Tutorial: High-speed low-power neuromorphic systems based on magnetic Josephson junctions

Abstract

Josephson junctions and single flux quantum (SFQ) circuits form a natural neuromorphic technology with SFQ pulses and superconducting transmission lines simulating action potentials and axons. Josephson junctions consist of superconducting electrodes with nanoscale barriers that modulate the coupling of the complex superconducting order parameter across the junction. When the order parameter undergoes a 2π phase jump, the junction emits a voltage pulse with an integrated amplitude of a flux quantum ϕ0 = h/(2e) = 2.068 × 10−15 V s. The coupling across a junction can be controlled and modulated by incorporating the nanoscale magnetic structure in the barrier. The magnetic state of embedded nanoclusters can be changed by applying small current or field pulses, enabling both unsupervised and supervised learning. The advantage of this magnetic/superconducting technology is that it combines natural spiking behavior and plasticity in a single nanoscale device and is orders of magnitude faster and lower energy than other technologies. Maximum operating frequencies are above 100 GHz, while spiking and training energies are ∼10−20 J and 10−18 J, respectively. This technology can operate close to the thermal limit, which at 4 K is considerably lower energy than in a human brain. The transition from deterministic to stochastic behavior can be studied with small temperature modifications. Here, we present a tutorial on the spiking behavior of Josephson junctions; the use of the nanoscale magnetic structure to modulate the coupling across the junction; the design and operation of magnetic Josephson junctions, device models, and simulation of magnetic Josephson junction neuromorphic circuits; and potential neuromorphic architectures based on hybrid superconducting/magnetic technology.