Correction of the second-order degree of coherence measurement

The measurement of the second-order degree of coherence [g（2）（t）] is one of the important methods used to study the dynamical evolution of photon-matter interaction systems. Here, we use a nitrogen-vacancy center in a diamond to compare the measurement of g（2） （t） with two methods. One is the prototype measurement process with a tunable delay. The other is a start-stop process based on the time-to-amplitude conversion （TAC） and multichannel analyzer （MCA） system, which is usually applied to achieve efficient measurements. The divergence in the measurement results is observed when the delay time is comparable with the mean interval time between two neighboring detected photons. Moreover, a correction function is presented to correct the results from the TAC-MCA system to the genuine g（2）（t）. Such a correction method will provide a way to study the dynamics in photonic systems for quantum information techniques.

Numerical investigation of unsteady cavitating turbulent flows around twisted hydrofoil from the Lagrangian viewpoint

Unsteady cavitating turbulent flow around twisted hydrofoil is simulated with Zwart cavitation model combined with the filter-based density correction model（FBDCM）.Numerical results simulated the entire process of the 3-D cavitation shedding including the re-entrant jet and side-entrant jet dynamics and were compared with the available experimental data.The distribution of finite-time Lyapunov exponent（FTLE） was used to analyze the 3-D behavior of the re-entrant jet from the Lagrangian viewpoint,which shows that it can significantly influence the particle trackers in the attached cavity.Further analysis indicates that the different flow behavior on the suction side with different attack angle can be identified with Lagrangian coherent structures（LCS）.For the area with a large attack angle,the primary shedding modifies the flow pattern on the suction side.With the decrease in attack angle,the attached cavity tends to be steady,and LCS A is close to the upper wall.A further decrease in attack angle eliminates LCS A in the boundary layer.The FTLE distribution also indicates that the decreasing attack angle induces a thinner boundary layer along the foil surface on the suction side.

A measurement-device-independent quantum key distribution protocol with a heralded single photon source

With a heralded single photon source(HSPS), a measurement-device-independent quantum key distribution(MDIQKD) protocol is proposed, combined with a three-intensity decoy-state method. HSPS has the two-mode characteristic, one mode is used as signal mode, and the other is used as heralded mode to reduce the influence of the dark count. The lower bound of the yield and the upper bound of the error rate are deduced and the performance of the MDI-QKD protocol with an HSPS is analyzed. The simulation results show that the MDI-QKD protocol with an HSPS can achieve a key generation rate and a secure transmission distance which are close to the theoretical limits of the protocol with a single photon source(SPS). Moreover, the key generation rate will improve with the raise of the senders' detection efficiency. The key generation rate of the MDI-QKD protocol with an HSPS is a little less than that of the MDI-QKD protocol with a weak coherent source(WCS) in the close range, but will exceed the latter in the far range. Furthermore, a farther transmission distance is obtained due to the two-mode characteristic of HSPS.