  The implementation of quantum technologies in electronics leads naturally to
the concept of coherent single-electron circuits, in which a single charge is
used coherently to provide enhanced performance. In this work, we propose a
coherent single-electron device that operates as an electrically-tunable
capacitor. This system exhibits a sinusoidal dependence of the capacitance with
voltage, in which the amplitude of the capacitance changes and the voltage
period can be tuned by electric means. The device concept is based on
double-passage Landau-Zener-St\"uckelberg-Majorana interferometry of a coupled
two-level system that is further tunnel-coupled to an electron reservoir. We
test this model experimentally by performing
Landau-Zener-St\"uckelberg-Majorana interferometry in a single-electron double
quantum dot coupled to an electron reservoir and show that the voltage period
of the capacitance oscillations is directly proportional to the excitation
frequency and that the amplitude of the oscillations depends on the dynamical
parameters of the system: intrinsic relaxation and coherence time, as well as
the tunneling rate to the reservoir. Our work opens up an opportunity to use
the non-linear capacitance of double quantum dots to obtain enhanced device
functionalities.
