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We present a new type of photoconductive GaAs switch, which can be integrated directly onto an already existing high-T/sub c/ circuit. Our test circuit consists of two coupled coplanar transmission lines made of gold on top of a YBa/sub 2/Cu/sub 3/O/sub x/ layer grown on a MgO substrate. When the photoconductive switch is irradiated with femtosecond laser pulses, subpicosecond (0.4 ps) electric pulses are created. These pulses are capable of propagating more that 1 mm along the transmission line. The pulses were detected with a photoconductive sampling technique, and results are presented at both room temperature and at 81 K. The photoconductive switch is useful both as an optical trigger generating pulses and in the sampling system detecting the pulses. This switch can be used to characterize high-T/sub c/ circuits e.g. SFQ logic in the subpicosecond time domain.