2,381 research outputs found
A Quantum Scattering Interferometer
The collision of two ultra-cold atoms results in a quantum-mechanical
superposition of two outcomes: each atom continues without scattering and each
atom scatters as a spherically outgoing wave with an s-wave phase shift. The
magnitude of the s-wave phase shift depends very sensitively on the interaction
between the atoms. Quantum scattering and the underlying phase shifts are
vitally important in many areas of contemporary atomic physics, including
Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic
clocks, and magnetically-tuned Feshbach resonances. Precise measurements of
quantum scattering phase shifts have not been possible until now because, in
scattering experiments, the number of scattered atoms depends on the s-wave
phase shifts as well as the atomic density, which cannot be measured precisely.
Here we demonstrate a fundamentally new type of scattering experiment that
interferometrically detects the quantum scattering phase shifts of individual
atoms. By performing an atomic clock measurement using only the scattered part
of each atom, we directly and precisely measure the difference of the s-wave
phase shifts for the two clock states in a density independent manner. Our
method will give the most direct and precise measurements of ultracold
atom-atom interactions and will place stringent limits on the time variations
of fundamental constants.Comment: Corrected formatting and typo
Satellites of the largest Kuiper Belt objects
We have searched the four brightest objects in the Kuiper Belt for the presence of satellites using the newly commissioned Keck Observatory Laser Guide Star Adaptive Optics system. Satellites are seen around three of the four objects: Pluto (whose satellite Charon is well-known and whose recently discovered smaller satellites are too faint to be detected), 2003 EL61 (where a second satellite is seen in addition to the previously known satellite), and 2003 UB313 (where a satellite is seen for the first time). The object 2005 FY9, the brightest Kuiper Belt object (KBO) after Pluto, does not have a satellite detectable within 0".4 with a brightness of more than 1% of the primary. The presence of satellites around three of the four brightest KBOs is inconsistent with the fraction of satellites in the Kuiper Belt at large at the 99.2% confidence level, suggesting a different formation mechanism for these largest KBO satellites. The two satellites of 2003 EL61, and the one satellite of 2003 UB313, with fractional brightnesses of 5% and 1.5%, and 2%, of their primaries, respectively, are significantly fainter relative to their primaries than other known KBO satellites, again pointing to possible differences in their origin
Strong Electric Fields From Positive Lightning Strokes in the Stratosphere
A balloon payload launched in Brazil has measured vector electric fields from lightning at least an order of magnitude larger than previously reported above 30 km in the stratosphere. During the flight hundreds of lightning events were recorded, including several positive cloud to ground lightning strokes. A two stroke flash, with small (15 kA peak current) and moderate (53 kA) positive strokes at a horizontal range of 34 km, produced field changes over 140 V/m at 34 km altitude. On-board optical lightning detection, recorded with GPS timing, coupled with ground based lightning location gives high time resolution for study of the electric field transient propagation. These measurements imply that lightning electric fields in the mesosphere over large thunderstorms may be much larger than previously measured
Sum Rules for Multi-Photon Spectroscopy of Ions in Finite Symmetry
Models describing one- and two-photon transitions for ions in crystalline
environments are unified and extended to the case of parity-allowed and parity-
forbidden p-photon transitions. The number of independent parameters for
characterizing the polarization dependence is shown to depend on an ensemble of
properties and rules which combine symmetry considerations and physical models.Comment: 16 pages, Tex fil
Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance
The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210âmeV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs
Crystal Structure of the Zorbamycin-Binding Protein ZbmA, the Primary Self-Resistance Element in Streptomyces flavoviridis ATCC21892
The bleomycins (BLMs), tallysomycins (TLMs), phleomycin, and zorbamycin (ZBM) are members of the BLM family of glycopeptide-derived antitumor antibiotics. The BLM-producing Streptomyces verticillus ATCC15003 and the TLM-producing Streptoalloteichus hindustanus E465-94 ATCC31158 both possess at least two self-resistance elements, an N-acetyltransferase and a binding protein. The N-acetyltransferase provides resistance by disrupting the metal-binding domain of the antibiotic that is required for activity, while the binding protein confers resistance by sequestering the metal-bound antibiotic and preventing drug activation via molecular oxygen. We recently established that the ZBM producer, Streptomyces flavoviridis ATCC21892, lacks the N-acetyltransferase resistance gene and that the ZBM-binding protein, ZbmA, is sufficient to confer resistance in the producing strain. To investigate the resistance mechanism attributed to ZbmA, we determined the crystal structures of apo and Cu(II)-ZBM-bound ZbmA at high resolutions of 1.90 and 1.65 Ă
, respectively. A comparison and contrast with other structurally characterized members of the BLM-binding protein family revealed key differences in the proteinâligand binding environment that fine-tunes the ability of ZbmA to sequester metal-bound ZBM and supports drug sequestration as the primary resistance mechanism in the producing organisms of the BLM family of antitumor antibiotics
Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice
The study of superfluid fermion pairs in a periodic potential has important
ramifications for understanding superconductivity in crystalline materials.
Using cold atomic gases, various condensed matter models can be studied in a
highly controllable environment. Weakly repulsive fermions in an optical
lattice could undergo d-wave pairing at low temperatures, a possible mechanism
for high temperature superconductivity in the cuprates. The lattice potential
could also strongly increase the critical temperature for s-wave superfluidity.
Recent experimental advances in the bulk include the observation of fermion
pair condensates and high-temperature superfluidity. Experiments with fermions
and bosonic bound pairs in optical lattices have been reported, but have not
yet addressed superfluid behavior. Here we show that when a condensate of
fermionic atom pairs was released from an optical lattice, distinct
interference peaks appear, implying long range order, a property of a
superfluid. Conceptually, this implies that strong s-wave pairing and
superfluidity have now been established in a lattice potential, where the
transport of atoms occurs by quantum mechanical tunneling and not by simple
propagation. These observations were made for unitarity limited interactions on
both sides of a Feshbach resonance. For larger lattice depths, the coherence
was lost in a reversible manner, possibly due to a superfluid to insulator
transition. Such strongly interacting fermions in an optical lattice can be
used to study a new class of Hamiltonians with interband and atom-molecule
couplings.Comment: accepted for publication in Natur
Patient-centric trials for therapeutic development in precision oncology
An enhanced understanding of the molecular pathology of disease gained from genomic studies is facilitating the development of treatments that target discrete molecular subclasses of tumours. Considerable associated challenges include how to advance and implement targeted drug-development strategies. Precision medicine centres on delivering the most appropriate therapy to a patient on the basis of clinical and molecular features of their disease. The development of therapeutic agents that target molecular mechanisms is driving innovation in clinical-trial strategies. Although progress has been made, modifications to existing core paradigms in oncology drug development will be required to realize fully the promise of precision medicine
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