A circuit is provided for sampling and accurately reproducing unknown signals which could be electrical, optical, x-ray, gamma ray or particle signals with picosecond resolution. The circuit comprises a superconductive sampling gate having at least two states which are distinguishable from one another and switching circuitry to switch the state of the sampling gate. The switching circuitry includes a sampling pulse source and a bias current source which are combined with the unknown signal to change the state of the monitor gate. A step generator utilizing Josephson junction technology is connected to the source of the unknown signal and sends a signal to the source of the unknown signal in order to initiate the outputting of the unknown signal and thus the sampling. Timing circuitry, also utilizing Josephson junction technology, provides an adjustable delay between the step signal generation and the sampling pulse generation.