To build large-scale superconducting quantum computers, it is crucial to develop cryogenic qubit controllers that can manipulate many qubits inside a dilution refrigerator. In this paper, we report the recent progress and challenges of qubit controllers. Specifically, we review various qubit controllers composed of cryogenic complementary metal-oxide-semiconductor (cryo-CMOS) logic and superconductor logic (such as single-flux-quantum logic and adiabatic quantum-flux-parametron logic). A detailed comparison of the qubit controllers reveals the advantages and disadvantages of each controller and indicates future challenges for both cryo-CMOS and superconductor logic-based controllers.

Superconducting Josephson devices have been developed for various electronics fields such as highly sensitive magnetometers, voltage standards, and high-speed digital signal processing. In this paper, the author describes two kinds of superconducting digital-to-analog （D/A） converters that generate quantum voltages. One is a balanced-ternary D/A converter comprising asymmetric superconducting quantum interference devices, in which the generation of either a positive or a negative zero-crossing Shapiro step is controlled by an external magnetic flux. The other is a single-flux-quantum （SFQ） D/A converter, in which SFQ pulse-frequency modulation is employed. They generate quantum voltage defined by the reference microwave frequency and the AC Josephson effect.

A single-flux-quantum circuit is a promising device for a cryogenic signal processor for a superconducting nanostrip single photon detector （SNSPD） due to its high-speed operation, low power dissipation, and low timing jitter. The cryogenic signal processor based on the SFQ circuits can sense the output signals of multi-pixel SNSPDs with low timing jitter in a low temperature stage, which provides an advanced SNSPD system such as a high-count rate system, a two-photon coincidence system, a single-photon imaging system, and so forth. We have developed and demonstrated some SNSPD systems using an SFQ signal processor in a 0.1-W Gifford-McMahon （GM） cryocooler. In this review, we introduce a cryogenic signal processor based on SFQ circuits for advanced SNSPD systems.