Arrays of Vortex Transitional (VT) memory cells with functional density up to 1 Mbit/cm 2 have been designed, fabricated, and successfully demonstrated. This progress is due to recent advances in design optimization and in superconductor electronics fabrication achieved at MIT Lincoln Laboratory.As a starting point, we developed a demo array of VT cells for the 100-μA/μm 2 MIT LL fabrication process SFQ5ee with 8 niobium layers. The studied two-junction memory cell with a twojunction nondestructive readout occupied 168 μm 2 , resulting in an over 0.5 Mbit/cm 2 functional density. Then, we reduced the cell area down to 99 μm 2 (corresponding to over 0.9 Mbit/cm 2 functional density) by utilizing self-shunted Josephson Junctions (JJs) with critical current density, Jc of 600 µA/μm 2 and eliminating shunt resistors. The fabricated high-Jc memory cells were fully operational and possessed wide Read/Write current margins, quite close to the theoretically predicted values. We discuss approaches to further increasing the integration scale of superconductor memory and logic circuits: a) miniaturization of superconducting transformers by using soft magnetic materials; b) reduction of JJ area by using planar high-Jc junctions similar to variable thickness bridges.

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We have extended our previous study of SFQ balanced comparatom which use two overdamped Josephsoa junctions to (a) finite sampling rate and @) junctions with higher critical current density. The effective width AI, of the gray Zone of the comparators fabricated using the niobium-triiayer technology of HYPRES, Inc. &-l kA/cmz) and Stony Brook's domestic planarized process ( i p s kA/cmf) has been measured as a function of the SFQ pulse rate (from 2.5 to 55 GHz) and temperature (from 1.6 to 4.2 K), for various drivers which determine the SFQ pulse shape and external impedance. The data have been compared with available theories of Josephson junction dynamics in the presence of thermal and quantum fluctuations. We have found that Nx can be substantially reduced by using relatively broad ("soft") SFQ pulses. For high-j, comparators fed by short (-2-ps) SFQ pulses the temperature dependence is practically negligible, indicating the dominance of quantum fluctuations.