arXiv subjects

Arnaud Coatanhay

Publications and source records attributed to Arnaud Coatanhay.

10 recordsLinked to original sources

arXiv2026

Impact of a CSS quantum error correction code in underwater quantum key distribution

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.

quant-ph↗

arXiv2026

Performance Analysis of Underwater Quantum Key Distribution Protocols: BB84, SARG04, and BBM92

This study compares the quantum bit error rate (QBER) performance of BB84, SARG04, and the entanglement-based BBM92 protocol in non-turbulent underwater optical channels. Clear, coastal, and turbid water types are considered under different background-illumination and receiver configurations. For BB84 and SARG04, channel attenuation and background-induced detections determine the quantum gain and QBER. For BBM92, photon loss is included through the two arm detection efficiencies, whereas the polarization state conditioned on two-photon detection is described by local depolarizing Kraus channels. The BBM92 QBER is obtained from the correlations in both measurement bases and from an exact partition of recorded events into true and false coincidences. Stochastic numerical estimates provide consistency checks of the analytical implementation. The results show that water turbidity and optical background reduce the distance below the adopted QBER threshold. They also show that a low conditional BBM92 QBER may coexist with a very small coincidence probability; therefore, the QBER-threshold distance alone does not represent a complete operational or secret-key-rate bound.

quant-ph↗

arXiv2026

A Wigner-based volumetric transport framework for paraxial waves in random media

We develop a Wigner-based phase-space framework for mean paraxial wave propagation in random media. Starting from the random parabolic wave equation, we derive the exact evolution of the realization-dependent Wigner distribution and identify the ensemble-averaged Wigner function as the natural second-order state variable. The averaged equation contains a closure defect, given by a mixed field--medium correlation, which prevents a closed transport equation from being obtained without additional assumptions. We therefore organize the modelling as a hierarchy from the random wave equation to an exact Wigner formulation, then to a nonlocal kinetic closure, and finally to a local Fokker--Planck reduction in the small-angle regime. For the minimal homogeneous isotropic Fokker--Planck model, we derive closed evolution laws for the quadratic moments, exhibit the cubic-in-distance contribution to beam spreading, and obtain explicit Gaussian and Gauss--Schell propagation formulas. These analytical results are used to validate a phase-space splitting solver in one-dimensional transverse benchmarks. Comparisons with nonlocal kinetic models show that the diffusive approximation is accurate for narrow momentum-transfer kernels and loses validity in a controlled way as finite-jump effects become significant. Finally, we introduce a first atmospheric specialization based on a regularized turbulence spectrum, yielding an effective diffusion coefficient expressed in terms of standard atmospheric parameters.

physics.ao-ph↗

arXiv2026

Environment-aware transverse transport geometry for non-Gaussian oscillator states

Non-Gaussian oscillator states are highly sensitive to weak perturbations, but reversible phase rotations and irreversible environmental changes should not be weighted equally. We introduce a finite-dimensional, direction-dependent local transport cost on a truncated Fock space and quotient it by Hamiltonian tangent directions. The resulting transverse quantity is a seminorm, not a global distance. A weighted operator frame specifies the representation cost of physical fluxes. Although the frame ({a_M,a_M^\dagger}) already spans the full trace-zero sector, multiphoton and diagonal directions can represent selected Lindblad tangents much more efficiently. Numerical tests on compass-like states show that phase rotations are removed, dephasing becomes inexpensive only after diagonal directions are included, and two- and three-photon processes are naturally captured by multiphoton frames. Thermal weights satisfy an operator-modular covariance but do not define a complete KMS geometry. The construction is therefore an environment-aware local diagnostic with explicit cutoff, regularization, and normalization conventions.

quant-ph↗

arXiv2026

Stellar Braid Monodromy of Finite-Rank Non-Gaussian Photonic States

Finite-rank non-Gaussian bosonic states admit a holomorphic description in the Bargmann representation: after a zero-free Gaussian factor is separated off, their non-Gaussian structure is encoded by a finite stellar divisor. This article introduces a topological refinement of stellar rank for regular parameterized families of such states. Rather than only counting the zeros of the stellar divisor, we follow their motion under deformations of the state and record the associated braid monodromy. In the finite-Fock chart, a regular degree-r stellar state is represented by a monic polynomial with r simple zeros. The regular stratum is biholomorphic to the unordered configuration space of r points in the complex plane, and its fundamental group is the Artin braid group on r strands. Thus braid monodromy is an intrinsic invariant of loops in the regular finite-rank stellar state space. We then extend the construction to admissible finite stellar divisors of the form E_tau,mu(z) P(z); the zero-free Gaussian parameters form a contractible fiber over the same configuration-space base. Experimentally motivated finite-Fock families, especially the cubic subspace spanned by the first four Fock states, provide concrete laboratories, while trinomial slices yield explicit discriminants and local half-twists. The resulting invariant is post-tomographic and applies to preparation loops and parameterized families; it complements Wigner negativity, stellar rank, approximate stellar rank, and other scalar diagnostics of non-Gaussianity.

quant-ph↗

arXiv2026

Free-Space CV-QKD with Single-Mode Fiber Reception: Effective Coupling Statistics and Protocol-Dependent Reference Noise

We study free-space continuous-variable quantum key distribution (CV-QKD) with single-mode fiber (SMF) reception under atmospheric turbulence. The optical channel is modeled by split-step propagation through random phase screens, followed by finite-aperture collection and projection onto the guided receiving mode. We first examine the standard GG02 setting and ask which receiver-side observable is sufficient for effective key-rate prediction. We show that a mean-loss description is generally too optimistic, whereas a scalar effective law for the SMF coupling efficiency provides an accurate downstream Gaussian-channel description within the effective model considered here. We then extend the optical model to a pilot-assisted architecture in which the signal and pilot propagate through correlated but non-identical turbulent realizations generated by a frozen-flow construction. In this case, the signal coupling law alone is no longer sufficient: signal--pilot phase mismatch and loss of post-coupling coherence produce an additional protocol-dependent reference-noise penalty. The results distinguish two regimes: a scalar coupling description is largely adequate for GG02, while transmitted-reference architectures require an additional differential reference observable beyond the signal coupling statistics.

quant-ph↗

arXiv2026

Non-Maximally Entangled States for Quantum Key Distribution in Underwater Channels: BBM92 Protocol via Kraus Operators

Underwater optical channels pose significant challenges to the security and reliability of quantum communication systems due to absorption and scattering. In this paper, we investigate the BBM92 entanglement-based quantum key distribution (QKD) protocol under realistic underwater channel conditions. Photon pairs are prepared in non-maximally entangled states, and the underwater propagation medium is modeled as a quantum channel incorporating both amplitude-damping and depolarizing effects, described within the Kraus operator formalism. The protocol performance is evaluated in terms of quantum bit error rate (QBER) and secret key rate (SKR), analyzed as functions of the entanglement degree and channel degradation parameters. Closed-form analytical expressions for the QBER and SKR are derived for the proposed channel model and validated through Monte Carlo simulations. The proposed framework is then applied to various realistic underwater scenarios, considering different water types, namely clear ocean, coastal, and turbid water, as well as varying atmospheric conditions.

quant-ph↗

arXiv2026

CV-QKD over Turbulence Channels with Virtual Photon Subtraction and Quantum Multiple-Symbol Detection for Underwater Quantum Communications

Continuous-variable quantum key distribution (CV-QKD) is a promising approach for secure underwater quantum communications (UQCs), where propagation loss, scattering, turbulence, and receiver thermal noise can severely degrade the transmission of quantum states. In this paper, we propose an underwater CV-QKD system with virtual photon subtraction (VPS), implemented through post-selection of Alice's measurement outcomes, without requiring channel state information (CSI) at the receiver. Three VPS-based system configurations are analyzed, corresponding to homodyne detection (VPS-HD), quantum maximum-likelihood detection (VPS-QMLD), and quantum multiple-symbol detection (VPS-QMSD). System performance is evaluated in terms of the accepted-only quantum bit error rate (QBER), where underwater turbulence is modeled by an Erlang distribution. Analytical and semi-closed-form QBER expressions are derived for the three configurations and validated through Monte Carlo simulations for different water types and system parameters. The results show close agreement between analytical and simulation results and demonstrate that VPS-QMSD provides the best robustness against underwater turbulence, achieving the lowest QBER compared with VPS-QMLD and VPS-HD.

quant-ph↗

arXiv2026

From State-Space Transport to Measurement-Aware Distinguishability in Quantum Sensing

Overlap-based distinguishability measures, such as fidelity- or Chernoff-type quantities, play a central role in quantum sensing and quantum illumination. In strongly lossy and fluctuating environments, however, these quantities may become numerically compressed and therefore less informative for optimization, monitoring, or adaptive control. In this work, we investigate transport-based distinguishability criteria for lossy quantum sensing. We first introduce an isotropic Gaussian transport metric defined on first and second moments and compare it with a fidelity-based benchmark in a thermal-loss model. We then show analytically that, within an isotropic thermal-reference geometry, this metric locally disfavors squeezing relative to coherent displacement, thereby distinguishing global phase-space robustness from directional metrological advantage. We next introduce a projected transport metric adapted to quadrature-resolved measurements and show that its optimization over the measurement quadrature is analytically tractable, reducing to a boundary choice between the principal axes of the output noise ellipse. We further extend the framework to a measurement-aware metric defined on detector output statistics, and derive an explicit Gaussian formula for a noisy quadrature measurement chain. Finally, in a fading setting, we show that the isotropic metric and the projected metric aligned with the coherent displacement retain first-order sensitivity to the transmissivity in the strong-loss regime, whereas the orthogonal projected metric is compressed to second order. These results support a hierarchical view of transport-based distinguishability in quantum sensing, ranging from global robustness indicators to measurement-adapted operational metrics.

quant-ph↗

arXiv2013

Qualitative Analysis of the Time-Frequency Signature Induced by a Reflected L-Band Signal from Time Evolving Sea Surfaces

Passive remote sensing techniques have become more and more popular for detection and characterization purposes. The advantage of using the Global Navigation Satellite Systems (GNSS) are the well known signals emitted and the availability in most areas on Earth. In the present paper, L-Band signals (including GNSS signals) are considered for oceanographic purposes. The main interest in this contribution is the analysis of the signal reflected by an evolving sea surface using time-frequency transforms. The features which occur in this domain are examined in relation to the physical phenomena: interaction of the electromagnetic waves with the moving sea surface.

physics.ao-ph↗