Building Blocks for Nanocryotron Logic

Nanocryotrons have emerged in recent years as a candidate for superconducting electronics. Their inherent spiking behavior and robustness against magnetic fields make them suitable devices for implementing reliable low-power systems at cryogenic temperatures. Although some proof-of-concept devices based on nanocryotrons have been demonstrated, the lack of reliable standard cells that combine memory and logic functions has hindered the design of larger circuits. In this work, a logic family based on this technology is proposed. This family constitutes a complete set of cells that combine sequential and combinatorial functions. In particular, the set of gates and memory cells offers a sound basis for the design of any finite state machine. All the gates share the same structure, consisting of a superconducting loop, in which the information is stored, and a number of nanocryotrons that can modify the state of the cell. The elementary building block is a destructive readout memory cell, containing two input nanocryotrons, for writing and reading operations. The other devices derive directly from the memory: by adding an input nanocryotron, an OR gate can be made, while swapping the positions of read and write terminals produces inverting logic functions such as NOT and NOR, necessary to create a functionally complete set. The devices were designed, simulated with SPICE, and fabricated out of a single niobium nitride thin film. All the single-cell circuits were experimentally demonstrated and characterized at 4.2 K, in liquid helium. Moreover, two memory cells have been combined to make an equivalent delay flip-flop. The proper operation of this device shows the possibility of combining multiple cells to make larger systems. This work paves the way for the design of large-scale systems based on nanocryotrons. Diverse applications may benefit from the development of such systems. The possibility of coupling these devices could make them suitable for integrated control and processing of superconducting nanowire single-photon detectors. Moreover, thanks to their structural and operational robustness, standalone systems based on nanocryotrons can be envisioned as low-power digital technology in harsh environments such as deep space.