  High-fidelity quantum operations are a key requirement for fault-tolerant
quantum information processing. In electron spin resonance, manipulation of the
quantum spin is usually achieved with time-dependent microwave fields. In
contrast to the conventional dynamic approach, adiabatic geometric phase
operations are expected to be less sensitive to certain kinds of noise and
field inhomogeneities. Here, we investigate such phase gates applied to
electron spins both through simulations and experiments, showing that the
adiabatic geometric phase gate is indeed inherently robust against
inhomogeneity in the applied microwave field strength. While only little
advantage is offered over error-correcting composite pulses for modest
inhomogeneities <=10%, the adiabatic approach reveals its potential for
situations where field inhomogeneities are unavoidably large.
