1. A high gain non-linear threshold input logic circuit, comprising: a supply current source, a low reference voltage, an output branch connected in series between said supply current source and said low reference voltage, said output branch comprising an output branch resistor in series with an output Josephson junction device, an input branch connected in series between said supply current source and said low reference voltage, said input branch comprising a branch resistor, an input Josephson junction device and a sink resistor in series, an input node between said input Josephson junction device and said sink resistor, non-linear threshold input logic circuit means coupled to said input node of said input branch, said non-linear threshold input logic circuit means comprising a plurality of input gate circuits coupled to said input node, biasing means for applying a biasing current to said input node, and said non-linear threshold input logic circuit means being coupled to said input node and said biasing means for providing a non-linear input threshold characteristic.