Quasi-lumped Element Bandpass Filter for a Low-Temperature Superconducting Qubit Control-Pulse Generator

This paper discusses a class of filters utilized in a low-power superconductor rapid single-flux-quantum (rapid SFQ, RSFQ) microwave pulse generator for controlling quantum bits (qubits). Here, the filter plays an important role to convert SFQ pulse trains (generated by RSFQ digital gates) and output a microwave control pulse. Characteristic of the output pulse can be adjusted by the in-band and roll-off performances of the filter, which is analyzed through numerical examples. Moreover, a novel in-line topology lumped network is proposed for the filter implementation, which can satisfy complex terminal cases at source (connecting an RSFQ digital block) and load (connecting a qubit). For the validation, we carry out a fourth-order bandpass filter using multilayer niobium (Nb) then-film technology, then incorporate it into an RSFQ control-pulse generator, and finally confirm correct functionality by both simulation and measurement.