Cryogenic Digital Readout of Superconducting Qubits*

We present schemes for interfacing the result of superconducting qubit measurements with Single-Flux-Quantum (SFQ) logic, a superconducting cryogenic digital logic family. Using a Josephson Photomultiplier (JPM), we are able to read out a superconducting qubit coupled to a cavity by mapping qubit states onto cavity photon occupation, then mapping the cavity photon occupation onto different classical circulating current states of the JPM. This scheme provides fast, high fidelity readout at the same temperature stage as the qubit device without the need for cryogenic isolators or amplifiers. The results of this readout can then be encoded into SFQ logic by coupling the JPM to an underdamped Josephson Transmission Line (JTL) circuit. Simulations of this protocol, as well as recent JPM measurement results are discussed.