Degenerate polarization entangled photon source based on a single Ti-diffusion lithium niobate waveguide in a polarization Sagnac interferometer

Combining a Ti-diffusion periodically poled lithium niobate(PPLN)waveguide with a Sagnac interferometer,two opposite directions type-II spontaneous parametric down conversions(SPDC)occur coherently and yield a high brightness,high stability polarization entanglement source.The source produces degenerate photon pairs at 1540.4 nm with a brightness of B=(1.36±0.03)×10^(6) pairs/(s·nm·m W).We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F=0.983±0.001.The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.

Neither Incomplete nor Non-Local, Just Entangled

Bell-state experiments with pairs of polarization-entangled photons are interpreted without any hinge on non-local mechanisms. The presented model rests on a careful analysis of published experimental findings. These foundations are implemented into a standard quantum mechanical treatment that obeys the purely local nature of each polarization preparation in the course of a measurement process. Polarization entanglement is ascribed to the generation of indiscernible photon pairs while undistorted propagation maintains this interrelation. Thus, the proposed approach assigns the essential characteristics of polarization entanglement to each constituent of an entangled pair. Accordingly, space-time separated polarization preparations lead to consistent probabilities of joint detection events. The obtained results agree with those of previous non-local models and thus reproduce the experimentally required violations of the Bell inequality. Since the presented approach lacks any non-local phenomenon, hidden variables are rendered superfluous, too.

Experimental long-distance quantum secure direct communication

Quantum secure direct communication(QSDC) is an important quantum communication branch, which realizes the secure information transmission directly without encryption and decryption processes.Recently, two table-top experiments have demonstrated the principle of QSDC. Here, we report the first long-distance QSDC experiment, including the security test, information encoding, fiber transmission and decoding. After the fiber transmission of 0.5 km, quantum state fidelities of the two polarization entangled Bell states are 91% and 88%, respectively, which are used for information coding. We theoretically analyze the performance of the QSDC system based on current optical communication technologies,showing that QSDC over fiber links of several tens kilometers could be expected. It demonstrates the potential of long-distance QSDC and supports its future applications on quantum communication networks.