  Extremely long coherence times, excellent single-qubit gate fidelities and
two-qubit logic have been demonstrated with silicon metal-oxide-semiconductor
spin qubits, making it one of the leading platforms for quantum information
processing. Despite this, a long-standing challenge in this system has been the
demonstration of tunable tunnel coupling between single electrons. Here we
overcome this hurdle with gate-defined quantum dots and show couplings that can
be tuned on and off for quantum operations. We use charge sensing to
discriminate between the (2,0) and (1,1) charge states of a double quantum dot
and show excellent charge sensitivity. We demonstrate tunable coupling up to 13
GHz, obtained by fitting charge polarization lines, and tunable tunnel rates
down to below 1 Hz, deduced from the random telegraph signal. The demonstration
of tunable coupling between single electrons in a silicon
metal-oxide-semiconductor device provides significant scope for high-fidelity
two-qubit logic toward quantum information processing with standard
manufacturing.
