Integrated synapse devices for fluxon neural networks

提出了一种基于squid的神经元超导电路和两种可变突触电路。约瑟夫森电路似乎更适合神经网络的VLSI，因为它可以在极低的功耗下高速运行。采用单结SQUID和双结SQUID的组合，实现了一个具有良好输入输出隔离和陡阈值特性的神经元电路。单结SQUID的量子态代表神经元状态，双结SQUID的输出电压为s型函数，工作在并联电阻的非锁存模式下。其中一个可变突触电路通过数字方式改变其电导值。另一种可变突触电路是可变电流源，其输出电流可以数字改变。两个突触回路都由多个分流的双结squid组成。除了电路特性的数值模拟外，我们还利用Nb/AIOx/Nb Josephson结技术制作了超导神经芯片。成功地演示了各元件和3位A/D转换器的基本工作原理。该A/D转换器的模拟输入频率高达100kHz，受我们的高增益测量设备的限制。仿真表明，该网络对100MHz以上的模拟输入有响应。以如此高的速度测试网络是我们未来的挑战之一。讨论了一种基于Hebb规则的变电流源型突触学习系统。

Novel superconducting circuits for a neuron and two types of variable synapses, which are based on SQUIDs, are presented. Josephson circuits seem to be superior for VLSI of the neural networks because of the high-speed operation under very low power dissipation. A neuron circuit with good input-output isolation and steep threshold characteristics is accomplished using a combination of a single-junction SQUID coupled to a double-junction SQUID.The quantum state of the single-junction SQUID represents the neuron state, and output voltage of the double-junction SQUID,which is operated under a nonlatching mode with shunt resistors, is a sigmoid-shaped function. One of the variable synapse circuits changes its conductance value digitally. Another variable synapse circuit is a variable current source in which the output current can change digitally. Both synapse circuits consist of multiple shunted double-junction SQUIDs. Besides numerical simulations of the circuit characteristics, we have fabricated superconducting neural chips using a Nb/AIOx/Nb Josephson junction technology. The fundamental operation of each element and 3-bit A/D converter are successfully demonstrated. This A/D converter was operated with analog input frequency as high as 100kHz, limited by our high-gain measurement equipment. Simulation shows that this network responds to analog input at over 100MHz. Testing the network at such high speeds is one of our challenges in the future. A learning system based on Hebb's rule with variable-current-source type of synapse is also discussed.