  In quantum computation, information is processed by gates that must
coherently couple separate qubits. In many systems the qubits are naturally
coupled, but such an always-on interaction limits the algorithms that may be
implemented. Coupling interactions may also be directed in devices and circuits
that are provided with additional control wiring. This can be achieved by
adjusting the gate voltage in a semiconductor device or an additional flux in a
superconducting device. Such control signals must be applied adiabatically
(limiting the speed) and the additional wiring provides pathways for noise,
which leads to decoherence. Here we demonstrate an alternative approach to
coupling by exploiting the nonlinear behaviour of a degenerately doped silicon
transistor. A single transistor can exhibit a large number of individual
resonances, which are seen as changes in the source-drain current of a
dc-biased device. These resonances may be addressed in frequency space due to
their high quality factors. Two widely separated resonances are addressed and
coupled in three-frequency spectroscopy by ensuring that the third frequency
corresponds to the difference between the two individual resonances. The
nonlinearity causes the generation of additional driving signals with
appropriate frequency and phase relationships to ensure coupling, resulting in
additional spectroscopic features that could be exploited for rapid state
manipulation and gate operations.
