Coherent control of a semiconductor quantum dot qubit, encoded by valley-states in Si

Traditionally, spin and charge of individual electrons are used to encode a qubit in gated-defined semiconductor quantum dots. Valley states of electrons in silicon represents another degree of freedom in addition to spin and charge degree of freedoms. In this talk, we demonstrate the coherent control of a semiconductor quantum dot qubit, encoded by valley-states in Si. A double quantum dot device, fabricated on a SiGe hetero-structure, is used for the experiment. We found that either one of the quantum dots can be used to encode a qubit. The left dot has a valley splitting of 5.3 GHz (or 22 ueV), while the right dot has a different splitting of 8.2 GHz (or 34 ueV). The x-axis control of the qubit is done by either timed x-rotation near the anti-crossing point of the two valley eigenstates, or by the Landau-Zener effect using a slowly rising/falling pulse. The z-axis control is done at the region when the energy separation of the two valley levels is nearly detuning independent, which offers a protection against charge noise. Fast qubit operations, in the range of a few GHz, for both x and z rotations, are realized.