SFQ control circuits for Josephson junction qubits

Devices of three types: magnetic pulse generators, read-out circuits, and digital circuits controlling them are needed to support scalable operation of quantum logic based on the Josephson junction qubits. The most natural framework for implementation of such a classical interface to quantum circuits is provided by the RSFQ technology. In this work, we argue, however, that specific qubit requirements can not be satisfied with the conventional RSFQ approach and describe the necessary modifications. We suggest a new structure of the pulse generator based on two long underdamped Josephson junctions which is characterized by the reduced power consumption and reduced output noise. We also show how to trade off power consumption and speed of the digital control circuits by using Josephson junctions with similar values of the shunt resistors but very different critical currents. Finally, we suggest a new read-out circuit which enables dynamic compensation of the backaction down to the level approaching fundamental quantum-mechanical limit.