digital mixers which permit mixing of asynchronous signals may be constructed of rapid single flux quantum logic elements . the logic elements may include an rsfq non - destructive readout cell , an rsfq d flip - flop , an rsfq xor circuit , and an rsfq t flip - flop . a binary tree arrangement of t flip - flops can be used to provide in - phase and quadrature phase - divided replicas of a reference signal . the mixing elements can be either an xor circuit , a dual port ndro circuit functioning as a multiplexer or an rs type ndro functioning as an and gate . the rsfq logic elements utilize josephson junctions which operate in superconducting temperature domains .