  Velleytronics as a new electronic conception is an emerging exciting research
field with wide potential applications, which is attracting great research
interests for their extraordinary properties. The localized electronic spins by
optical generation of valley polarization with spin-like quantum numbers are
promising candidates for implementing quantum-information processing in solids.
It is expected that a single qubit preparation can be realized optically by
using combination of left- and right-circularly polarized lights. Significantly
in a series of experiments, this has already been well achieved by linearly
polarized laser representing equal weights of left- and right-circular
components resulting in formation of a valley exciton as a specific pseudo-spin
qubit with equal amplitudes for spin up and spin down. Further researches on
the control of valley pseudospin using longitudinal magnetic field and optical
Stark effect have been reported. However, a general qubit preparation has not
yet been demonstrated. Moreover as a platform for quantum information
processing, the precise readout of a qubit state is necessary, for which the
state tomography is a standard method in obtaining all information of a qubit
state density matrix.
