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The memory-assisted measurement device-independent quantum key distribution (MDI-QKD), which requires less stringent conditions on the memory modules than that of quantum repeaters scheme, offers a practical mid-term solution to long-distance quantum key distribution. In this paper, considering the high cost and the high multi-photon probability, respectively, of single-photon source (SPS) and weak coherent source (WCS), we present schemes on implementing the parametric down-conversion sources, including the heralded single-photon source (HSPS) and single-photon-added coherent source (SPACS), in the memory-assisted MDI-QKD. By numerical simulations, we show that HSPS and SPACS scheme have apparent superiorities both in the key generation rate and the required minimal coherence time of quantum memory compared to WCS scheme. Moreover, we find that the robustness of SPACS against intensity fluctuations is better than WCS, but still worse than HSPS.

A photonic approach to realizing anti-chromatic dispersion transmission for a frequency and bandwidth-doubling dual-chirp microwave waveform is proposed and experimentally demonstrated. The system has no requirement on polarization devices or optical filters for only the integrated dual-drive dual-parallel Mach–Zehnder modulator employed. To overcome chromatic dispersion, the carrier frequency suppression approach is proposed. The anti-chromatic dispersion process is accomplished in a central station and independent to carrier frequency, fiber length, and dispersion coefficients. An experiment is conducted to verify the analysis. Dual-chirp waveforms at 13 GHz with a bandwidth of 0.8 GHz and time duration of 1 μs are obtained. After 25 km fiber transmission, the proposed approach shows a relatively flat curve in a frequency-power diagram, while the normally carrier-suppressed double-sideband modulation method experiences a significant power fading for fiber dispersion.

In recent years, more attention has been paid to FSO(free space optical) communication system for the advantages of high security, easy installation and deployment and high transmission data compare to conventional frequency wireless communication. FSO communication has a wider prospect in application of civilian and military. However, the FSO link is easily affected by the atmospheric effects such as cloud, rain, fog and so on, which leads to the decline of the performance of the communication system. Orthogonal frequency division multiplexing (OFDM) is a kind of multicarrier transmission in which high data rate streams are split into lower rate streams and then transmitted simultaneously over several narrow-band subcarriers. OFDM subcarrier can use many different modulation modes. One of the main modulation modes is multilevel quadrature amplitude modulation (MQAM). OFDM is known for its increased robustness against frequency selective fading, narrow-band interference, and high channel efficiency and it is widely used in broadband wireless communication systems. Meanwhile, most of the current studies about free space optical communication are based on atmospheric turbulence models with lognormal, Gamma-Gamma and M distribution. There are not appropriate for aperture averaging reception condition. In 2012, Barrios R and Dios F proposed a new Exponentiated Weibull atmospheric turbulence model for the first time. Exponentiated Weibull atmospheric turbulence model is suitable for the weak to strong turbulence and the average diameter of the aperture. Therefore, this paper aimed at the combined effects of the Exponentiated Weibull atmosphere turbulence, geometric spread and pointing errors on airborne FSO communication system, the bit error rate (BER) performance of the OFDM airborne FSO communication link is investigated. The OFDM optical communication link model based on M-QAM is built, and the electrical carrier to noise ratio for OFDM optical communication link is obtained. The closed form mathematical expression for the total average BER performance is theoretically derived. The relationship between the BER performance and the transmitted optical power under different parameters such as the atmosphere turbulence, the normalized jitter standard deviation and the normalized beam-width is analyzed by simulation. The simulation results show that with transmission optical power increased, the performance of optical link which only under the influence of atmospheric turbulence is better improved than which combined effect of atmospheric turbulence and pointing error. The pointing error has a obvious deterioration in the performance of the system. The performance of the bit error rate improved by increasing the transmission optical power when not considered pointing error is 3 orders of magnitude higher than when considered pointing error. The bit error rate increases with the increase of turbulence intensity, the normalized jitter standard deviation and the normalized beam-width. The airborne system performance is similarly improved in different modulation orders by increasing the transmitted optical power. The BER performance is obviously improved by increasing the transmitted optical power when the normalized jitter standard deviation is less than 0.7. In practical application, the derived average error rate closed expression can be used to estimate the performance of the system and provide reference for the design of the airborne FSO communication system.

In consideration of the time-dependent transmittance caused by atmospheric turbulence, we combine real-time selection (RTS) method with measurement device independent quantum key distribution (MDI-QKD). The modified scheme filters out the intervals with high channel transmittance and thus in turn increases the secure key rate. The optimal threshold of post-selection is determined by an iterative algorithm in advance, which balances the decrease in the total number of signals and the increase in average transmittance. Simulation results show that our modified scheme has apparent advances in both maximum tolerant loss and secure key rate compared to the original MDI-QKD protocol. The MDI-QKD with RTS even performs better when the level of turbulence becomes larger. Furthermore, the modified scheme is more stable against the statistical fluctuation as well.

This paper discusses the performance of an improved dual-hop mixed radio frequency (RF) / free space optical (FSO) relay communication system in the present of multiple co-channel interferences (CCIs) at the relay node. Multi-user diversity(MUD) technology has been adopted to improve the RF link performance. In order to improve the performance of the FSO link, multi-aperture receiver scheme has been implemented. The multi-user RF link experiences Nakagami-m fading, while the FSO link is subjected to Exponentiated Weibull distributed atmospheric turbulence. Moreover, the CCIs at the relay node are assumed to undergo Nakagami-m fading. For this proposed system, the exact closed-form expression for the outage probability (OP) has been derived. To quickly understand the effects of different parameters on the system performance, the asymptotic expression for the case of high signal-to-noise (SNR) has been further presented. Results show that the diversity order of this proposed system is min (αβM/2,m 1 K), where M represents the number of receiver apertures; K denotes the number of users; α, β and m 1 are the channel parameters. Additionally, the effects of various parameters on system performance have been investigated. Finally, the numerical results have been validated by Monte-Carlo simulations.

Measurement device independent Quantum Key Distribution (MDI-QKD), is immune to all attacks on detection and achieve immense improvement with respect to quantum key distribution system security. However, Bell state measurement (BSM), the kernel processing in MDI-QKD, can only identify two of the four Bell states, which limits the efficiency of the protocol. In this paper, a modified MDI-QKD with hybrid qubit is proposed to provide a major step towards answering this question. The hybrid qubits, which are composed of single photon qubit qubits and coherent qubit, are sent to the quantum relay to perform parallel BSMs synchronously and bit flip can be easily operated to complete the whole key distribution process. The secure key rate can be improved with our modified protocol owing to the higher success probability of BSM, which is increased by adding the parity check of coherent qubit. Furthermore, though our protocol requires photon number resolving detectors, the BSM of coherent state could be in...

In this paper, by combining measurement-device-independent quantum key distribution (MDI-QKD) scheme with entangled photon sources, we present a modified MDI-QKD scheme with pairs of vector vortex(VV) beams, which shows a structure of hybrid entangled entanglement corresponding to intrasystem entanglement and intersystem entanglement. The former entanglement, which is entangled between polarization and orbit angular momentum within each VV beam, is adopted to overcome the polarization misalignment associated with random rotations in quantum key distribution. The latter entanglement, which is entangled between the two VV beams, is used to perform entangled-based MDI-QKD protocol with pair of VV beams to inherit the merit of long distance. The numerical simulations show that our modified scheme can tolerate 97dB with practical detectors. Furthermore, our modified protocol only needs to insert q-plates in practical experiment.

An effective approach to design miniaturized-element frequency selective surface (FSS) is presented. The FSS is based on an original Jerusalem cross-shaped slot one, whereas there is a metallic patch array with the same period laid over it. An equivalent circuit model analysis is presented to provide an insightful design procedure for the miniaturization. Numerical simulations indicate that one prototype of the designed FSS has been much compacter in size, the unit size is miniaturized to only 0.0371λ × 0.0371λ, where λ represents the resonant wavelength in free space; moreover, the miniaturization performance is stable with respect to different polarizations and incident angles. In addition, after the amelioration of the unit cell structure, the unit size is consumedly miniaturized to 0.0094λ × 0.0094λ. Finally, the prototype FSS is fabricated and measured, a good agreement between the measured and simulated results is observed. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2572–2576, 2015

The vector vortex (VV) beam, originally introduced to exhibit a form of single-particle quantum entanglement between different degrees of freedom, has specific applications for quantum-information protocols. In this paper, by combining measurement-device-independent quantum key distribution (MDIQKD) with a spontaneous parametric-downconversion source (SPDCS), we present a modified MDIQKD scheme with pairs of VV beams, which shows a structure of hybrid entangled entanglement corresponding to intrasystem entanglement and intersystem entanglement. The former entanglement, which is entangled between polarization and orbit angular momentum within each VV beam, is adopted to overcome the polarization misalignment associated with random rotations in quantum key distribution. The latter entanglement, which is entangled between the two VV beams, is used to perform the MDIQKD protocol with SPDCS to inherit the merit of the heralded process. The numerical simulations show that our modified scheme has apparent advances both in transmission distance and key-generation rate compared to the original MDIQKD. Furthermore, our modified protocol only needs to insert $q$ plates in a practical experiment.

In order to study the effect of nonlinear phase noise on the performance of DQPSK modulation system, based on the analysis of mathematical model of nonlinear phase noise, the property of bit-error rate(BER) varying with optical signal-to-noise ratio(OSNR) at transmitter is simulated in 40Gb/s DPSK and DQPSK system. The result shows: compared with DPSK system, DQPSK system is more sensitive to the effect of nonlinear phase noise, and the BER will obviously increase when the nonlinearity is big enough, by which communication quality of the system will be surely seriously influenced.

A triple-band low frequency ultra-compact metamaterial absorber (MA) is proposed, which is composed of a periodic array of 3D structures constructed in lossy FR4-epoxy dielectric slab, and another ultra-thin grounded dielectric substrate is attached to its bottom. The unit cell can be counted as a combiner of three 3D magnetic resonators, which have similar resonant mechanisms and can all be ultra-compact in size. By adjusting structural parameters of the unit cell, the proposed MA can perform well in absorptions at each low resonant frequency. For example, one MA with unit cell width 10.92 mm exhibits three absorption peaks at 2.151 GHz, 2.283 GHz, and 2.378 GHz with the corresponding absorptivities of 96.5%, 96.8%, and 99.6%, respectively; for the three low frequencies, the width of the unit cell is only 7.8%, 8.3%, or 8.6% of the free space wavelength, respectively. Furthermore, the three absorption peaks can be lowered to 1.220 GHz, 1.281 GHz, and 1.343 GHz when the relative permittivity of the ultra-thin grounded substrate is increased to 9.8. Finally, one waveguide experiment was conducted to verify the proposed MA effectively.

We present a design of a low frequency ultra-thin compact and polarization-insensitive metamaterial absorber (MA). The designed MA is a two-layer structure, a periodic array of novel split-ring resonators (SRRs), which are constructed in an FR4 dielectric layer, and another ultra-thin grounded sheet is attached to the bottom. Numerical simulated results show that the proposed MA can realize effective absorption at the frequency 281.9 MHz, and its overall thickness is just only 0.29% of the resonant wavelength, the unit space is only 2.57%, and the absorbance is kept well for a wide range of incident angles for different polarizations. In addition, the proposed MA is changed into a more compact one when the inter-digital structures are introduced in the SRRs. One convenient experiment is carried out in a rectangular waveguide simulator.

In this paper, we design a varactor-tunable metamaterial absorber (MA). The tunable MA is based on a mushroom-type high impedance surface (HIS), in which varactors are loaded between adjacent metal patches to adjust the capacitance and tune the resonance frequency, the primary ground plane is etched as the bias network to bias all of the varactors in parallel, and another ultra-thin grounded film is attached to the bottom. Its absorption characteristics are realized for electrically dielectric loss. The simulated values of a sample indicate that a tunable frequency range from 2.85 GHz to 2.22 GHz is achieved by adjusting the varactor capacitance from 0.1 pF to 2.0 pF, and better than 0.97 absorbance is realized; in addition, the tunable frequency range is expanded from 4.12 GHz to 1.70 GHz after optimization.

An effective approach to designing a tunable electromagnetic absorber is presented and experimentally verified; it is based on an idea that an existing frequency selective surface (FSS) absorber is regarded as a loaded receiving antenna array. The existing absorber is effectively simplified by withdrawing half of the loaded resistors; a more compact one is obtained when lumped capacitors are introduced. Building on this, a varactor-tunable absorber with a proper bias network is proposed. Numerical simulations of one tunable absorber with 1.6 mm in thickness show that a wide tuning range from 3.05 GHz to 1.96 GHz is achieved by changing the capacitance of the loaded varactor from 0.5 pF to 5.0 pF. An experiment is carried out using a rectangular waveguide measurement setup and excellent agreement between the simulated and measured results is demonstrated.