  Silicon offers the enticing opportunity to integrate hybrid quantum-classical
computing systems on a single platform. For qubit control and readout,
high-frequency signals are required. Therefore, devices that can facilitate its
generation are needed. Here, we present a quantum dot-based radiofrequency
multiplier operated at cryogenic temperatures. The device is based on the
non-linear capacitance-voltage characteristics of quantum dot systems arising
from their low-dimensional density of states. We implement the multiplier in a
multi-gate silicon nanowire transistor using two complementary device
configurations: a single quantum dot coupled to a charge reservoir and a
coupled double quantum dot. We study the harmonic voltage conversion as a
function of energy detuning, multiplication factor and harmonic phase noise and
find near ideal performance up to a multiplication factor of 10. Our results
demonstrate a method for high-frequency conversion that could be readily
integrated into silicon-based quantum computing systems and be applied to other
semiconductors.
