With the substantial progress of terrestrial fiber-based quantum networks and satellite-based quantum nodes, airborne quantum key distribution (QKD) is now becoming a flexible bond between terrestrial fiber and satellite, which is an efficient solution to establish a mobile, on-demand and real-time coverage quantum network. However, the boundary layer (BL) normally adhere to the surface of the aircraft when its speed is higher than Mach 0.3. The BL would change local refractive index and energy flux density drastically, thus lowering the coupling efficiency and infidelity of quantum states. Here, we investigate the airborne QKD performance with the BL effects, which has been rarely mentioned in existing research. Through simulations and modeling, we present the relation between divergence angle and secure key rate. With the increase of flight speed v, relative flight altitude h and the shortest projection distance d, the key-rate curve is obviously jitter, and the QKD performance is continuously reduced. Simulation results show that, under several typical circumstances, the BL will affect the communicating distance, the transmission efficiency and the generation of secure key rate in varying degrees, which is helpful for future airborne experimental designs.
Reference-frame-independent quantum key distribution (RFI-QKD) has been proven to be intrinsically robust against slowly varying reference frames under realistic environment. In this paper, we propose a simplified scheme for the passive decoy-state RFI-QKD considering the afterpulsing effect. We investigated the system performance with the weak coherent source (WCS) and parametric down conversion (PDC) source, respectively. Numerical simulation results show that the simplified scheme can achieve comparable secure key rate with the active decoy-state scheme. Our scheme is advantageous to simplify the system implementation and reduces the time cost. Besides, it is proven that the afterpulsing effect plays dominant role in the system. Our work could provide a useful solution for RFI-QKD in practical situations.
The integration of quantum key distribution (QKD) devices with the existing optical fiber networks is of great significance in reducing the deployment costs and saving fiber resources. Wavelength division multiplexing (WDM) is expected to be a desirable approach to fulfill this ultimate task. In this paper, we analyze the dominant noises in WDM-based QKD system and optimize the key parameters based on a modified model with 200 GHz channel spacing. Then, an appropriate decoy-state method is adopted to estimate the system performance considering statistical fluctuations. Finally, a three-layer artificial neural network is used to train and predict the optimal mean photon numbers within different situations. Our work provides a useful method for the parameters optimization of WDM-QKD system and accelerates the practical development of QKD that coexists with the current backbone fiber infrastructure.
In order to realize classical-quantum signals simultaneous transmission sharing a same fiber in quantum communication, it is proposed that signals are transmitted over multiple separate channels. The channels are divided by different wavelengths, pseudo-codes, and spatial modes. Based on the development status of quantum communication over optical fiber, this paper expounds the demand of quantum communication for multiplexing technology, and introduces the basic principle and research status of three existing multiplexing technologies. In this paper, the key technologies involved in the multiplexing system based on few mode fiber are discussed in depth, and the problems in signal transmission are analyzed.
A multi-dimensional modulation scheme in inter-satellite quantum communication based on pulse position modulation and polarization encoding is put forward and designed. By this way we can send additional classical signal simultaneously when transmitting quantum signal. The basic principle of multi-dimensional modulation scheme is introduced. Simulation experiment is performed using Optisystem and Matlab simultaneously. The binary sequence before modulation and that after demodulation are compared and analyzed. The error bit rate, eye diagram and quality factor of classical optical date are obtained. Stabilities of classical optical signal during the transmission are tested. The results show that this multi-dimensional modulation scheme can decrease the numbers of information channels needed, increase information transmission efficiency and reduce the demands for power load systematic complexity in communication terminals. The proposed coexistence system is confirmed to be effective and applicable.
A free space quantum communication multi-dimensional modulation system based on pulse position modulation and polarization encoding is put forward and designed and is used to send quantum information and classical information simultaneously. Atmospheric channel model and coexistence system model is built. We perform a simulation experiment for transmitting signal in atmospheric, using Optisystem and Matlab together. The binary sequence before modulation and that after demodulation are compared and analyzed. The error bit rate, eye diagram and quality factor of classical optical date are obtained. Stabilities of classical optical signal during the transmission are tested. The results show that this multi-dimensional modulation system can decrease the numbers of information channels needed and reduce the demands for power load and systematic complexity in communication terminals. The proposed quantum communication system is confirmed to be effective and applicable.
In order to study the influence of atmospheric turbulence on the polarization state of the free space quantum communication, the relationship between the refractive index and altitude, the refractive index structure constant and the turbulence dimension is deduced based on two different atmospheric refractive index structural constants models. The turbulence intensity factor κ is introduced and the equation of the variation of the quantum polarization degree with turbulence intensity is established. Through the simulation of the turbulent refractive index and the performance of four different polarization states in the low altitude turbulence environment, the results show that the atmospheric turbulence in the near ground will affect the fluctuation of the degree of polarization, and the degree of polarization varies linearly with the change of turbulence intensity. In the case of polarization |H>, the range of polarization |H> varies from 0 to 0.14 with the change of turbulence intensity. The influence of atmospheric turbulence on four different polarization states is different, and the degree of |H> and |V> depolarization is greater in the daytime and back. The depolarization degree of |-> at night is greater. The relationship between the degree of polarization and the change of turbulence intensity is analyzed by mathematical modeling, which is helpful to select the reasonable experimental scheme and compensate the change of polarization state in the aviation quantum Secure communication channel.
In this paper, a new proposal was presented to probabilistically transmit an unknown quantum state from a sender to a remote receiver with the aid of a controller when the non-maximally entangled state is only available for the sender. Meanwhile, the whole successful probability and classical communication of this scheme for controlled teleportation are calculated. The concrete processes of our proposal are elaborated in detail. This scheme would be used to expand the research range of quantum controlled teleportation.
In order to improve the detection efficiency in QKD system, this paper has put forward a new quantum key distribution scheme based on the single photon frequency up-conversion detection technology and decoy-state BB84 protocol. A long wavelength pump light is adopted in single photon detector (SPD) to avoid the noise caused by spontaneous parametric down conversion (SPDC), thus an overall 28% detection efficiency is achieved which is five times of 4.5% in conventional InGaAs-based detectors. Moreover, the propagating distances has reached 90km and 150km with weak coherent pulse (WCP) and decoy weak coherent pulse (DWCP) respectively, which is 1.3 times and 1.05 times of the conventional InGaAs-based detectors of 70km and 140km.
State of polarization(SOP) of single photon is the information carrier of polarization encoding BB84 protocol. SOP of quantum must be interrupted when quantum key distribution(QKD) propagates with classical signal in the same fiber. Coexistence schemes of quantum-classical signal based on two-channel and four-channel have been built respectively on Optisystem in this paper. The influence on SOP of quantum signal in BB84 protocol from different classical optical signal channel has been analyzed and compared by Stokes vector method and Poincare sphere method. SOP of 100 photons in QKD have been simulated. The results show that more channels will increase the bit error rate of QKD, and this paper has described the SOP changes well in coexistence scheme of quantum-classical signal based on wavelength division multiplexing.
A model of coupled dynamics for supersonic airbreathing propulsion is developed to simulate the development process of the thruster’s flowfield. The influence of the inflow speed, from 1 to 4 Mach, on the laser propulsion performance is investigated numerically. The results indicate that the inflow speed has a marked effect on the propulsion efficiency. Under the same laser parameters, the larger inflow speed leads to lower thruster and impulse coupling coefficient. By analyzing the phenomena in the complex unsteady flowfield, we found that the bow shock occurring out of the surface of the thruster due to supersonic inflow is the real cause of the performance worsening and air drag reduction becomes a very important task at supersonic laser propulsion.
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