QUROPE Quantum Information Processing and Communication in Europe

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers.

We investigate single-photon generation from individual self-assembled InGaAs quantum dots coupled to the guided optical mode of a GaAs photonic crystal waveguide. By performing confocal microscopy measurements on single dots positioned within the waveguide, we locate their positions with a precision better than 0.5  μm. Time-resolved photoluminescence and photon autocorrelation measurements are used to prove the single-photon character of the emission into the propagating waveguide mode.

We report on the optical emission properties of catalyst-free, molecular beam epitaxy grownInAs nanowires (NW) on Si (111) using photoluminescence spectroscopy. InAs NW ensembles with similar density, length, and crystal structure (wurtzite-phase with stacking faults) but substantially different NW diameter (40–135 nm) are investigated, and the role of diameter on band-edge emission elucidated.

Nonlinear Zeeman splitting of neutral excitons is observed in composition-engineered InxGa1−xAs self-assembled quantum dots, and its microscopic origin is explained. Eight-band k·p simulations, performed using realistic dot parameters extracted from cross-sectional scanning tunneling microscopy measurements, reveal that a quadratic contribution to the Zeeman energy originates from a spin-dependent mixing of heavy- and light-hole orbital states in the dot.

We demonstrate optical single electron spin-initialization, -storage and -readout in a single self-assembled InGaAs quantum dot using a spin memory device. Single electron spin relaxation is monitored over timescales exceeding ≥30μs, defined only by extrinsic experimental parameters such as the optical detection efficiency. The selective generation of a single electron in the dot is performed by resonant optical excitation and subsequent partial exciton ionization; the hole is removed from the dot whilst the electron remains stored.

We report the realization of a silicon three-dimensional photonic crystal nanocavity containing self-assembled germanium-island emitters. The three-dimensional woodpile photonic crystal was assembled layer by layer by micromanipulation using silicon plates grown by molecular beam epitaxy. An optical nanocavity was formed in the center of the photonic crystal by introducing a point defect into one of the plates.

We present investigations of the propagation length of guided surface plasmon polaritons along Au waveguides on GaAs and their coupling to near surface InGaAs self-assembled quantum dots. Our results reveal surface plasmon propagation lengths ranging from 13.4 ± 1.7 μm to 27.5 ± 1.5 μm as the width of the waveguide increases from 2-5 μm. Experiments performed on active structures containing near surface quantum dots clearly show that the propagating plasmon mode excites the dot, providing a new method to spatially image the surface plasmon mode.

We present transport measurements of cleaved edge overgrowth GaAs quantum wires. The conductance of the first mode reaches 2 e^2/h at high temperatures T > 10 K, as expected. As T is lowered, the conductance is gradually reduced to 1 e^2/h, becoming T-independent at T < 0.1 K, while the device cools far below 0.1 K. This behavior is seen in several wires, is independent of density, and not altered by moderate magnetic fields B. The conductance reduction by a factor of two suggests lifting of the electron spin degeneracy in absence of B.

We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronic experiments, and use it to investigate metallic Coulomb blockadethermometers (CBTs) with various resistances R. The high-R devices cool to slightly lower T, consistent with better isolation from the noise environment, and exhibit electron-phonon cooling ∝ T 5 and a residual heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly weaker T-dependence, deviating from the electron-phonon mechanism.