  The co-location of memory and processing is a core principle of neuromorphic
computing. A local memory device for synaptic weight storage has long been
recognized as an enabling element for large-scale, high-performance
neuromorphic hardware. In this work, we demonstrate programmable
superconducting synapses with integrated memories for use in superconducting
optoelectronic neural systems. Superconducting nanowire single-photon detectors
and Josephson junctions are combined into programmable synaptic circuits that
exhibit single-photon sensitivity, memory cells with more than 400 internal
states, leaky integration of input spike events, and 0.4 fJ programming
energies (including cooling power). These results are attractive for
implementing a variety of supervised and unsupervised learning algorithms and
lay the foundation for a new hardware platform optimized for large-scale
spiking network accelerators.
