Hybridization of electronic states in quantum dots through photon emission

The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as 'artificial atoms', because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons (bound electron-hole pairs) or with electrons. The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter. At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots, motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently. We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.     

Resolving photon number states in a superconducting circuit

Electromagnetic signals are always composed of photons, although in the circuit domain those signals are carried as voltages and currents on wires, and the discreteness of the photon's energy is usually not evident. However, by coupling a superconducting quantum bit (qubit) to signals on a microwave transmission line, it is possible to construct an integrated circuit in which the presence or absence of even a single photon can have a dramatic effect. Such a system can be described by circuit quantum electrodynamics (QED)-the circuit equivalent of cavity QED, where photons interact with atoms or quantum dots. Previously, circuit QED devices were shown to reach the resonant strong coupling regime, where a single qubit could absorb and re-emit a single photon many times. Here we report a circuit QED experiment in the strong dispersive limit, a new regime where a single photon has a large effect on the qubit without ever being absorbed. The hallmark of this strong dispersive regime is that the qubit transition energy can be resolved into a separate spectral line for each photon number state of the microwave field. The strength of each line is a measure of the probability of finding the corresponding photon number in the cavity. This effect is used to distinguish between coherent and thermal fields, and could be used to create a photon statistics analyser. As no photons are absorbed by this process, it should be possible to generate non-classical states of light by measurement and perform qubit-photon conditional logic, the basis of a logic bus for a quantum computer.     

The signature of orbital motion from the dayside of the planet τ Boötis b

The giant planet orbiting τ Bo?tis (named τ Bo?tis b) was amongst the first extrasolar planets to be discovered. It is one of the brightest exoplanets and one of the nearest to us, with an orbital period of just a few days. Over the course of more than a decade, measurements of its orbital inclination have been announced and refuted, and have hitherto remained elusive. Here we report the detection of carbon monoxide absorption in the thermal dayside spectrum of τ Bo?tis b. At a spectral resolution of ～100,000, we trace the change in the radial velocity of the planet over a large range in phase, determining an orbital inclination of 44.5°?±?1.5° and a mass 5.95?±?0.28 times that of Jupiter, demonstrating that atmospheric characterization is possible for non-transiting planets. The strong absorption signal points to an atmosphere with a temperature that is decreasing towards higher altitudes, in contrast to the temperature inversion inferred for other highly irradiated planets. This supports the hypothesis that the absorbing compounds believed to cause such atmospheric inversions are destroyed in τ Bo?tis b by the ultraviolet emission from the active host star.     

Shiga-like toxins are neutralized by tailored multivalent carbohydrate ligands

The diseases caused by Shiga and cholera toxins account for the loss of millions of lives each year. Both belong to the clinically significant subset of bacterial AB5 toxins consisting of an enzymatically active A subunit that gains entry to susceptible mammalian cells after oligosaccharide recognition by the B5 homopentamer. Therapies might target the obligatory oligosaccharide-toxin recognition event, but the low intrinsic affinity of carbohydrate-protein interactions hampers the development of low-molecular-weight inhibitors. The toxins circumvent low affinity by binding simultaneously to five or more cell-surface carbohydrates. Here we demonstrate the use of the crystal structure of the B5 subunit of Escherichia coli O157:H7 Shiga-like toxin I (SLT-I) in complex with an analogue of its carbohydrate receptor to design an oligovalent, water-soluble carbohydrate ligand (named STARFISH), with subnanomolar inhibitory activity. The in vitro inhibitory activity is 1-10-million-fold higher than that of univalent ligands and is by far the highest molar activity of any inhibitor yet reported for Shiga-like toxins I and II. Crystallography of the STARFISH/Shiga-like toxin I complex explains this activity. Two trisaccharide receptors at the tips of each of five spacer arms simultaneously engage all five B subunits of two toxin molecules.     

Protection of macaques from vaginal SHIV challenge by vaginally delivered inhibitors of virus-cell fusion

Human immunodeficiency virus type 1 (HIV-1) continues to spread, principally by heterosexual sex, but no vaccine is available. Hence, alternative prevention methods are needed to supplement educational and behavioural-modification programmes. One such approach is a vaginal microbicide: the application of inhibitory compounds before intercourse. Here, we have evaluated the microbicide concept using the rhesus macaque 'high dose' vaginal transmission model with a CCR5-receptor-using simian-human immunodeficiency virus (SHIV-162P3) and three compounds that inhibit different stages of the virus-cell attachment and entry process. These compounds are BMS-378806, a small molecule that binds the viral gp120 glycoprotein and prevents its attachment to the CD4 and CCR5 receptors, CMPD167, a small molecule that binds to CCR5 to inhibit gp120 association, and C52L, a bacterially expressed peptide inhibitor of gp41-mediated fusion. In vitro, all three compounds inhibit infection of T cells and cervical tissue explants, and C52L acts synergistically with CMPD167 or BMS-378806 to inhibit infection of cell lines. In vivo, significant protection was achieved using each compound alone and in combinations. CMPD167 and BMS-378806 were protective even when applied 6 h before challenge.     

The vault complex

Vaults are large ribonucleoprotein particles found in eukaryotic cells. They are composed of multiple copies of a M _r 100,000 major vault protein and two minor vault proteins of M _r 193,000 and 240,000, as well as small untranslated RNAs of 86–141 bases. The vault components are arranged into a highly characteristic hollow barrel-like structure of 35 × 65 nm in size. Vaults are predominantly localized in the cytoplasm where they may associate with cytoskeletal elements. A small fraction of vaults are found to be associated with the nucleus. As of yet, the precise cellular function of the vault complex is unknown. However, their distinct morphology and intracellular distribution suggest a role in intracellular transport processes. Here we review the current knowledge on the vault complex, its structure, components and possible functions.Received 23 January 2003; received after revision 13 March 2003; accepted 26 March 2003     

Extracellular proteins as enterobacterial thermometers

Biological thermometers are cellular components or structures which sense increasing temperatures, interaction of the thermometer and the thermal stress bringing about the switching-on of inducible responses, with gradually enhanced levels of response induction following gradually increasing temperatures. In enterobacteria, for studies of such thermometers, generally induction of heat shock protein (HSP) synthesis has been examined, with experimental studies aiming to establish (often indirectly) how the temperature changes which initiate HSP synthesis are sensed; numerous other processes and responses show graded induction as temperature is increased, and how the temperature changes which induce these are sensed is also of interest. Several classes of intracellular component and structure have been proposed as enterobacterial thermometers, with the ribosome and the DnaK chaperone being the most favoured, although for many of the proposed intracellular thermometers, most of the evidence for their functioning in this way is indirect. In contrast to the above, the studies reviewed here firmly establish that for four distinct stress responses, which are switched-on gradually as temperature increases, temperature changes are sensed by extracellular components (extracellular sensing components, ESCs) i.e. there is firm and direct evidence for the occurrence of extracellular thermometers. All four thermometers described here are proteins, which appear to be distinct and different from each other, and on sensing thermal stress are activated by it to four distinct extracellular induction components (EICs), which interact with receptors on the surface of organisms to induce the appropriate responses. It is predicted that many other temperature-induced processes, including the synthesis of HSPs, will be switched-on following the activation of similar extracellular thermometers by thermal stimuli.