Subharmonic generators and error detection by using the quantum flux parametron (QFP)

Several subharmonic generators are presented, the larger of which consists of four shift registers connected serially around a loop. Low-frequency operation was demonstrated for four different devices using one, two, three, and four shift registers, respectively. The circuits presented use a standing wave for powering the QFPs, guaranteeing that all the QFPs are perfectly synchronized and that no power is consumed in the clock lines. If all the QFPs along a clock line are to be power within 20% margin, the maximum operating frequency is limited by the clock lines´ length. Concerning the high-speed operation, a frequency divide-by-two circuit was able to work at a clock frequency of 8 GHz. For checking that no error occurs during the operation of the QFPs, a four-bit shift register was used. The latter device was able to work at frequencies up to 3.1 GHz, and all of the four-bit patterns were stored during experimental times of the order of one hour without observing the occurrence of an error. Thus, about 10¹⁴ error free operations per QFP were demonstrated