Highly stable power control for chip-based continuous-variable quantum key distribution system.

Quantum key distribution allows secret key generation with information theoretical security. It can be realized with photonic integrated circuits to benefit the tiny footprints and the large-scale manufacturing capacity. Continuous-variable quantum key distribution is suitable for chip-based integration due to its compatibility with mature optical communication devices. However, the quantum signal power control compatible with the mature photonic integration process faces difficulties on stability, which limits the system performance and causes the overestimation of a secret key rate that opens practical security loopholes. Here, a highly stable chip-based quantum signal power control scheme based on a biased Mach-Zehnder interferometer structure is proposed, theoretically analyzed, and experimentally implemented with standard silicon photonic techniques. Simulations and experimental results show that the proposed scheme significantly improves the system stability, where the standard deviation of the secret key rate is suppressed by an order of magnitude compared with the system using traditional designs, showing a promising and practicable way to realize a highly stable continuous-variable quantum key distribution system on chip.

Error rate analysis of few-mode fiber based free-space optical communication.

Few-mode fiber (FMF) based receiver emerges as a promising solution for free-space optical (FSO) communication due to its excellent performance in the presence of turbulence. We propose a theoretical model that uses the coupling efficiency of FMF to evaluate the performance of FMF-based FSO system in the presence of turbulence. The series solutions and asymptotic solutions to bit-error rate (BER) of such system are derived for maximal-ratio combining (MRC) scheme and equal gain combining (EGC) scheme over the Gamma-Gamma turbulence channels. Simulation results show that for the FMF-based FSO system, the asymptotic BER of MRC and EGC are highly accurate in the large transmitted optical power regimes.

Unidimensional Two-Way Continuous-Variable Quantum Key Distribution Using Coherent States.

We propose a unidimensional two-way continuous-variable quantum key distribution protocol with coherent states, where the sender modulates a single quadrature of the coherent states rather than both quadratures to simplify the structure of a two-way system. Security analysis is performed with a general attack strategy, known as two-mode attack, which helps to reduce limitations in the analysis. The performance of the protocol under all accessible two-mode attacks at fixed distance is illustrated. Further, two typical two-mode attack strategies are obtained from it, which are one-mode attack strategy and optimal two-mode attack strategy. Between them, the one-mode attack is the simplest form of the two-mode attack, while the optimal two-mode attack is the most complicated one. Simulations show that though the system is simplified, the performance of the two-way protocol with unidimensional modulation is still comparable to that of the counterpart with Gaussian modulation even against the optimal two-mode attack when Eve's ability is maximized. Thus, the proposed protocol simplifies the two-way system while guaranteeing its performance to a certain extent. Especially in a practical system with short transmission distance and high excess noise, the protocol has a good application prospect.

Performance enhancement of free-space optical communications under atmospheric turbulence using modes diversity coherent receipt.

This paper numerically and experimentally investigates the performance of free-space optical receiver using modes diversity coherent receipt under moderate-to-strong turbulence. By utilizing a three-mode photonic lantern with digital maximum ratio combining, a 40 Gbps QPSK optical signal is received. The turbulence strength is measured by the ratio of beam diameter to atmospheric coherence length, D/r0. The larger the D/r0, the stronger the turbulence is, and vice versa. Compared with conventional single mode fiber based receipt, the required transmitted power can reduce by 4.6 dB and 4 dB at 10% interruption probability under turbulence strength D/r0 = 8 and 16. The required transmitted power at bit error ratio of 2.2 × 10-2 can relax by 4.2 dB and 5 dB under turbulence strength D/r0 = 8 and 16.

[The effect of different dispersive methods on flexural strength nano-ZrO(2) reinforced denture polymethyl methacrylate].

PURPOSE: To study the effect of different dispersive methods on flexural strength of nano-ZrO(2)/polymethl methacrylate(PMMA) composite samples. METHODS: The nanometer ZrO2 particles which were modified by coupling agent Z-6030 were mixed to PMMA as the filler materials (3%) by means of handle disperse, ball milling disperse and ultrasonic wet disperse, respectively. Pure PMMA was used as blank control .With reference to the standard of ISO-1567:1999, standard samples were made and tested by three-point bending test. Then the data of flexural strength of the four groups were analyzed statistically with SAS 6.12 software package for one-way ANOVA(Newman-Keuls test). And the optimized synthesis way of the PMMA composite materials was selected. RESULTS: The flexural strength of the ball milling group and the ultrasonic group were significantly higher than the handle group and blank group(P<0.05), with an increase of 17%,10% compared with the blank group. There was no significant difference between the ball milling group and the ultrasonic group (P>0.05). CONCLUSION: Dispersal ways have effect on the flexural strength of PMMA composite materials which are filled with ZrO(2) particles.

[Influence of different amount of silane coupling agent on the flexural strength of PMMA/nanometer ZrO2 composites].

PURPOSE: To study the influence of different amount of silane coupling agent Z-6030 on the flexural strength of PMMA/nanometer ZrO(2) composites. METHODS: Nanometer ZrO(2) particles were modified by silane coupling agent Z-6030 with different percentages.In situ polymerization method was used to combine PMMA/ nanometer ZrO(2) composites of denture base plate. With reference to the standard of ISO1567:1999, standard samples of the experimental group were made. PMMA II type powder and PMMA II type liquor were used,which was not added nanometer ZrO(2) and silane coupling agent to make the standard samples of common denture base plate. PMMA II type powder and PMMA II type liquor were used,which was added nanometer ZrO(2) unmodified by silane coupling agent to make the standard samples of the control group. The data of flexural strength of the base plate materials were tested with three-point bending test. One-way ANOVA was adopted to analyze the data using SAS 6.12 software package. RESULTS: When the percentage of silane coupling agent Z-6030 was 3.5%,the flexural strength of PMMA/nanometer ZrO(2) composite resin material of denture base plate was strongest, which was significantly higher than that of the common denture base plate group,the control group,2.0% group,4.0% group,4.5% group and 5.0% group(P<0.05). 0.5% group,1.0% group,1.5% group,2.5% group,3.0% group and 3.5% group had no significant difference with each other(P>0.05). 0.5% group,1.0% group,1.5% group,2.0% group,2.5% group,3.0% group,4.0% group,4.5% group,5.0% group,the common denture base plate group and the control group had no significant difference with each other(P>0.05). CONCLUSION: Using proper percentage of silane coupling agent Z-6030 can improve the flexural strength of PMMA/nanometer ZrO(2) composite resin material of denture base plate. This study showed that the best range of the percentage of silane coupling agent Z-6030 was 3.5%.

[Clinical analysis of colour distribution in tetracycline teeth].

PURPOSE: To analyze the colour range and distribution of tetracycline teeth, and the difference from normal teeth. METHODS: 142 cases with 468 anterior tetracycline teeth and 100 cases with 200 normal anterior teeth were collected. The color was measured by a computer-aided Shade-Eye NCC colorimeter, and expressed in terms of 3 coordinate values (L,a,b) of the CIE-1976-Lab color system. Chroma (Cab)and hue (h(ab) degrees)were calculated according to the value of a and b. The data were analyzed with SAS6.12 software package for description, Student's t test and Duncan test. RESULTS: The range of L, a, b Cab, h(ab) degrees of tetracycline teeth was 42.33-77.00, -0.6-9.6, 2.67-31.5, 5.24-31.89, 38.62 degrees-95.47 degrees, respectively. Significant difference of L value and hue (h(ab) degrees) was found between tetracycline teeth and normal teeth (P < 0.01), which indicated that tetracycline teeth were darker and redder than normal teeth. There was no difference of chroma (Cab) between tetracycline teeth and normal teeth (P > 0.05). Moreover there was significant difference of L value between canine and central incisor, and of H value (h(ab) degrees) between canine and lateral incisor in tetracycline teeth, which indicated that canine was darker and redder than incisor, and redder than lateral incisor, but there was no significant difference in chroma (Cab) among all anterior teeth and no difference in L, a, b between central incisor and lateral incisor. CONCLUSIONS: The study showed the color distribution of tetracycline teeth was wider and more dispersive than normal teeth, and much darker and redder than normal teeth. Supported by Shanghai Leading Academic Discipline