Efficient generation of polarized single or entangled photons is a crucial requirement for the implementation
of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of
emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection.
We realized highly efficient single photon sources (SPS) based on well established semiconductor technology: In
a pin structure a single electron and a single hole are funneled into a single InAs quantum dot using a submicron
AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an
all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral
filtering is demonstrated. Out-coupling efficiency and emission rate are increased by embedding the SPS into a
micro-cavity of Q = 140. The design of the micro-cavity is based on detailed modeling to optimize its performance.
The resulting resonant single-QD diode generates single polarized photons at a repetition rate of 1 GHz exhibiting
a second order correlation function of g<sup>(2)</sup>(0) = 0.
Eventually, QDs grown on (111) oriented substrate are proposed as source of entangled photon pairs. Intrinsic
symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this
substrate orientation. As a result the XX → X → 0 recombination cascade of a QD can be used for the generation
of entangled photons without further tuning of the finestructure splitting via QD size and/or shape. We present
first micro-photoluminescence studies on QDs grown on (111) GaAs, demonstrating a fine structure splitting less
than the spectral resolution of our set-up.
Efficient generation of polarized single photons or entangled photon pairs is a crucial requirement for the implementation
of quantum key distribution (QKD) systems  . In this context, self-organized semiconductor quantum dots (QDs)
  play a decisive role as they are capable of emitting only one polarized photon at a time using appropriate
electrical current injection  . Therefore, a single QD embedded in a LED can be used as a single photon source. By
tuning the electronic structure it is possible to use QD as source for entangled photons. Resonant cavity-induced
enhancement of spontaneous emission and out-coupling efficiency can improve external quantum efficiency of quantum
dot based single photon sources dramatically. In order to optimise the device geometry detailed numerical device
modelling must be performed. The modelling of the electromagnetic field were done using eigenmode-techniques. The
essential design parameters, such as cavity length, aperture diameter, position and thickness were systematically varied.
We designed and fabricated optimized resonant cavity light emitting diodes combined with a submicron oxide current
aperture, to pump individual InGaAs/GaAs QDs electrically. These devices demonstrates more than ten times increased
single photon rate in comparison to the simple LED design. Pulsed correlation measurements demonstrated true single
photon emission with g<sup>2</sup>(0) = 0 at a rate of 1 GHz.
Low transparency current density and improved temperature stability with a large characteristic temperature
T<sub>0</sub> > 650 K up to 80 °C are demonstrated for 1.3 μm MBE grown InGaAs quantum dot (QD) edge emitting
lasers. Digital modulation with an open eye pattern up to 12 Gb/s at room temperature and bit error rate below
10<sup>-12</sup> for 10 Gb/s modulation was realized for this wavelength. Semiconductor optical amplifiers based on
InGaAs QD gain media achieved a chip gain of 26 dB. A conventionally doped semiconductor DBR QD-VCSEL
containing 17 p-modulation doped QD layers demonstrated a cw output power of 1.8 mW and a
differential efficiency of 20 % at 20 °C. The maximum -3dB modulation bandwidth at 25 °C was 3 GHz. First
MOCVD-grown QD-VCSELs with selectively oxidized DBRs and 9 QD-layers were realized, emitting at 1.1
μm. A cw multimode output power of 1.5 mW, 6 mW in pulsed operation, and an cw external efficiency of 45 %
were achieved at 20 °C. The minimum threshold current of a device with 2 μm aperture was 85 μA.
We report on a miniature solid state emitter structure, which allows electrical pumping of only one single InAs quantum
dot (<i>QD</i>) grown in the Stranski-Krastanow mode. The emitter is based on a single layer of low density (~10<sup>8</sup> cm<sup>-2</sup>)<i> QDs </i>
grown by Molecular Beam Epitaxy and a submicron AlOX current aperture defined by selective oxidation of high
aluminium content AlGaAs layers. The device demonstrates strongly monochromatic polarized emission of the single
<i>QD</i> exciton at subnanoampere current pumping. No other emission is observed across a spectral range of 500 nm, proving that indeed just one single<i> QD </i>is contributing. Correlation measurements of the emitted photons show a clear