18,072 research outputs found
Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions
By using a symmetry motivated basis to evaluate local distortions against
pair distribution function data (PDF), we show without prior bias, that the
off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone
centred and rhombohedral-like in nature across its known ferroelectric and
paraelectric phases. With our newly-gained insight we construct a simple Monte
Carlo (MC) model which captures our main experimental findings and demonstrate
how the rich crystallographic phase diagram of BaTiO3 emerges from correlations
of local symmetry-breaking distortions alone. Our results strongly support the
order-disorder picture for these phase transitions, but can also be reconciled
with the soft-mode theory of BaTiO3 that is supported by some spectroscopic
techniques.Comment: 5 pages, 3 figure
High-fidelity trapped-ion quantum logic using near-field microwaves
We demonstrate a two-qubit logic gate driven by near-field microwaves in a
room-temperature microfabricated ion trap. We measure a gate fidelity of
99.7(1)\%, which is above the minimum threshold required for fault-tolerant
quantum computing. The gate is applied directly to Ca "atomic clock"
qubits (coherence time ) using the microwave
magnetic field gradient produced by a trap electrode. We introduce a
dynamically-decoupled gate method, which stabilizes the qubits against
fluctuating a.c.\ Zeeman shifts and avoids the need to null the microwave
field
Absence of an embryonic stem cell DNA methylation signature in human cancer.
BackgroundDifferentiated cells that arise from stem cells in early development contain DNA methylation features that provide a memory trace of their fetal cell origin (FCO). The FCO signature was developed to estimate the proportion of cells in a mixture of cell types that are of fetal origin and are reminiscent of embryonic stem cell lineage. Here we implemented the FCO signature estimation method to compare the fraction of cells with the FCO signature in tumor tissues and their corresponding nontumor normal tissues.MethodsWe applied our FCO algorithm to discovery data sets obtained from The Cancer Genome Atlas (TCGA) and replication data sets obtained from the Gene Expression Omnibus (GEO) data repository. Wilcoxon rank sum tests, linear regression models with adjustments for potential confounders and non-parametric randomization-based tests were used to test the association of FCO proportion between tumor tissues and nontumor normal tissues. P-values of < 0.05 were considered statistically significant.ResultsAcross 20 different tumor types we observed a consistently lower FCO signature in tumor tissues compared with nontumor normal tissues, with 18 observed to have significantly lower FCO fractions in tumor tissue (total n = 6,795 tumor, n = 922 nontumor, P < 0.05). We replicated our findings in 15 tumor types using data from independent subjects in 15 publicly available data sets (total n = 740 tumor, n = 424 nontumor, P < 0.05).ConclusionsThe results suggest that cancer development itself is substantially devoid of recapitulation of normal embryologic processes. Our results emphasize the distinction between DNA methylation in normal tightly regulated stem cell driven differentiation and cancer stem cell reprogramming that involves altered methylation in the service of great cell heterogeneity and plasticity
High-fidelity quantum logic gates using trapped-ion hyperfine qubits
We demonstrate laser-driven two-qubit and single-qubit logic gates with
fidelities 99.9(1)% and 99.9934(3)% respectively, significantly above the
approximately 99% minimum threshold level required for fault-tolerant quantum
computation, using qubits stored in hyperfine ground states of calcium-43 ions
held in a room-temperature trap. We study the speed/fidelity trade-off for the
two-qubit gate, for gate times between 3.8s and 520s, and develop a
theoretical error model which is consistent with the data and which allows us
to identify the principal technical sources of infidelity.Comment: 1 trap, 2 ions, 3 nines. Detailed write-up of arXiv:1406.5473
including single-qubit gate data als
High-fidelity preparation, gates, memory and readout of a trapped-ion quantum bit
We implement all single-qubit operations with fidelities significantly above
the minimum threshold required for fault-tolerant quantum computing, using a
trapped-ion qubit stored in hyperfine "atomic clock" states of Ca.
We measure a combined qubit state preparation and single-shot readout fidelity
of 99.93%, a memory coherence time of seconds, and an average
single-qubit gate fidelity of 99.9999%. These results are achieved in a
room-temperature microfabricated surface trap, without the use of magnetic
field shielding or dynamic decoupling techniques to overcome technical noise.Comment: Supplementary Information included. 6 nines, 7 figures, 8 page
Microwave control electrodes for scalable, parallel, single-qubit operations in a surface-electrode ion trap
We propose a surface ion trap design incorporating microwave control
electrodes for near-field single-qubit control. The electrodes are arranged so
as to provide arbitrary frequency, amplitude and polarization control of the
microwave field in one trap zone, while a similar set of electrodes is used to
null the residual microwave field in a neighbouring zone. The geometry is
chosen to reduce the residual field to the 0.5% level without nulling fields;
with nulling, the crosstalk may be kept close to the 0.01% level for realistic
microwave amplitude and phase drift. Using standard photolithography and
electroplating techniques, we have fabricated a proof-of-principle electrode
array with two trapping zones. We discuss requirements for the microwave drive
system and prospects for scalability to a large two-dimensional trap array.Comment: 8 pages, 6 figure
Consistent Gravitationally-Coupled Spin-2 Field Theory
Inspired by the translational gauge structure of teleparallel gravity, the
theory for a fundamental massless spin-2 field is constructed. Accordingly,
instead of being represented by a symmetric second-rank tensor, the fundamental
spin-2 field is assumed to be represented by a spacetime (world) vector field
assuming values in the Lie algebra of the translation group. The flat-space
theory naturally emerges in the Fierz formalism and is found to be equivalent
to the usual metric-based theory. However, the gravitationally coupled theory,
with gravitation itself described by teleparallel gravity, is shown not to
present the consistency problems of the spin-2 theory constructed on the basis
of general relativity.Comment: 16 pages, no figures. V2: Presentation changes, including addition of
a new sub-section, aiming at clarifying the text; version accepted for
publication in Class. Quantum Grav
Probing Qubit Memory Errors at the Part-per-Million Level
Robust qubit memory is essential for quantum computing, both for near-term
devices operating without error correction, and for the long-term goal of a
fault-tolerant processor. We directly measure the memory error for
a Ca trapped-ion qubit in the small-error regime and find
for storage times t\lesssim50\,\mbox{ms}. This exceeds
gate or measurement times by three orders of magnitude. Using randomized
benchmarking, at t=1\,\mbox{ms} we measure ,
around ten times smaller than that extrapolated from the time,
and limited by instability of the atomic clock reference used to benchmark the
qubit.Comment: 8 pages, 5 figure
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