We demonstrate the fabrication of highly efficient sources of entangled photon pairs by inserting a semiconductor
quantum dot in an optical cavity. Two electron-hole pairs trapped in a quantum dot (QD) radiatively recombine emitting
a cascade of two polarization entangled photons. To extract both photons, we use a photonic molecule consisting of two
identical micropillars, one empty, the other embedding a chosen QD. By adjusting the diameter of the pillars and their
relative distance, we ensure that both optical transitions of the QD are simultaneously resonant to cavity modes. The
emitted photon pairs are efficiently extracted thanks to Purcell effect. Doing so, we obtain the brightest sources of
entangled photon pairs to date. We further show that the implementation of Purcell effect allows increasing the fidelity
of the two photon state by reducing spin induced phase shift during the radiative cascade.
The talk summarizes results of our recent optical studies related to spin states in II-VI and III-V semiconductor quantum dot (QD) systems. First the influence of in-plane anisotropy on the QD excitonic spin states is recalled. Then various ways of circumventing, compensating, or exploiting this influence are discussed. Short lifetime of neutral excitons (governed by inter-dot tunneling) allowed us to transfer their spin polarization to another QD before its destruction by the anisotropic exchange interaction. This spin polarization, as well as single carrier spin memory effects in quantum dots are demonstrated using trion states, negligibly perturbed by the anisotropy. Modification of the anisotropy by external perturbations (electric and magnetic field) is shown. In particular, full compensation of the anisotropy by in-plane electric field is demonstrated using optical orientation of neutral excitons. Finally, the influence of the anisotropy will be exploited to achieve circular-to-linear and linear-to-circular polarization conversion in single QDs and in coupled QD pairs.
This work is devoted to correlation spectroscopy of individual II-VI CdTe/ZnTe QDs in view to determine non-resonant
excitation mechanisms and provide information on spin relaxation of QD states. Second order photon autocorrelations
and cross-correlations were measured in a Hanbury-Brown and Twiss setup for neutral and charged exciton and
biexciton transitions, excited by pulses of a frequency-doubled femtosecond Ti:Sapphire laser. Some of the
measurements were circular- or linear polarization resolved and performed in magnetic field. Besides, measurements of
photoluminescence excited by pairs of laser pulses revealed fast excitation phenomena in the range of tens of ps. The
results of measurements without polarization resolution were interpreted using a simple rate equation model and allowed
us to establish the dominant role of single carrier capture in the non-resonant excitation of the QD. Polarization-dependent
correlation measurements were used to study the magnetic field controlled transition between anisotropic QD
exciton eigenstates active in linear polarization and those active in circular polarization. The same measurements
provided information on spin relaxation of the carriers left in the dot after charged exciton recombination.