The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation [1][2][3][4][5][6][7] . However, this advantage is offset by the hyperfine interaction between the electron spin and the 10 4 to 10 6 spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds [8][9][10][11][12][13][14] . Spin-echo techniques have been used to mitigate the hyperfine interaction 14,15 , but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this 16,17 but is very difficult to realize in practice 12,18,19 . Exploring materials with zero-spin nuclei is another option, and carbon nanotubes 20 , graphene quantum dots 21 and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak 22,23 . We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks 24 that can intra-convert the spin state with the polarization of a photon.Our scheme to initialize a single hole spin is presented in Fig. 1. The quantum dot contains a single hole. The strong in-built strain in an InAs quantum dot shifts the valence light hole states with spin J 53/2, J z 5 61/2 away from the fundamental gap such that the uppermost valence states have heavy hole character with spin J 53/2, J z 5 63/2. The corresponding hole spin states are represented as X j i and Y j i. A s z -polarized laser drives the Y j i hole to an exciton state with spin S z 5 21/2, XY,; j i, containing a spin-up, spin-down hole pair and a spin-down electron. Unlike the hole spin, the electron spin interacts with the nuclear spins through the contact hyperfine interaction. The electron spin experiences a small magnetic field, ,20 mT (refs 8-12), as a result of the incomplete cancellation of the random nuclear spins in the quantum dot. The component of the magnetic field in the plane, B xy nuclei , causes the electron spin in the excited state to precess with a period of ,1 ns. The coherence of the precession is destroyed by spontaneous emission with a characteristic ...

The accumulation of smooth muscle and endothelial cells is essential for remodeling and repair of injured blood vessel walls. Bone marrow–derived progenitor cells have been implicated in vascular repair and remodeling; however, the mechanisms underlying their recruitment to the site of injury remain elusive. Here, using real-time in vivo fluorescence microscopy, we show that platelets provide the critical signal that recruits CD34+ bone marrow cells and c-Kit+ Sca-1+ Lin− bone marrow–derived progenitor cells to sites of vascular injury. Correspondingly, specific inhibition of platelet adhesion virtually abrogated the accumulation of both CD34+ and c-Kit+ Sca-1+ Lin− bone marrow–derived progenitor cells at sites of endothelial disruption. Binding of bone marrow cells to platelets involves both P-selectin and GPIIb integrin on platelets. Unexpectedly, we found that activated platelets secrete the chemokine SDF-1α, thereby supporting further primary adhesion and migration of progenitor cells. These findings establish the platelet as a major player in the initiation of vascular remodeling, a process of fundamental importance for vascular repair and pathological remodeling after vascular injury.

We show how the optical properties of a single semiconductor quantum dot can be controlled with a small dc voltage applied to a gate electrode. We find that the transmission spectrum of the neutral exciton exhibits two narrow lines with approximately 2 mueV linewidth. The splitting into two linearly polarized components arises through an exchange interaction within the exciton. The exchange interaction can be turned off by choosing a gate voltage where the dot is occupied with an additional electron. Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system. Our experiments show that the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a mueV energy scale.

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of an applied uniaxial stress. The spectrum of the excitonic Rayleigh scattering was measured in reflectivity using high-resolution laser spectroscopy while the sample was submitted to a tunable uniaxial stress along its ͓110͔ crystal axis. We show that using this stretching technique, the quantum dot potential is elastically deformable such that the exciton fine structure splitting can be substantially reduced.

In human cancer, early systemic spread of tumor cells is recognized as a leading cause of death. Adjuvant therapies are administered to patients after complete resectioning of their primary tumors to eradicate the few residual and latent metastatic cells. These therapeutic regimens, however, are currently designed without direct information about the presence or nature of the latent cells. To address this problem, we developed a PCR-based technique to analyze the transcriptome of individual tumor cells isolated from the bone marrow of cancer patients. From the same cells, genomic aberrations were identified by comparative genomic hybridization. The utility of this approach for understanding the biology of occult disseminated cells and for the identification of new therapeutic targets is demonstrated here by the detection of frequent extracellular matrix metalloproteinase inducer (EMMPRIN; CD147) expression which was verified by immunostaining.

The coherence time of an excess electron spin confined in a quantum dot structure is expected to be orders of magnitude longer than the typical timescales required for its coherent manipulation. Motivated by this observation, several groups have proposed to use single quantum dot spins as quantum bits (qubits), and to manipulate, couple and measure individual spins using either transport or optical techniques. In the case of quantum dots with stronger confinement along the growth direction, lowest energy elementary optical transitions are those arising from excitation of a mz=3/2 (mz =-3/2) valence band electron to a mz =1/2 (mz =-1/2) conduction band state. If the quantum dot already has an excess conduction band electron, only one of these optical (trion) transitions is allowed; the other is spin (Pauli) blocked. In contrast, a neutral quantum dot always has a pair of exciton excitations. It has been suggested that Pauli blocking of absorption or fluorescence can be used to implement high efficiency all-optical single-spin measurements and conditional spin dynamics. In this talk, we will review our work aimed at experimental demonstration of Pauli blocking and single-spin measurement in single-electron-charged self-assembled quantum dots. 133

The ductus arteriosus (DA) is a fetal shunt vessel between the pulmonary artery and the aorta that closes promptly after birth. Failure of postnatal DA closure is a major cause of morbidity and mortality particularly in preterm neonates. The events leading to DA closure are incompletely understood. Here we show that platelets have an essential role in DA closure. Using intravital microscopy of neonatal mice, we observed that platelets are recruited to the luminal aspect of the DA during closure. DA closure is impaired in neonates with malfunctioning platelet adhesion or aggregation or with defective platelet biogenesis. Defective DA closure resulted in a left-to-right shunt with increased pulmonary perfusion, pulmonary vascular remodeling and right ventricular hypertrophy. Our findings indicate that platelets are crucial for DA closure by promoting thrombotic sealing of the constricted DA and by supporting luminal remodeling. A retrospective clinical study revealed that thrombocytopenia is an independent predictor for failure of DA closure in preterm human newborns, indicating that platelets are likely to contribute to DA closure in humans.

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2431758͔There is currently great interest in producing entangled photons on demand for applications in quantum information processing. 1 One proposal which spurred much research is using radiative biexciton-exciton cascade in semiconductor quantum dots ͑QDs͒ to produce pairs of polarization entangled photons. 2 In an idealized QD, the bright exciton states ͑M = ±1͒ are degenerate. In this case the two decay paths from the biexciton to the vacuum state via the intermediate single exciton are indistinguishable in energy; thus photons emitted in the radiative cascade are polarization entangled. However, in practice, the rotational symmetry of a self-assembled QD is broken and the electron-hole exchange interaction mixes the bright exciton states into a nondegenerate doublet ͓referred to as a fine-structure splitting ͑FSS͔͒. This leads to an energetically distinguishable recombination path for the biexciton-exciton cascade. Polarization correlations are observed in the linear basis but polarization entanglement is destroyed due to the FSS. 3,4 For photons to be polarization entangled using this scheme, the requirement that the FSS be less than the homogeneous linewidth must be met. The FSS is typically 10-100 eV, while the homogeneous linewidth of self-assembled InGaAs QDs is ϳ1 eV. 5 Techniques used to actively tune the FSS include an inplane electric 6 or magnetic 7 field and an in situ uniaxial stress. 8 Also, QDs which are smaller due to the growth process 9 or subsequent annealing 10 have a smaller FSS. Unfortunately, such QDs are higher in energy and the QD photons become difficult to distinguish from those produced in the wetting layer. Recently, polarization entangled photons have been reported from specific QDs with energetically overlapping bright exciton states 11 and initially nondegenerate states tuned via a magnetic field. 12 However, a robust approach that would allow one to actively tune the FSS from a large value to zero for each QD is still necessary to realize an event ready entangled photon pair source. To this end we further explore the effect of a lateral electric field on the FSS.There are three basic characteristics of an exciton in a lateral electric field attributed to the quantum confined Stark effect, as has been investigated for quantum wells 13 and QDs: 14 a redshift in recombination energy, decreased oscillator strength, and an increase in nonradiative carrier tunneling probability. Additionally, electric fi...