V. Stavarache

Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.

Tailored quantum dots for entangled photon pair creation

Entangled photon pairs are a key requirement for the implementation of quantum teleportation schemes. [1] Typically, such photon pairs are created by parametric down conversion of a strongly attenuated laser beam in a non-linear optical crystal, with limited efficiency. Recently, the decay of a biexciton complex confined in a quantum dot (QD) has been suggested as an efficient source for polarization entangled photon pairs. [2] This concept was based on the assumption of an idealistic QD structure for which the valence band ground state has pure heavy hole character with angular momentum projections Jh,z = ±3/2 along the heterostructure growth direction. When an electron-hole pair is injected, the momenta of the carriers become coupled by the exchange interaction. If the dot has perfect D2d-symmetry, angular momentum is a good quantum number: the optically active states with momenta M = ±1 are degenerate, and their decay leads to emission of � ± -circularly polarized photons. If the dot ground states are occupied by two electrons and two holes, each with opposite spin orientations, a spin singlet biexciton X2 is formed, for whose decay two channels exist, as shown in Fig. 1 (upper panel left). The first photon is emitted with either � + or � − -polarization, and then the second photon with opposite polarization, as long as no spin flip occurs after the first process. Unless a polarization measurement is performed, the two photon polarization state is therefore described by | 2i = (| +i 1 | −i 2+ | −i 1 | +i 2)/ √ 2, forming an entangled state. A key requirement is that the photons emitted at each stage of the cascade are quasi-degenerate within their homogeneous linewidth, such that they cannot be distinguished by an energy measurement. Experiments have failed up to now to demonstrate such an entanglement, as only classical correlations were observed. [3] While some of the idealizations of the original proposal are well fulfilled, for example, for strongly confined self-assembled (In,Ga)As/GaAs quantum dots (such as the long exciton spin relaxation time as com

Control of quantum dot excitons by lateral electric fields

The control of exciton wave functions in (In,Ga)As∕GaAs quantum dots through lateral electric fields has been studied by photoluminescence (PL) spectroscopy. p-i-n and n-i-n lateral gate structures were fabricated by ion implantation and subsequent thermal annealing. While single dot spectroscopy shows only small exciton energy shifts when external bias is varied, time-resolved PL reveals strong changes of the exciton lifetime and therefore of the underlying electron-hole overlap, as a consequence of significant lateral carrier displacements within the dots.