S. M. Ulrich

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization- entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

Post-Selected Indistinguishable Photons from the Resonance Fluorescence of a Single Quantum Dot in a Microcavity

Applying continuous-wave pure resonant s-shell optical excitation of individual quantum dots in a high-quality micropillar cavity, we demonstrate the generation of post-selected indistinguishable photons in resonance fluorescence. Close to ideal visibility contrast of 90% is verified by polarization-dependent Hong-Ou-Mandel two-photon interference measurements. Furthermore, a strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (T2/(2T1)=0.91) single photons even at 80% of the emission saturation level.

Non-resonant dot|[ndash]|cavity coupling and its potential for resonant single-quantum-dot spectroscopy

Mechanisms of distinct resonance in microcavities driven by strongly detuned single quantum dots are not well understood. Investigation of non-resonant dot–cavity coupling of individual quantum dots in micropillars now suggests a dominant role of phonon-mediated dephasing. This new perspective may have implications for single-photon sources, quantum information applications and spectroscopy.

Dephasing of triplet-sideband optical emission of a resonantly driven InAs/GaAs quantum dot inside a microcavity.

Detailed properties of resonance fluorescence from a single quantum dot in a micropillar cavity are investigated, with particular focus on emission coherence in the dependence on optical driving field power and detuning. A power-dependent series over a wide range reveals characteristic Mollow triplet spectra with large Rabi splittings of |Ω|≤15  GHz. In particular, the effect of dephasing in terms of systematic spectral broadening ∝Ω(2) of the Mollow sidebands is observed as a strong fingerprint of excitation-induced dephasing. Our results are in excellent agreement with predictions of a recently presented model on phonon-dressed quantum dot Mollow triplet emission in the cavity-QED regime.

Triggered polarization-correlated photon pairs from a single CdSe quantum dot

We report on the generation of polarization-correlated photon pairs by the radiative biexciton (XX)-exciton (X) cascade of single CdSe quantum dots (QDs). Under nonresonant optical pulsed excitation (76 MHz) at low temperature (4 K), a high collinear correlation degree of 74.5% was observed from cross-correlation measurements between single XX and X emissions, which reflects an asymmetry-induced exciton fine-structure splitting. In consideration of the excitonic radiative lifetime (250 ps) this effect allows for direct conclusions about the relaxation time (T⩾480 ps) between the corresponding sublevels. Our results also suggest that the biexciton-exciton cascade in a CdSe QD is well suited for triggered single photon and/or photon pair generation rates above 1 GHz.

Cascaded single-photon emission from the Mollow triplet sidebands of a quantum dot

Researchers demonstrate that an individual Mollow sideband channel of the resonance fluorescence from an InGaAs quantum dot can act as an efficient single-photon source. The central frequency of the bright and narrow sideband emission can be changed by laser detuning over a range spanning 15 times the emission linewidth.

Triggered indistinguishable single photons with narrow line widths from site-controlled quantum dots.

In this Letter, we present narrow line width (7 μeV), nearly background-free single-photon emission (g((2))(0) = 0.02) and highly indistinguishable photons (V = 0.73) from site-controlled In(Ga)As/GaAs quantum dots. These excellent properties have been achieved by combining overgrowth on ex situ pit-patterned substrates with vertical stacking of spectrally distinct quantum dot layers. Our study paves the way for large-scale integration of quantum dots into quantum photonic circuits as indistinguishable single-photon sources.

Influence of lateral electric fields on multiexcitonic transitions and fine structure of single quantum dots

The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As∕GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Perot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs.

Correlated photon pairs from single (In,Ga)As∕GaAs quantum dots in pillar microcavities

We demonstrate the triggered generation of photon pairs by the cascaded biexciton-exciton emission from a single (In,Ga)As∕GaAs quantum dot (QD) in a pillar microcavity. Photon cross-correlation measurements between the biexciton and exciton decay reveal highly asymmetric features under continuous wave excitation and a bunching effect under pulsed excitation due to the cascaded nature of the process. An enhancement of the QD photoluminescence (PL) intensity by a factor of 40 was found for pillar microcavities (0.6μm diameter) compared to the excitonic PL intensity in bulk semiconductors, thus reflecting the enhanced photon collection effect out of the cavity structure. The observed pillar mode structure is well understood on the basis of an extended transfer-matrix method.

Phonon-assisted incoherent excitation of a quantum dot and its emission properties

We present a detailed study of a phonon-assisted incoherent excitation mechanism of single quantum dots. A spectrally detuned continuous-wave laser couples to a quantum dot transition by mediation of acoustic phonons, whereby excitation efficiencies up to 20