Space charge behavior and conduction current in polyethylene under dc stress were investigated. One of the reasons for the different breakdown property in crosslinked polyethylene (XLPE) from that in low-density polyethylene (LDPE) may be based on the existence of cross-linking by-products in XLPE. Furthermore, a thermal history in cross-linking process for XLPE may also cause of the difference. It is generally accepted that the existence of the cross-linking by-products increase the conduction current in XLPE under dc stress. It is also said that an anneal treatment in air atmosphere may affect to the electrical properties under dc stress. Therefore, we investigated the effect of the cross-linking by-products and the anneal treatment on space charge behavior and conduction current in polyethylene under dc stress. In our research, it is thought that the increasing dissipation power in XLPE is the cause of the breakdown in it under dc stress. Therefore, to calculate the dissipation power in the bulk of test sample, we measured the space charge distribution and the external circuit current simultaneously. Based on the results, we discussed the reason of the difference of the space charge properties in XLPE and LDPE focusing on the cross-linking by-products and the oxidation of the test samples. C⃝ 2016 Wiley Periodicals, Inc. Electr Eng Jpn, 198(3): 19-26, 2017; Published online in Wiley Online Library (wileyonlinelibrary.com).

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In long-distance quantum communication using quantum repeaters with quantum memories, entangled photons at telecommunication wavelength that can be coupled to the quantum memory with high efficiency are required. Typically, entangled photons are generated via spontaneous parametric down conversion (SPDC). However, the phase matching bandwidth of SPDC is more than 100 GHz which is much broader than the bandwidth of a Pr3+:Y2SiO5 quantum memory (overall bandwidth is ~10 GHz while bandwidth of each frequency channel is ~ 10 MHz) suitable for frequency-multiplexed quantum repeaters. In this study, nondegenerate SPDC (1550 nm and 995 nm) inside an optical cavity is used to obtain narrow linewidth and cluster width of SPDC to match the Pr3+:Y2SiO5 bandwidth. We also developed a cavity control mechanism that can fulfill doubly-resonant condition. The developed two-photon source can maximize the coupling efficiency with a Pr3+:Y2SiO5 by introducing wavelength conversion and is promising for quantum repeater.