  Universal quantum computation requires high fidelity single qubit rotations
and controlled two qubit gates. Along with high fidelity single qubit gates,
strong efforts have been made in developing robust two qubit logic gates in
electrically-gated quantum dot systems to realize a compact and
nano-fabrication-compatible architecture. Here, we perform measurements of
state-conditional coherent oscillations of a charge qubit. Using a quadruple
quantum dot formed in a Si/SiGe heterostructure, we show the first
demonstration of coherent two-axis control of a double quantum dot charge qubit
in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We
extract the strength of the capacitive coupling between double quantum dots by
measuring the detuning energy shift ($\approx$ 75 $\mu$eV) of one double dot
depending on the excess charge configuration of the other double dot. We
further demonstrate that the strong capacitive coupling allows fast conditional
Landau-Zener-Stuckelberg interferences with conditonal $\pi$ phase flip time
about 80 ps, showing promising pathways toward multi-qubit entanglement control
in semiconductor quantum dots.
