The evaluation of interferences and noise in quantum key distribution (QKD) links that coexist with classical links is fundamental in the design of QKD deployments. In this work, we provide an experimental assessment of the effects of amplified spontaneous emission (ASE) noise resulting from the copropagation of weak coherent pulses (WCPs) and erbium-doped fiber amplifier (EDFA)-amplified classical signals through an outdoor free-space optical link. We achieve an average estimated lower bound for the quantum bit error rate (QBER) due to ASE noise below 4% in all the scenarios that we evaluate, thus making the QKD link feasible. Our study explores the need to take into consideration both the ASE noise and the interference from the amplified signal when designing a discrete variable (DV)-QKD scheme that coexists with the propagation of classical channels. By adding filters on the receiver side, the direct interference caused by the amplified classical signal can be reduced. At the transmitter side, ASE suppression filters must be used to address false detections caused by ASE noise, and the EDFA must operate at the best pump current possible to achieve the desired gain while minimizing noise. For a 10 dB increase in the EDFA’s gain, we have measured an increase in the estimated lower bound for the QBER of 1.71%. This establishes a tradeoff between the amount of launched power required and the amount of ASE noise that degrades the quality of the quantum signal.
The evaluation of interferences and noise in quantum key distribution (QKD) links that coexist with classical links is fundamental in the design of QKD deployments. In this work, we provide an experimental assessment of the effects of amplified spontaneous emission (ASE) noise resulting from the copropagation of weak coherent pulses (WCPs) and erbium-doped fiber amplifier (EDFA)-amplified classical signals through an outdoor free-space optical link. We achieve an average estimated lower bound for the quantum bit error rate (QBER) due to ASE noise below 4% in all the scenarios that we evaluate, thus making the QKD link feasible. Our study explores the need to take into consideration both the ASE noise and the interference from the amplified signal when designing a discrete variable (DV)-QKD scheme that coexists with the propagation of classical channels. By adding filters on the receiver side, the direct interference caused by the amplified classical signal can be reduced. At the transmitter side, ASE suppression filters must be used to address false detections caused by ASE noise, and the EDFA must operate at the best pump current possible to achieve the desired gain while minimizing noise. For a 10 dB increase in the EDFA’s gain, we have measured an increase in the estimated lower bound for the QBER of 1.71%. This establishes a tradeoff between the amount of launched power required and the amount of ASE noise that degrades the quality of the quantum signal.
A version of an erbium doped fiber amplifier (EDFA) application scheme designed to increase the efficiency of the simultaneous quantum key distribution session and transmission of information by classical channels in a single optical fiber is explored.A theoretical study of the possibility to use EDFA in the explored way was conducted by numerical simulation methods.The mathematical model is based on the EDFA dynamics equations and the equations that determine the secure key generation rate in case of the subcarrier-wave quantum key distribution.A method for determining the optimal parameters of the scheme under study is described and the evaluation of the feasibility of using EDFA in the explored way is performed.A comparative analysis of the subcarrier-wave quantum key distribution system performance when integrated into an optical network is made in terms of the secure key generation rate for the cases when EDFA is either used or not.The results obtained demonstrate high efficiency of the scheme under study, i.e., the maximum achievable distance of the secure key distribution is increased while maintaining the efficiency of the information transmission.
Classical-quantum coexistence enables cost-effective transmission of data and quantum signals over the same fiber-optic channel. Nevertheless, weak quantum-key distribution (QKD) signals are susceptible to non-linear interference generated from the classical traffic, primarily spontaneous Raman scattering (SpRS) and four-wave-mixing (FWM), as well as to unfiltered noise. In QKD protocols, increased channel loss and excess noise both reduce the secret key rates (SKRs), as illustrated in this work for the two-state BB84 and Gaussian-modulated coherent-states (GMCS) protocols. In this study, we derive closed-form expressions for evaluating the accumulated interference power from coexisting classical signals in a quantum frequency channel. Our model enables effective design of classical-quantum systems in single-mode fibers (SMFs), capturing the evolution of interference arising from the relevant physical phenomena. We utilize the model to examine frequency allocation in multiband transmission systems, demonstrating that, contrary to common practice of allocating QKD channels in the O-band, increased SKR is achieved by placing quantum channels in the upper E-/lower S-band across the relevant scenarios.
A successful commercial deployment of quantum key distribution (QKD) technologies requires integrating QKD links into existing fibers and sharing the same fiber networks with classical data traffic. To mitigate the spontaneous Raman scattering (SpRS) noise from classical data channels, several quantum/classical coexistence strategies have been developed. O-band solutions place the QKD channel in the O-band for lower SpRS noise but with the penalty of higher fiber loss and can rarely reach beyond 80 km of fiber; another method is C-band coexistence with attenuated classical channels, which sacrifices the performance of classical channels for lower SpRS noise. In this work, a time-interleaving technique is demonstrated to enable the co- propagation of quantum and classical channels in the C-band without sacrificing either performance. By embedding QKD pulses in the gaps between classical data frames, the quantum channel is isolated from SpRS noise in both wavelength and time domains. C-band co- propagation of a polarization-encoding decoy-state BB84 QKD channel with a 100 Gb/s QPSK channel is experimentally demonstrated with quantum bit error rate (QBER) of 1.12%, 2.04%, and 3.81% and secure key rates (SKR) of 39.5 kb/s, 6.35 kb/s, and 128 b/s over 20, 50, and 100 km fibers, respectively. These results were achieved with the presence of classical launch power up to 10 dBm, which is at least one order of magnitude higher than reported works. We also demonstrated the co- propagation of a QKD channel with eight classical channels with total launch power up to 18-dBm (9-dBm per channel), which is the highest power of classical channels reported in C-band coexistence works.
Characteristics of noise in erbium doped fiber amplifier (EDFA) are theoretically analyzed and experimentally verified. Four discreet energy models are used for erbium ion transition. Spatially varying photon number for traveling optical field was defined in a spatially divided segment with finite length, whose length is decided by property of the spontaneous emission. Quantitative evaluation of the Langevin noise sources becomes possible by this model. The amplified spontaneous emission was included in terms of discrete longitudinal mode, which is defined for whole length of the EDFA. It was found that the relative intensity noise (RIN) in the EDFA hardly changes with noise frequency in the region higher than several kHz. The phase noise in EDFA also hardly changes with the noise frequency. The frequency noise of the signal light in the EDFA increases proportional to squared value of the noise frequency. The amount of RIN and the phase noise decrease in higher input optical power. Good correspondence between theoretical and experimental results was obtained.
The first semi-quantum noise randomized cipher based on amplified spontaneous emission (ASE) light source employing Y-00 protocol is proposed and demonstrated by a proof-of-principle experiment. As the signal carrier, ASE light can provide another fundamental uncertainty, namely, self-beating noise of ASE signal, which is much bigger than quantum shot noise of mesoscopic coherent state and also inevitable. By incorporating both the shot noise and beat noise of ASE signals, the security will be improved with a larger number of masked signals (NMS) under intrinsic noise. After formulating NMS and Q-factor in theory, we investigate the impacts of key system parameters on the security and transmission performances, respectively, in order to optimize the system design. To evidence the theoretical results, an experimental setup with balanced photodetector (BPD) as decoder is constructed. The local light modulated by M/2-ary running key is applied as optical threshold signal, which is fed into BPD together with the M-ary ASE signal, outputting the binary signal. Eventually, a 128-level Y-00 realization based on ASE source is realized at 2.5Gb/s over 100-km fiber. The experimental results agree with the theory in the trend, which indicate the validity and feasibility of the proposed scheme.
We investigate the crosstalk noise, especially the spontaneous Raman scattering, in the optical fiber of a co-propagation system between quantum key distribution (QKD) and classical communications. Although many methods have been proposed, such as increasing the wavelength spacing and narrowband filtering technique, to suppress Raman scattering noise, these methods greatly affect the performance of QKD. One way to solve the obstacle restricting the coexistence is to decrease the classical signal power. Based on the high gain of the gated avalanche photodiode and pulse position modulation, we demonstrate that the co-propagation system works effectively with only a small effect on long-haul fibers, which has great significance for the practical widespread commercialization of QKD.