Voltage-sensor activation with a tarantula toxin as cargo

The opening and closing of voltage-activated Na+, Ca2+ and K+ (Kv) channels underlies electrical and chemical signalling throughout biology, yet the structural basis of voltage sensing is unknown. Hanatoxin is a tarantula toxin that inhibits Kv channels by binding to voltage-sensor paddles, crucial helix-turn-helix motifs within the voltage-sensing domains that are composed of S3b and S4 helices. The active surface of the toxin is amphipathic, and related toxins have been shown to partition into membranes, raising the possibility that the toxin is concentrated in the membrane and interacts only weakly and transiently with the voltage sensors. Here we examine the kinetics and state dependence of the toxin-channel interaction and the physical location of the toxin in the membrane. We find that hanatoxin forms a strong and stable complex with the voltage sensors, far outlasting fluctuations of the voltage sensors between resting (closed) conformations at negative voltages and activated (open) conformations at positive voltages. Toxin affinity is reduced by voltage-sensor activation, explaining why the toxin stabilizes the resting conformation. We also find that when hanatoxin partitions into membranes it is localized to an interfacial region, with Trp 30 positioned about 8.5 A from the centre of the bilayer. These results demonstrate that voltage-sensor paddles activate with a toxin as cargo, and suggest that the paddles traverse no more than the outer half of the bilayer during activation.     

Photon blockade in an optical cavity with one trapped atom

At low temperatures, sufficiently small metallic and semiconductor devices exhibit the 'Coulomb blockade' effect, in which charge transport through the device occurs on an electron-by-electron basis. For example, a single electron on a metallic island can block the flow of another electron if the charging energy of the island greatly exceeds the thermal energy. The analogous effect of 'photon blockade' has been proposed for the transport of light through an optical system; this involves photon-photon interactions in a nonlinear optical cavity. Here we report observations of photon blockade for the light transmitted by an optical cavity containing one trapped atom, in the regime of strong atom-cavity coupling. Excitation of the atom-cavity system by a first photon blocks the transmission of a second photon, thereby converting an incident poissonian stream of photons into a sub-poissonian, anti-bunched stream. This is confirmed by measurements of the photon statistics of the transmitted field. Our observations of photon blockade represent an advance over traditional nonlinear optics and laser physics, into a regime with dynamical processes involving atoms and photons taken one-by-one.     

Generation of nonclassical photon pairs for scalable quantum communication with atomic ensembles

Quantum information science attempts to exploit capabilities from the quantum realm to accomplish tasks that are otherwise impossible in the classical domain. Although sufficient conditions have been formulated for the physical resources required to achieve quantum computation and communication, there is a growing understanding of the power of quantum measurement combined with the conditional evolution of quantum states for accomplishing diverse tasks in quantum information science. For example, a protocol has recently been developed for the realization of scalable long-distance quantum communication and the distribution of entanglement over quantum networks. Here we report the first enabling step in the realization of this protocol, namely the observation of quantum correlations for photon pairs generated in the collective emission from an atomic ensemble. The nonclassical character of the fields is demonstrated by the violation of an inequality involving their normalized correlation functions. Compared to previous investigations of non-classical correlations for photon pairs produced in atomic cascades and in parametric down-conversion, our experiment is distinct in that the correlated photons are separated by a programmable time interval (of about 400 nanoseconds in our initial experiments).     

Experimental realization of a one-atom laser in the regime of strong coupling

Conventional lasers (from table-top systems to microscopic devices) typically operate in the so-called weak-coupling regime, involving large numbers of atoms and photons; individual quanta have a negligible impact on the system dynamics. However, this is no longer the case when the system approaches the regime of strong coupling for which the number of atoms and photons can become quite small. Indeed, the lasing properties of a single atom in a resonant cavity have been extensively investigated theoretically. Here we report the experimental realization of a one-atom laser operated in the regime of strong coupling. We exploit recent advances in cavity quantum electrodynamics that allow one atom to be isolated in an optical cavity in a regime for which one photon is sufficient to saturate the atomic transition. The observed characteristics of the atom-cavity system are qualitatively different from those of the familiar many-atom case. Specifically, our measurements of the intracavity photon number versus pump intensity indicate that there is no threshold for lasing, and we infer that the output flux from the cavity mode exceeds that from atomic fluorescence by more than tenfold. Observations of the second-order intensity correlation function demonstrate that our one-atom laser generates manifestly quantum (nonclassical) light, typified by photon anti-bunching and sub-poissonian photon statistics.