We have evaluated the propagation speed of impulses used for cell selection in the impulse-driven matrix memory. The memory has a special feature of high-speed operation exceeding tens of GHz. The feature originates from a single π-junction-SQUID used as a storage cell. The single π-junction-SQUID has two stable states with clockwise or counterclockwise circulating currents under no bias currents. The two states correspond to the binary signal stored in the memory cell.
In the actual design, we use a π-junction with sufficiently large critical current coupled with a conventional 0-junction, which acts as a single π-junction with a well-defined critical current. The barrier height is controlled by an impulse propagating on a word- and a bit-line to change the state. The maximum repetition frequency of the memory is determined by the time required to transmit those word and bit lines.
We fabricated the ring oscillator which contained a transmitter of an impulse, a receiver, and a 1250-micron-long PTL coupled with 8 memory cells through the bit-write line. The π-junctions were made with a Nb/Pd89Ni11/Nb structure on an Nb-based integrated circuit prepared with the AIST QuFab HSTP2 process. By fitting the numerical results, the speed of impulses is obtained to be 124 micron/ps. From this speed, we estimate the maximum repetition frequency of the 1kb-impulse-driven memory to be 78 GHz if we can use 1-micron process.