Although the schematic above could seem rather complicated, the device is actually very simple. Its purpose is to monitor the internal state of the  T flip-flop  build around junctions J0 … J6 as a DC voltage across the junction J8 (see waveforms below, where /M0/ABS is the average value of the U(/M0/J8) voltage).  A straightforward approach to the problem of  monitoring the inner state of an RSFQ gate would be to inductively couple it with a DC SQUID. The main idea of such an approach is that when there is no flux inside the gate, the SQUID is in a superconducting state and produces 0 voltage drop.  When there is a flux quantum inside the RSFQ gate, the resulting current inductively biases the SQUID into the normal state and we can observe a finite voltage (of the order of Ic*Rn) across it. The SFQ/DC  converter above represents a further  development of this approach. The J7-J8 pair behaves exactly as an output SQUID. But instead of inductive coupling,  it is directly connected to the quantizing inductance of the T flip-flop.

The device was simulated and optimized with a phase source source and a matching JTL at the input and a resistive load at the output:

References

The  device is very similar to the one described in:

V. K. Kaplunenko, V. P. Koshelets, K. K. Likharev, V. V. Migulin, O. A. Mukhanov, G. A. Ovsyannikov, V. K. Semenov, I. L. Serpuchenko, and A. N. Vystavkin, “Experimental Study of the RSFQ Logic Circuits,” in: Extended Abstracts of ISEC’87, Tokyo, pp. 127-130, Aug. 1987. K. Likharev and V. Semenov,  “RSFQ logic/memory family: A new Josephson-junction technology for sub-terahertz clock-frequency digital systems”,  IEEE Trans. Appl. Supercond., vol. 1, pp. 3-28, March 1991.