  A design of subkelvin single flux quantum (SFQ) circuits with reduced power
dissipation and additional cooling of shunt resistors for superconducting qubit
control circuits has been developed and characterized. We demonstrate operation
of SFQ comparators with current resolution of 40 nA at 2 GHz sampling rate. Due
to improved cooling the electron temperature in shunt resistors of a SFQ
comparator is below 50 mK when the bath temperature is about 30 mK.
