661 research outputs found
Spectral Engineering of Slow Light, Cavity Line Narrowing, and Pulse Compression
More than 4 orders of magnitude of cavity-linewidth narrowing in a
rare-earth-ion-doped crystal cavity, emanating from strong intracavity
dispersion caused by off-resonant interaction with dopant ions, is
demonstrated. The dispersion profiles are engineered using optical pumping
techniques creating significant semipermanent but reprogrammable changes of the
rare-earth absorption profiles. Several cavity modes are shown within the
spectral transmission window. Several possible applications of this phenomenon
are discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1304.445
Extracting high fidelity quantum computer hardware from random systems
An overview of current status and prospects of the development of quantum
computer hardware based on inorganic crystals doped with rare-earth ions is
presented. Major parts of the experimental work in this area has been done in
two places, Canberra, Australia and Lund, Sweden, and the present description
follows more closely the Lund work. Techniques will be described that include
optimal filtering of the initially inhomogeneously broadened profile down to
well separated and narrow ensembles, as well as the use of advanced
pulse-shaping in order to achieve robust arbitrary single-qubit operations with
fidelities above 90%, as characterized by quantum state tomography. It is
expected that full scalability of these systems will require the ability to
determine the state of single rare-earth ions. It has been proposed that this
can be done using special readout ions doped into the crystal and an update is
given on the work to find and characterize such ions. Finally, a few aspects on
the possibilities for remote entanglement of ions in separate
rare-earth-ion-doped crystals are considered.Comment: 19 pages, 9 figures. Written for The Proceedings of the
Nobelsymposium on qubits for future quantum computers, Gothenburg, May-0
Scalable designs for quantum computing with rare-earth-ion-doped crystals
Due to inhomogeneous broadening, the absorption lines of rare-earth-ion
dopands in crystals are many order of magnitudes wider than the homogeneous
linewidths. Several ways have been proposed to use ions with different
inhomogeneous shifts as qubit registers, and to perform gate operations between
such registers by means of the static dipole coupling between the ions.
In this paper we show that in order to implement high-fidelity quantum gate
operations by means of the static dipole interaction, we require the
participating ions to be strongly coupled, and that the density of such
strongly coupled registers in general scales poorly with register size.
Although this is critical to previous proposals which rely on a high density of
functional registers, we describe architectures and preparation strategies that
will allow scalable quantum computers based on rare-earth-ion doped crystals.Comment: Submitted to Phys. Rev.
Changes in face-specific neural processes explain reduced cuteness and approachability of infants with cleft lip
The current study investigated whether changes in the neural processing of faces of infants with a facial abnormality – a cleft lip – mediate effects of the cleft lip on judgments of infant cuteness and approachability. Event-related potentials (ERPs) in response to pictures of faces of healthy infants and infants with a cleft lip, and ratings of cuteness and approachability of these infant faces, were obtained from 30 females. Infants with a cleft lip were rated as less attractive (less cute and approachable) than healthy infants, and both the N170 and P2 components of the ERP were of reduced amplitude in response to pictures of infants with a cleft lip. Importantly, decreased configural processing of infant faces with a cleft lip, as evidenced by reduced N170 amplitudes, mediated the reduced attractiveness ratings for infants with a cleft lip compared to healthy infants. Our findings help elucidate the mechanisms behind the less favorable responses to infants with a cleft lip, highlighting the role of face-specific rather than domain-general neural processes
Attractiveness and neural processing of infant faces: Effects of a facial abnormality but not dopamine
Education and Child Studie
Slow light for deep tissue imaging with ultrasound modulation
Slow light has been extensively studied for applications ranging from optical delay lines to single photon quantum storage. Here, we show that the time delay of slow-light significantly improves the performance of the narrowband spectral filters needed to optically detect ultrasound from deep inside highly scatteringtissue. We demonstrate this capability with a 9 cm thick tissue phantom, having 10 cm^(−1) reduced scattering coefficient, and achieve an unprecedented background-free signal. Based on the data, we project real time imaging at video rates in even thicker phantoms and possibly deep enough into real tissue for clinical applications like early cancer detection
Entropy-driven genome organization
DNA and RNA polymerases active on bacterial and human genomes in the crowded environment of a cell are modeled as beads spaced along a string. Aggregation of the large polymerizing complexes increases the entropy of the system through an increase in entropy of the many small crowding molecules; this occurs despite the entropic costs of looping the intervening DNA. Results of a quantitative cost/benefit analysis are consistent with observations that active polymerases cluster into replication and transcription “factories” in both pro- and eukaryotes. We conclude that the second law of thermodynamics acts through nonspecific entropic forces between engaged polymerases to drive the self-organization of genomes into loops containing several thousands (and sometimes millions) of basepairs
Protein Localization with Flexible DNA or RNA
Localization of activity is ubiquitous in life, and also within sub-cellular compartments. Localization provides potential advantages as different proteins involved in the same cellular process may supplement each other on a fast timescale. It might also prevent proteins from being active in other regions of the cell. However localization is at odds with the spreading of unbound molecules by diffusion. We model the cost and gain for specific enzyme activity using localization strategies based on binding to sites of intermediate specificity. While such bindings in themselves decrease the activity of the protein on its target site, they may increase protein activity if stochastic motion allows the acting protein to touch both the intermediate binding site and the specific site simultaneously. We discuss this strategy in view of recent suggestions on long non-coding RNA as a facilitator of localized activity of chromatin modifiers
Statistical-mechanical lattice models for protein-DNA binding in chromatin
Statistical-mechanical lattice models for protein-DNA binding are well
established as a method to describe complex ligand binding equilibriums
measured in vitro with purified DNA and protein components. Recently, a new
field of applications has opened up for this approach since it has become
possible to experimentally quantify genome-wide protein occupancies in relation
to the DNA sequence. In particular, the organization of the eukaryotic genome
by histone proteins into a nucleoprotein complex termed chromatin has been
recognized as a key parameter that controls the access of transcription factors
to the DNA sequence. New approaches have to be developed to derive statistical
mechanical lattice descriptions of chromatin-associated protein-DNA
interactions. Here, we present the theoretical framework for lattice models of
histone-DNA interactions in chromatin and investigate the (competitive) DNA
binding of other chromosomal proteins and transcription factors. The results
have a number of applications for quantitative models for the regulation of
gene expression.Comment: 19 pages, 7 figures, accepted author manuscript, to appear in J.
Phys.: Cond. Mat
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
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