Three-Dimensional Electrical Control of the Excitonic Fine Structure for a Quantum Dot in a Cavity.

The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose and implement a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

Bright Polarized Single-Photon Source Based on a Linear Dipole.

Semiconductor quantum dots in cavities are promising single-photon sources. Here, we present a path to deterministic operation, by harnessing the intrinsic linear dipole in a neutral quantum dot via phonon-assisted excitation. This enables emission of fully polarized single photons, with a measured degree of linear polarization up to 0.994±0.007, and high population inversion-85% as high as resonant excitation. We demonstrate a single-photon source with a polarized first lens brightness of 0.50±0.01, a single-photon purity of 0.954±0.001, and single-photon indistinguishability of 0.909±0.004.

Hong-Ou-Mandel Interference with Imperfect Single Photon Sources.

Hong-Ou-Mandel interference is a cornerstone of optical quantum technologies. We explore both theoretically and experimentally how unwanted multiphoton components of single-photon sources affect the interference visibility, and find that the overlap between the single photons and the noise photons significantly impacts the interference. We apply our approach to quantum dot single-photon sources to access the mean wave packet overlap of the single-photon component. This study provides a consistent platform with which to diagnose the limitations of current single-photon sources on the route towards the ideal device.

The effect of bleaching agents on the DNA analysis of bloodstains on different floor coverings.

Blood at crime scenes is one of the most significant traces of evidence in investigation proceedings. Cleaning up these traces with household cleaning products, often containing bleaching agents, inhibits or complicates the detection of DNA. In this study, human blood was applied onto different floor coverings (carpet, laminate, parquet, PVC, tile) and subsequently cleaned with water and bleaching agents (hydrogen peroxide, sodium hypochlorite, DanKlorix®, Vanish Oxi Action®) at different times. Samples have been collected afterwards from the floors. The samples underwent a quantitative and qualitative DNA analysis. Cleaning smooth surfaces with water is usually sufficed to prohibit retrieving a DNA profile in most of the cases. Cleaning carpets was more difficult due to their absorbent surface whereas the use of bleaching agents caused an additional reduction of verifiable DNA concentrations. Retrieving partial or complete profiles after the use of bleaching agents was only possible when cleaning with low concentrations of 3% hydrogen peroxide.

Detection of blood and DNA traces after thermal exposure.

The analysis of blood traces is often of significant reconstructive and evidence-gathering importance. Perpetrators deliberately set fires to destroy evidence. There is little literature regarding the effect of fire and extreme heat on blood and the detection of blood. Blood and DNA are believed to be no longer traceable after exposure to a temperature of 1000 °C. This study exposed different objects of a standardized procedure to temperatures of 300, 700, and 1000 °C. It documented the influence of heat on blood traces through the use of luminol. DNA analysis confirmed that fewer DNA profiles can be created with increasing temperature. However, even after exposure up to a max. of 1000 °C, it was still possible to produce a complete DNA pattern from approx. 60% of the samples. Consequently, crime scenes that have been destroyed by fire should be evaluated with the same attention to detail as the unburned areas.

Blood Trace Evidence on Washed Textiles - a systematic approach.

Luminol has been used for a long time for detecting latent blood traces during police investigations because it is easy to use and does not pose any health risks, while providing trace evidence for DNA analysis. It is often the method of choice for examining clothing. Clothes worn during the offense are often destroyed or washed afterwards by the offenders. The purpose of this study is to show the possibilities of blood and DNA detection on washed clothes by documenting the macroscopic results and their chemiluminescence after washing. The tests comprised different fabrics and laundry detergents including different washing and drying methods. Chemiluminescence was detected on almost all blood-marked samples (95.9%), even after all traces visible to the naked eye have been removed by washing. Evidence of a complete DNA profile or individual alleles could be confirmed in almost all of the test cases (93.3%).

Giant optical polarisation rotations induced by a single quantum dot spin.

In the framework of optical quantum computing and communications, a major objective consists in building receiving nodes implementing conditional operations on incoming photons, using a single stationary qubit. In particular, the quest for scalable nodes motivated the development of cavity-enhanced spin-photon interfaces with solid-state emitters. An important challenge remains, however, to produce a stable, controllable, spin-dependent photon state, in a deterministic way. Here we use an electrically-contacted pillar-based cavity, embedding a single InGaAs quantum dot, to demonstrate giant polarisation rotations induced on reflected photons by a single electron spin. A complete tomography approach is introduced to extrapolate the output polarisation Stokes vector, conditioned by a specific spin state, in presence of spin and charge fluctuations. We experimentally approach polarisation states conditionally rotated by [Formula: see text], π, and [Formula: see text] in the Poincaré sphere with extrapolated fidelities of (97 ± 1) %, (84 ± 7) %, and (90 ± 8) %, respectively. We find that an enhanced light-matter coupling, together with limited cavity birefringence and reduced spectral fluctuations, allow targeting most conditional rotations in the Poincaré sphere, with a control both in longitude and latitude. Such polarisation control may prove crucial to adapt spin-photon interfaces to various configurations and protocols for quantum information.

Optically induced coupling of two magnetic dopant spins by a photoexcited hole in a Mn-doped InAs/GaAs quantum dot.

We report evidence of a photoinduced coupling between two spins provided by Mn dopants in their neutral acceptor state A(0) in a single InAs/GaAs quantum dot. The coupling occurs due to simultaneous exchange interactions between each of the two dopant spins and a photocreated hole. Microphotoluminescence spectroscopy achieved both in longitudinal and perpendicular magnetic fields reveals the splitting of the four spin configurations |J(1) = ± 1,J(2) = ± 1} due to the 2A(0)-hole exchange interaction. We obtain a comprehensive interpretation of the experimental data with a simplified spin Hamiltonian model, which more specifically shows that the hole-mediated coupling is similar to a ε(12)-70 µeV exchange interaction between both A(0) spins.

Optical nonlinearity for few-photon pulses on a quantum dot-pillar cavity device.

Giant optical nonlinearity is observed under both continuous wave and pulsed excitation in a deterministically coupled quantum dot-micropillar system, in a pronounced strong-coupling regime. Using absolute reflectivity measurements we determine the critical intracavity photon number as well as the input and output coupling efficiencies of the device. Thanks to a near-unity input-coupling efficiency, we demonstrate a record nonlinearity threshold of only 8 incident photons per pulse. The output-coupling efficiency is found to strongly influence this nonlinearity threshold. We show how the fundamental limit of single-photon nonlinearity can be attained in realistic devices, which would provide an effective interaction between two coincident single-photons.

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot.

We report on the resonant optical pumping of the | ± 1⟩ spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable device. The experiment relies on a W scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1⟩ or |-1⟩ state characterized by a polarization near or above 50% is evidenced.

Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses.

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.

Anomalous Hanle effect due to optically created transverse overhauser field in single InAs/GaAs quantum dots.

We report on experimental observations of an anomalous Hanle effect in individual self-assembled InAs/GaAs quantum dots. A sizable electron spin polarization photocreated under constant illumination is maintained in transverse magnetic fields as high as approximately 1 T, up to a critical field where it abruptly collapses. These striking anomalies of the Hanle curve point to a novel mechanism of dynamic nuclear spin polarization giving rise to an effective magnetic field generated perpendicular to the optically injected electron spin polarization. This transverse Overhauser field, confirmed by the cancellation of electron Zeeman splitting below the critical field, is likely to be a consequence of the strong inhomogeneous quadrupolar interactions typical for strained quantum dots.

Coherent manipulation of a solid-state artificial atom with few photons.

In a quantum network based on atoms and photons, a single atom should control the photon state and, reciprocally, a single photon should allow the coherent manipulation of the atom. Both operations require controlling the atom environment and developing efficient atom-photon interfaces, for instance by coupling the natural or artificial atom to cavities. So far, much attention has been drown on manipulating the light field with atomic transitions, recently at the few-photon limit. Here we report on the reciprocal operation and demonstrate the coherent manipulation of an artificial atom by few photons. We study a quantum dot-cavity system with a record cooperativity of 13. Incident photons interact with the atom with probability 0.95, which radiates back in the cavity mode with probability 0.96. Inversion of the atomic transition is achieved for 3.8 photons on average, showing that our artificial atom performs as if fully isolated from the solid-state environment.

Somatic and germ cell mutagenesis in lambda lacZ transgenic mice treated with acrylamide or ethylnitrosourea.

The transgenic Muta Mouse in vivo mutagenesis assay was employed to determine the activity of acrylamide and ethylnitrosourea in liver and germ cells after 3, 10 and 100 days following treatment. Each cell of the Muta Mouse carries 80 copies of the lambda gt10 phage including the bacterial lacZ gene, which act as the target gene for the mutagenesis assay. Groups of Muta Mice were given a single intraperitoneal injection of 80 or 160 mg/kg ethylnitrosourea or 50 or 100 mg/kg acrylamide. The tissues were prepared 3, 10 or 100 days post treatment. The liver genomic DNA was extracted with the manufacturer's standard protocol, while the genomic germ cell DNA was extracted with 4 different methods due to problems encountered in DNA yields and packaging efficiency. The mutation analysis of the lacZ gene was carried out by the positive selective assay method [Gossen et al. (1989) Proc. Natl. Acad. Sci. USA, 86, 7971-7975; Dean and Myhr (1994) Mutagenesis, 9, 183-185]. There was a slight increase due to treatment of the observed mutation frequencies in the acrylamide liver group for all three assay times. From the day 3 group to the day 100 group a time dependent decrease in all the absolute mutant frequencies was detectable. The ethylnitrosourea liver group showed a time- and dose-dependent increase in the mutant frequencies from day 3 to day 100. No meaningful results were obtained for the germ cell tissue assays due to the low amount of genomic DNA extracted which was not packageable in the lambda lacZ assay. At present for the mutagenesis assay of isolated spermatozoa in our laboratory we would be forced to pool tissues from animals to obtain enough DNA for an assay. Since 'jackpot'-animals may exist [Heddle et al. (1992) Mutation Res., 272, 195-203] the individual animals of such a pooled analysis group must be tested before pooling.

Comparison of lacI and lacZ transgenic mouse mutation assays: an EU-sponsored interlaboratory study.

As part of an EU Environment Programme sponsored study, the performance of two transgenic mouse mutation assays has been evaluated in three laboratories using common liver samples. The systems studied were the lacI-(Big Blue) mutation assay, and the GalE- positive selection modified lacZ- (Muta Mouse) assay. The liver samples compared were derived from mice treated with either saline or dimethylnitrosamine (DMN; 14 day recovery). Each assay gave an increased mutation frequency (MF) for the DMN-treated livers when compared to the saline control MFs. Sources of variability in the assays are discussed, and it is concluded that whole liver should be homogenised before DNA extraction, and that concurrent controls should be processed in parallel with test samples.

Nuclear magnetization in gallium arsenide quantum dots at zero magnetic field.

Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in semiconductors is commonly achieved in the presence of a stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain-free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse components of the nuclear spin polarization in the absence of lattice strain--that is, in dots with strongly reduced static nuclear quadrupole effects, as reproduced by our model calculations.

Optically probing the fine structure of a single Mn atom in an InAs quantum dot.

We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A0 whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mueV, but vanishingly small for electrons.