  A device called qubit energy tuner (QET) based on single flux quantum (SFQ)
circuits is proposed for Z control of superconducting qubits. Created from the
improvement of flux digital-to-analog converters (flux DACs), a QET is able to
set the energy levels or the frequencies of qubits, especially flux-tunable
transmons, and perform gate operations requiring Z control. The circuit
structure of QET is elucidated, which consists of an inductor loop and flux
bias units for coarse tuning or fine tuning. The key feature of a QET is
analyzed to understand how SFQ pulses change the inductor loop current, which
provides external flux for qubits. To verify the functionality of the QET,
three simulations are carried out. The first one verifies the responses of the
inductor loop current to SFQ pulses. The results show that there is about 4.2%
relative deviation between analytical solutions of the inductor loop current
and the solutions from WRSpice time-domain simulation. The second and the third
simulations with QuTip show how a Z gate and an iSWAP gate can be performed by
this QET, respectively, with corresponding fidelities 99.99884% and 99.93906%
for only once gate operation to specific initial states. These simulations
indicate that the SFQ-based QET could act as an efficient component of
SFQ-based quantum-classical interfaces for digital Z control of large-scale
superconducting quantum computers.
