230 research outputs found
Colloidal particles at a nematic-isotropic interface: effects of confinement
When captured by a flat nematic-isotropic interface, colloidal particles can
be dragged by it. As a result spatially periodic structures may appear, with
the period depending on a particle mass, size, and interface
velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two
substrates, the interface takes a wedge-like shape, accommodating the
interface-substrate contact angle and minimizing the director distortions on
its nematic side. Correspondingly, particles move along complex trajectories:
they are first captured by the interface and then `glide' towards its vertex
point. Our experiments quantify this scenario, and numerical minimization of
the Landau-de Gennes free energy allow for a qualitative description of the
interfacial structure and the drag force.Comment: 7 pages, 9 figure
Two-Qubit Gate Set Tomography with Fewer Circuits
Gate set tomography (GST) is a self-consistent and highly accurate method for
the tomographic reconstruction of a quantum information processor's quantum
logic operations, including gates, state preparations, and measurements.
However, GST's experimental cost grows exponentially with qubit number. For
characterizing even just two qubits, a standard GST experiment may have tens of
thousands of circuits, making it prohibitively expensive for platforms. We show
that, because GST experiments are massively overcomplete, many circuits can be
discarded. This dramatically reduces GST's experimental cost while still
maintaining GST's Heisenberg-like scaling in accuracy. We show how to exploit
the structure of GST circuits to determine which ones are superfluous. We
confirm the efficacy of the resulting experiment designs both through numerical
simulations and via the Fisher information for said designs. We also explore
the impact of these techniques on the prospects of three-qubit GST.Comment: 46 pages, 13 figures. V2: Minor edits to acknowledgment
Precision tomography of a three-qubit electron-nuclear quantum processor in silicon
Nuclear spins were among the first physical platforms to be considered for
quantum information processing, because of their exceptional quantum coherence
and atomic-scale footprint. However, their full potential for quantum computing
has not yet been realized, due to the lack of methods to link nuclear qubits
within a scalable device combined with multi-qubit operations with sufficient
fidelity to sustain fault-tolerant quantum computation. Here we demonstrate
universal quantum logic operations using a pair of ion-implanted P
nuclei in a silicon nanoelectronic device. A nuclear two-qubit controlled-Z
gate is obtained by imparting a geometric phase to a shared electron spin, and
used to prepare entangled Bell states with fidelities up to 94.2(2.7)%. The
quantum operations are precisely characterised using gate set tomography (GST),
yielding one-qubit gate fidelities up to 99.93(3)%, two-qubit gate fidelity of
99.21(14)% and two-qubit preparation/measurement fidelities of 98.95(4)%. These
three metrics indicate that nuclear spins in silicon are approaching the
performance demanded in fault-tolerant quantum processors. We then demonstrate
entanglement between the two nuclei and the shared electron by producing a
Greenberger-Horne-Zeilinger three-qubit state with 92.5(1.0)% fidelity. Since
electron spin qubits in semiconductors can be further coupled to other
electrons or physically shuttled across different locations, these results
establish a viable route for scalable quantum information processing using
nuclear spins.Comment: 27 pages, 14 figures, plus 20 pages supplementary information. v2
includes new and updated references, and minor text change
Change of tRNA identity leads to a divergent orthogonal histidyl-tRNA synthetase/tRNAHis pair
Mature tRNAHis has at its 5′-terminus an extra guanylate, designated as G−1. This is the major recognition element for histidyl-tRNA synthetase (HisRS) to permit acylation of tRNAHis with histidine. However, it was reported that tRNAHis of a subgroup of α-proteobacteria, including Caulobacter crescentus, lacks the critical G−1 residue. Here we show that recombinant C. crescentus HisRS allowed complete histidylation of a C. crescentus tRNAHis transcript (lacking G−1). The addition of G−1 did not improve aminoacylation by C. crescentus HisRS. However, mutations in the tRNAHis anticodon caused a drastic loss of in vitro histidylation, and mutations of bases A73 and U72 also reduced charging. Thus, the major recognition elements in C. crescentus tRNAHis are the anticodon, the discriminator base and U72, which are recognized by the divergent (based on sequence similarity) C. crescentus HisRS. Transplantation of these recognition elements into an Escherichia coli tRNAHis template, together with addition of base U20a, created a competent substrate for C. crescentus HisRS. These results illustrate how a conserved tRNA recognition pattern changed during evolution. The data also uncovered a divergent orthogonal HisRS/tRNAHis pair
Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits
Scalable quantum processors require high-fidelity universal quantum logic
operations in a manufacturable physical platform. Donors in silicon provide
atomic size, excellent quantum coherence and compatibility with standard
semiconductor processing, but no entanglement between donor-bound electron
spins has been demonstrated to date. Here we present the experimental
demonstration and tomography of universal 1- and 2-qubit gates in a system of
two weakly exchange-coupled electrons, bound to single phosphorus donors
introduced in silicon by ion implantation. We surprisingly observe that the
exchange interaction has no effect on the qubit coherence. We quantify the
fidelity of the quantum operations using gate set tomography (GST), and we use
the universal gate set to create entangled Bell states of the electrons spins,
with fidelity ~ 93%, and concurrence 0.91 +/- 0.08. These results form the
necessary basis for scaling up donor-based quantum computers
Tyrosyl-tRNA synthetase: the first crystallization of a human mitochondrial aminoacyl-tRNA synthetase.
Human mitochondrial tyrosyl-tRNA synthetase and a truncated version with its C-terminal S4-like domain deleted were purified and crystallized. Only the truncated version, which is active in tyrosine activation and Escherichia coli tRNA(Tyr) charging, yielded crystals suitable for structure determination. These tetragonal crystals, belonging to space group P4(3)2(1)2, were obtained in the presence of PEG 4000 as a crystallizing agent and diffracted X-rays to 2.7 A resolution. Complete data sets could be collected and led to structure solution by molecular replacement.journal articleresearch support, non-u.s. gov't2007 Apr 012007 03 30importe
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