Eight-fold signal amplification of a superconducting nanowire single-photon detector using a multiple-avalanche architecture

Superconducting nanowire avalanche single-photon detectors (SNAPs) with n parallel nanowires are advantageous over single-nanowire detectors because their output signal amplitude scales linearly with n. However, the SNAP architecture has not been viably demonstrated for n > 4. To increase n for larger signal amplification, we designed a multi-stage, successive-avalanche architecture which used nanowires, connected via choke inductors in a binary-tree layout. We demonstrated an avalanche detector with n = 8 parallel nanowires and achieved eight-fold signal amplification, with a timing jitter of 54 ps

Molecular mimicry in the decoding of translational stop signals

The structures of the ribosome and its subunits are now available at atomic detail, as well as those of several factors that bind to its active center. Of particular interest are the protein release factors that decode stop signals. In contrast to the codons specifying the different amino acids, the stop signals are not decoded by RNA molecules, the tRNAs. The tRNA analogue hypothesis (1994) for the decoding of stop signals was proposed to explain how the release factors might mimic a tRNA to span the decoding site of the small subunit and the enzyme center of the large subunit of the ribosome. The specific term "molecular mimicry" was applied soon after to include proteins or their domains that enter the tRNA binding sites on the ribosome. The solution crystal structures of the two release factors already solved (one eubacterial and one eukaryotic), although quite distinct in their folds, each resembles the shape of a tRNA. The eukaryotic factor, like a tRNA, seems to have specific motifs at the tips of two of its domains that interact with the decoding site and the enzyme center as predicted in the tRNA analogue model. Biochemical and genetic studies had identified two analogous motifs in the bacterial factors, but these are quite close together in the solution structure, suggesting a major conformational change may take place when the factor binds to the ribosome. Indeed, reconstructed cryoelectron microscopic images support an unfolding of the structure. A second class of release factor functions as a translational G-protein in the same manner as the two elongation factors and forms part of the termination mimicry complex. Undoubtedly, the molecular mimicry concept will be refined as the conformational changes that take place in the active center of the ribosome and in the proteins that bind to it are better understood

Superconducting- nanowire single-photon-detector linear array

We designed, fabricated, and tested a one-dimensional array of superconducting-nanowire singlephoton detectors, integrated with on-chip inductors and resistors. The architecture is suitable for monolithic integration on a single chip operated in a cryogenic environment, and inherits the characteristics of individual superconducting-nanowire single-photon detectors. We demonstrated a working array with four pixels showing position discrimination and a timing jitter of 124 ps. The electronic crosstalk between the pixels in the array was negligible

Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition

Thin superconducting films form a unique platform for geometrically confined, strongly interacting electrons. They allow an inherent competition between disorder and superconductivity, which in turn enables the intriguing superconducting-to-insulating transition and is believed to facilitate the comprehension of high-Tc superconductivity. Furthermore, understanding thin film superconductivity is technologically essential, e.g., for photodetectors and quantum computers. Consequently, the absence of established universal relationships between critical temperature (Tc), film thickness (d), and sheet resistance (Rs) hinders both our understanding of the onset of the superconductivity and the development of miniaturized superconducting devices. We report that in thin films, superconductivity scales as dTc(Rs). We demonstrated this scaling by analyzing the data published over the past 46 years for different materials (and facilitated this database for further analysis). Moreover, we experimentally confirmed the discovered scaling for NbN films, quantified it with a power law, explored its possible origin, and demonstrated its usefulness for nanometer-length-scale superconducting film-based devices

Translational termination efficiency in both bacteria and mammals is regulated by the base following the stop codon

The translational stop signal and polypeptide release factor (RF) complexed with Escherichia coli ribosomes have been shown to be in close physical contact by site-directed photochemical cross-linking experiments. The RF has a protease-sensitive site in a highly conserved exposed loop that is proposed to interact with the peptidyltransferase center of the ribosome. Loss of peptidyl-tRNA hydrolysis activity and enhanced codon-ribosome binding by the cleaved RF is consistent with a model whereby the RF spans the decoding and peptidyltransferase centers of the ribosome with domains of the RF linked by conformational coupling. The cross-link between the stop signal and RF at the ribosomal decoding site is influenced by the base following the termination codon. This base determines the efficiency with which the stop signal is decoded by the RF in both mammalian and bacterial systems in vivo. The wide range of efficiencies correlates with the frequency with which the signals occur at natural termination sites, with rarely used weak signals often found at recoding sites and strong signals found in highly expressed genes. Stop signals are found at some recoding sites in viruses where -1 frame-shifting occurs, but the generally accepted mechanism of simultaneous slippage from the A and P sites does not explain their presence here. The HIV-1 gag-pol -1 frame shifting site has been used to show that stop signals significantly influence frame-shifting efficiency on prokaryotic ribosomes by a RF-mediated mechanism. These data can be explained by an E/P site simultaneous slippage mechanism whereby the stop codon actually enters the ribosomal A site and can influence the event