Single photon emission with high degree of circular polarization from a single quantum dot under zero magnetic field

Single photon emission with high degree of circular polarization (DCP) from a positively charged exciton (trion) state in a single quantum dot (QD) is demonstrated under zero magnetic field. Obtained high DCP as large as 0.6 is a clear manifestation of highly effective mutual polarization conversion between single photon and single electron spin. This finding also reveals that the spin relaxation is strongly suppressed during capture and thermalization processes and also in trion ground states in a QD. Moreover, exciton charging state control is shown to be possible by modifying the electron capture probability through the excitation energy tuning, which leads to an exclusive formation of the positive trion in a QD.