physical layers of security in optical lines A$_{i}$-A'$_{i}$ and/or B'$_{i}$-A$_{i}$.
• Figure 2: (a) The four weak coherent states for $N$$=$$2$ modes represented in the two-dimensional first quadrature space $\mathcal{E}_{1}\mathcal{E}_{2}$. The states are centred on $(\mathcal{E}_{1}\mathcal{E}_{2})=(\pm a \pm a \rangle$. The values $\nu_{1}^{B}\nu_{2}^{B}$ for $N$$=$$2$ are indicated within the threshold regions with squared frontiers in the two-dimensional first quadrature space $\mathcal{E}_{1}\mathcal{E}_{2}$; the same thresholds $\pm\mathcal{E}_{o}$ has been chosen for each dimension. (b) The states in the basis $\mathcal{P}$, that is, the vacuum state $\vert 00\rangle$ is shown.
• Figure 3: (a) QBER for 1 and 2 modes (cross and star dots) as a function of mean photon number; IQBER is shown for completeness (solid and dashed lines). (b) Secret Key Rate (SKR) as a function of distance $L$ for 1 and 2 modes (cross and star dots) for a mean optical field $a=1.2$ and threshold $\mathcal{E}_{o}=0.3$.
• Figure 4: Sketch of the autocompensation system. It is shown a product coherent state with two polarizations one for the local oscillator and the other one for the signal. An arbitrary perturbation P is indicated. Note the location of the HWP (Half Wave Plate) in the closed cycle (enclosed in a dotted circle) and the location of the PD-OFD (Polarization Dependent Optical fibre Delay) at the start of the quantum line.
• Figure 5: Sketch of extra physical layer of security between A and B connected with one satellite as trusted node travelling from a point above A to a point above B.