Coherent probing and manipulation of valley states in silicon double quantum dot devices with fast pulses

We report the coherent manipulation of a qubit based on two valley states of an electron confined in a silicon quantum dot. Coherent evolution between the states, which have a relatively small energy splitting of 20 $\mu$eV, is excited by a fast electrical pulse and the phase information is projected to a charge state for read-out by a nearby charge sensing channel. Coherent control was demonstrated at multiple charge configurations of the same device. The energy dispersion as a function of detuning as well as the phase coherence time of the valley qubit is obtained by varying pulse height and duration. Such coherent manipulation also provides a method of measuring valley splittings which are too small to probe with conventional methods of magneto-spectroscopy. Using these same techniques, we have produced analogous results in a different device. Coherent time domain oscillations of roughly 350 MHz, corresponding to a valley-like splitting of 1.4 $\mu$eV, are observed. Coherence times of up to 15 ns, in excess of reported values for charge qubits, have been observed in this system when a Ramsey-like pulse shape is applied.