Underwater optical communication previously suffered from substantial photon loss degrading performance in quantum key distribution systems. Four-qubit Calderbank-Shor-Steane (CSS) quantum error correction codes improve these communications; specifically, the discard code variant achieves a signal-to-noise ratio gain of 4.5 dB at an eleven per cent security threshold. This improvement enables secure communication in scenarios previously considered infeasible due to high levels of noise and attenuation.

Employing specific error correction codes enhances secure underwater communications by addressing signal degradation caused by photon loss which limits communication distance. These four-qubit Calderbank-Shor-Steane (CSS) codes improve performance when signals become weak but add complexity impacting data transmission speed. Improvements to secure underwater communications use techniques borrowed from error correction found in digital data transmission; much like adding redundancy to a spoken message so it can be understood even amidst background noise, these methods address signal loss inherent in water-based optical systems.

Quantum key distribution (QKD) enables secure communication by creating encryption keys based on the laws of physics and ensuring any attempt at eavesdropping is immediately detectable but suffers sharply when light signals weaken due to absorption and scattering within water. Employing four-qubit Calderbank-Shor-Steane (CSS) codes, a technique for correcting errors in quantum information, enhances performance, achieving a 4.5 dB gain in signal strength at an eleven per cent security threshold.

Discarded CSS coding enhances security and range within challenging aquatic environments

A substantial improvement in underwater quantum communication has been achieved using the discard variant of a four-qubit Calderbank-Shor-Steane (CSS) code; it attains a 4.5 dB signal-to-noise ratio gain at an eleven percent security threshold, exceeding the 3 dB gain attained by the standard CSS code. Prior to this work, such communications were unattainable under conditions with excessive photon loss and noise inherent in water channels. Detailed analysis reveals that employing quantum error correction codes extends operational range but reduces data throughput, necessitating scenario-dependent deployment strategies for optimal performance.

The discarded version of a four-qubit Calderbank-Shor-Steane (CSS) code improves underwater quantum communication performance, yielding an unsecured signal-to-noise ratio gain when generating keys and extending operational range in challenging conditions. Simulations validated analytical studies of both qubit error rates and secure key generation rates following detailed modelling incorporating photon loss, geometric spreading and solar noise within vertical BB84 quantum key distribution protocols.

At a security threshold requiring eleven percent or fewer errors, the standard CSS code provides a three decibel improvement; however, its discard variation achieves 4.5 dB, a substantial increase for severely limited channels. Calculations also show that reducing the probability of successful photon arrival by factors of ten results in significant reductions to achievable data throughput with all tested configurations.

Quantum durability extends optical reach in demanding undersea environments

Securing communication beneath the waves is vital for protecting critical infrastructure such as offshore platforms and subsea cables; existing optical methods struggle with inherent photon loss and noise within water environments. Quantum error correction can extend viable transmission distances but introduces complexity impacting data speed. Acknowledging this trade-off is important, however, because it unlocks communication possibilities otherwise lost to signal degradation underwater.

A clear benefit has been demonstrated in challenging conditions where light weakens rapidly with distance, extending viable transmission ranges by several decibels at lower power levels. Consequently, secure communications are now feasible for critical infrastructure like offshore energy platforms or subsea networks even when visibility is poor and signals suffer interference. These extended secure underwater communication ranges result from employing quantum error correction techniques that combat signal weakening in aquatic environments.

While increasing complexity reduces data speed, these methods unlock previously unattainable links vital for both offshore energy platforms and subsea networks. Four-qubit Calderbank-Shor-Steane (CSS) codes represent a strong step towards reliable secure communication underwater; they extend viable transmission distances in challenging optical channels where photon loss and noise typically limit performance. Similar to clarifying a message spoken amidst background clamour, these CSS codes add redundancy to quantum information, enabling key distribution even when signals are weak but introducing the aforementioned impact on data speed.

The research demonstrated that employing four-qubit Calderbank-Shor-Steane (CSS) quantum error correction codes can improve secure communications under water by extending feasible transmission ranges by up to 4.5 dB at specific signal-to-noise ratios. The study highlights a trade-off between increased complexity, which reduces data throughput, and improved range; CSS codes prove beneficial when signals are weak but less so as power increases. Researchers identified operational depth windows where this approach enables communication previously considered impossible given the Jerlov water type III scenario tested.

👉 More information
🗞 Impact of a CSS quantum error correction code in underwater quantum key distribution
✍️ Juliette Florin, Nicolas Le Josse, Arnaud Coatanhay and Gilles Burel
🧠 ArXiv: https://arxiv.org/abs/2609.18666

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