Abstract-In this paper, we experimentally investigate the statistical distribution of intensity fluctuations for underwater wireless optical channels under different channel conditions, namely fresh and salty underwater channels with and without air bubbles. To do so, we first measure the received optical signal with a large number of samples. Based on the normalized acquired data the channel coherence time and the fluctuations probability density function (PDF) are obtained for different channel scenarios. Our experimental results show that salt attenuates the received signal while air bubbles mainly introduce severe intensity fluctuations. Moreover, we observe that log-normal distribution precisely fits the acquired data PDF for scintillation index (σ 2 I ) values less than 0.1, while Gamma-Gamma and K distributions aptly predict the intensity fluctuations for σ 2 I > 1. Since neither of these distributions are capable of predicting the received irradiance for 0.1 < σ 2 I < 1, we propose a combination of an exponential and a log-normal distributions to perfectly describe the acquired data PDF for such regimes of scintillation index.

Optimal wavelength assignment in dense-wavelength-division-multiplexing (DWDM) systems that integrate both quantum and classical channels is studied. In such systems, weak quantum key distribution (QKD) signals travel alongside intense classical signals on the same fiber, where the former can be masked by the background noise induced by the latter. Here, we investigate how optimal wavelength assignment can mitigate this problem. We consider different DWDM structures and various sources of crosstalk and propose several near-optimal wavelength assignment methods that maximize the total secret key rate of the QKD channels. Our numerical results show that the optimum wavelength assignment pattern is commonly consisted of several interspersed quantum and classical bands. Using our proposed techniques, the total secret key rate of quantum channels can substantially be improved, as compared to conventional assignment methods, in the noise dominated regimes. Alternatively, we can maximize the number of QKD users supported under certain key rate constraints.

Abstract-We propose orthogonal frequency division multiplexing (OFDM), as a spectrally efficient multiplexing technique, for quantum key distribution (QKD) at the core of trustednode quantum networks. Two main schemes are proposed and analyzed in detail, considering system imperfections, specifically, time misalignment issues. It turns out that while multiple service providers can share the network infrastructure using the proposed multiplexing techniques, no gain in the total secret key generation rate is obtained if one uses conventional passive all-optical OFDM decoders. To achieve a linear increase in the key rate with the number of channels, an alternative active setup for OFDM decoding is proposed, which employs an optical switch in addition to conventional passive circuits. We show that by using our proposed decoder the bandwidth utilization is considerably improved as compared to conventional wavelength division multiplexing techniques.Index Terms-Quantum key distribution, orthogonal frequency division multiplexing, quantum networks

Abstract-An efficient method for optimal allocation of wavelengths in a hybrid dense-wavelength-division-multiplexing system, carrying both quantum and classical data, is proposed. The transmission of quantum bits alongside intense classical signals on the same fiber faces major challenges arising from the background noise generated by classical channels. Raman scattering, in particular, is shown to have detrimental effects on the performance of quantum key distribution systems. Here, by using a nearly optimal wavelength allocation technique, we minimize the Raman induced background noise on quantum channels, hence maximize the achievable secret key generation rate for quantum channels. It turns out the conventional solution that would involve splitting the spectrum into only two bands, one for quantum and one for classical channels, is only a suboptimal one. We show that, in our optimal arrangement, we might need several quantum and classical bands interspersed among each other.I. Introduction Quantum key distribution (QKD) is a promising technology that offers unconditional security in applications with high security requirements. In the past three decades, there has been much progress in both theoretical and experimental aspects of QKD. Since the first experimental demonstration of QKD [1] up until now, QKD has seen considerable enhancement in reach and performance in point-to-point scenarios [2]. To further make QKD a cost-effective technology for large-scale applications, its adaptation to the infrastructure of existing classical communications networks is unavoidable [3], [4]. In particular, dense-wavelength-division-multiplexing (DWDM) techniques can enable the simultaneous transmission of both quantum and classical data on the same fiber. However, the transmission of quantum data alongside strong classical signals, in practice, faces some challenges due to nonlinear effects in fiber, such as Raman scattering and four-wave mixing [5], which may severely affect the operation of QKD links. Here, we consider a hybrid DWDM link with known numbers of quantum and classical channels and find a nearly optimal wavelength assignment method in the presence of Raman noise. Our proposed technique can be generalized to account for other sources of noise as well [6]. Raman noise has, however, been shown to be the dominant source of background noise in such hybrid setups [5].The key problem in integrating quantum and classical communications channels is the background noise induced by classical channels onto quantum ones. Even if we allocate different wavelengths to the quantum and classical channels, as in DWDM, some of the noise generated by classical signals,

We propose a low-complexity near-optimal wavelength allocation technique for quantum key distribution access networks that rely on wavelength division multiple access. Such networks would allow users to send quantum and classical signals simultaneously on the same optical fiber infrastructure. Users can be connected to the access network via optical wireless or wired links. We account for the background noise present in the environment, as well as the Raman noise generated by classical channels, and calculate the secret key generation rate for quantum channels in the finite-key setting. This allows us to examine the feasibility of such systems in realistic scenarios when the secret key exchange needs to be achieved in a limited time scale. Our numerical results show that, by proper choice of system parameters for this noisy system, it is possible to exchange a secret key in tens of seconds. Moreover, our proposed algorithm can enhance the key rate of quantum channels, especially in high noise and/or high loss regimes of operation.