Individual self-assembled quantum dots and quantum posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For quantum posts, switching occurs continuously from predominantly charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is nonmonotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which quantum dots nucleate. This limitation can be overcome for quantum posts where acoustically induced charge transport is highly efficient in a wide lateral matrix-quantum well.
We demonstrate for the first time that a matter physical two level system, a qubit, can be fully controlled using one ultrafast step. We show that the spin state of an optically excited electron, an exciton, confined in a quantum dot, can be rotated by any desired angle, about any desired axis, during such a step. For this we use a single, resonantly tuned, picosecond long, polarized optical pulse. The polarization of the pulse defines the rotation axis, while the pulse detuning from a nondegenerate absorption resonance, defines the magnitude of the rotation angle. We thereby achieve a high fidelity, universal gate operation, applicable to other spin systems, using only one short optical pulse. The operation duration equals the pulse temporal width, orders of magnitude shorter than the qubit evolution life and coherence times.PACS numbers: 03.67. Lx, 42.50.Dv, 78.67.Hc, 02.30.Yy Matter qubits are essential for any realization of quantum information processing. Spins of particles are promising candidates for qubits, since nuclear, atomic or electronic spins are natural, relatively protected, physical two level systems. Their spin state can be described as a coherent superposition of the two levels and thereby geometrically as a vector pointing from the centre of a unit sphere whose poles are formed by the two levels, to a point on the sphere surface (Bloch sphere). An important prerequisite for a qubit is the ability to fully control its state. A geometrical description of such a universal operation is a rotation of the qubit's state vector by any desired angle, about any desired axis [1, 2]. Naturally, a universal operation should be performed with very high fidelity and completed within a very short time. The control time should be orders of magnitude shorter than the qubit's life and decoherence times [3].If the two spin eigenstates are non-degenerate (e.g. in a magnetic field), the spin state evolves in time. This evolution is described as a precession of the state vector about an axis connecting the sphere's poles, at a frequency which equals the energy difference between the two eigenstates divided by the Planck constant.The control methods demonstrated thus far use a sequence of optical pulses, which induce fixed rotations of the qubit around axes which differ from the precession axis (Ramsey interference). A delay between the pulses allows the qubit to coherently precess between the pulses and thus a universal operation is achieved. Clearly, such a sequence of steps increases the time required to perform the operation, resulting in an operation time comparable to the precession period. Moreover, the operation fidelity equals the product of the fidelities of each step. In contrast, we demonstrate for the first time, that a single, picosecond, optical pulse can be utilized to achieve complete control of a matter qubit, composed of an optically excited electron (exciton) in a single semiconductor quantum dot [4-9]. Our demonstration is by no means unique to this technologically important system and is app...
The ectoparasitic mite Varroa destructor is a major global threat to the Western honeybee Apis mellifera. This mite was originally a parasite of A. cerana in Asia but managed to spill over into colonies of A. mellifera which had been introduced to this continent for honey production. To date, only two almost clonal types of V. destructor from Korea and Japan have been detected in A. mellifera colonies. However, since both A. mellifera and A. cerana colonies are kept in close proximity throughout Asia, not only new spill overs but also spill backs of highly virulent types may be possible, with unpredictable consequences for both honeybee species. We studied the dispersal and hybridisation potential of Varroa from sympatric colonies of the two hosts in Northern Vietnam and the Philippines using mitochondrial and microsatellite DNA markers. We found a very distinct mtDNA haplotype equally invading both A. mellifera and A. cerana in the Philippines. In contrast, we observed a complete reproductive isolation of various Vietnamese Varroa populations in A. mellifera and A. cerana colonies even if kept in the same apiaries. In light of this variance in host specificity, the adaptation of the mite to its hosts seems to have generated much more genetic diversity than previously recognised and the Varroa species complex may include substantial cryptic speciation.
A noninvasive fabrication process involving soft nanoimprint lithography is used to pattern a photonic crystal (PhC) in titania film for enhanced light extraction from a GaN light emitting diode (LED). This technique avoids damaging the LED structure by the etching process, while photoluminescence measurements show extracted modes emitted from the quantum wells which agree well with modeling. A light extraction improvement of 1.8 times is measured using this noninvasive PhC.
We present a comprehensive study of the optical transitions and selection rules of variably charged single self-assembled InAs/GaAs quantum dots. We apply high resolution polarization sensitive photoluminescence excitation spectroscopy to the same quantum dot for three different charge states: neutral and negatively or positively charged by one additional electron or hole. From the detailed analysis of the excitation spectra, a full understanding of the single-carrier energy levels and the interactions between carriers in these levels is extracted for the first time.
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