  Due to its significant application in reducing algorithm depth, fSim gates
have attracted a lot of attention. However, during the implementation of
quantum gates, fluctuations in control parameters and decoherence caused by the
environment may lead to a decrease in the fidelity of the gate. Implementing
the fSim gate that is robust to these factors in one step remains an unresolved
issue. In this manuscript, we propose a one-step implementation of the
nonadiabatic geometric fSim gate composed of a nonadiabatic holonomic
controlled phase (CP) gate and a nonadiabatic noncyclic geometric iSWAP gate
with parallel paths in a tunable superconducting circuit. Compared to the
composite nonadiabatic geometric fSim gate composed of a nonadiabatic holonomic
CP gate and a nonadiabatic geometric iSWAP gate, our scheme only takes half the
time and demonstrates robustness to parameter fluctuations, as well as to
environmental impacts. Moreover, the scheme does not require complex controls,
making it very easy to implement in experiments, and can be achieved in various
circuit structures. Our scheme may provide a promising path toward quantum
computation and simulation.
