Underwater quantum coding

Marco O. Lanzagorta

Joanna Śliwa, Konrad Wrona, Tsvetelina Shabanska +1 more

This paper focuses on defining specific scenarios for quantum-safe underwater communication to drive security requirements. The maritime environment in general and the underwater domain in particular have unique constraints and require the use of specialized cryptographic techniques. The two scenarios presented are underwater harbor protection and anti-submarine warfare. Employing post-quantum cryptographic algorithms in these use cases enhances security and maintains the integrity of sensor data. This research contributes to strengthening underwater communication security through advancements in acoustic cryptography, error correction codes, adaptive modulation, and energy-efficient algorithms.

Juliette Florin, Nicolas Le Josse, Arnaud Coatanhay +1 more

Quantum key distribution (QKD) enables secure underwater communications essential for maritime infrastructure. Underwater optical channels introduce substantial photon loss (erasures) and ambient noise that degrade QKD performance. This paper investigates whether two four-qubit Calderbank-Shor-Steane (CSS) quantum error correction codes (QECC) mitigate these impairments in vertical underwater communication BB84 QKD protocol. After developing a comprehensive stochastic channel model incorporating photon loss, geometric spreading, and solar noise, we assess the viability of QECC through the analytical study of the quantum bit error rate (QBER) and the secure key rate (SKR) with and without security depending on the signal-to-noise ratio (SNR) validated against Monte Carlo simulations. The standard four-qubit CSS code achieves a 3 dB SNR gain at the QBER 11% security threshold; the discard code variant achieves 4.5 dB. However, QECC includes an encoding overhead that reduces the SKR. We demonstrate a crucial relationship between the SKR and the probability of arrival of the sent photon. Analysis shows that QECC is beneficial exclusively in marginal SNR regimes; at high SNR, raw BB84 dominates. For an ocean Type III Jerlov water scenario with sunlight coming from the sun located at the top of the atmosphere, we identify operational depth-range windows where QECC enables communication otherwise infeasible. This analysis establishes that error correction deployment must be scenario-dependent: extend operational range at the cost of throughput when SNR is marginal, or prioritize key generation rates at high SNR.

Michael Reck, Klaus Mattle, Harald Weinfurter +2 more

Michael Reck, Klaus Mattle, Harald Weinfurter +1 more

Zhiqing Wu, Shanchuan Dong, 尚震 Zhen Shang +8 more

To address the challenges of frame drift and link interruptions in underwater quantum key distribution (QKD), we design a polarization-encoded reference-frame-independent quantum key distribution (RFI-QKD) system. Temporal coding of synchronized optical pulses combined with quantum signal discrimination enables the system to re-establish timing synchronization after a link interruption and resume key generation. With an average output photon number of 0.1, we experimentally demonstrate RFI-QKD over a 15.3 dB-loss channel, achieving a final key rate of 357 bps. Furthermore, over a 17 m underwater channel with an attenuation of 2.76 dB, the system attains a final key rate of 24.14 kbps.