90 research outputs found
Ultrafast optical rotations of electron spins in quantum dots
Coherent manipulation of quantum bits (qubits) on time scales much shorter
than the coherence time is a key prerequisite for quantum information
processing. Electron spins in quantum dots (QDs) are particularly attractive
for implementations of qubits. Efficient optical methods for initialization and
readout of spins have been developed in recent years. Spin coherence times in
the microsecond range have been demonstrated, so that spin control by
picosecond optical pulses would be highly desirable. Then a large number of
spin rotations could be performed while coherence is maintained. A major
remaining challenge is demonstration of such rotations with high fidelity. Here
we use an ensemble of QD electron spins focused into a small number of
precession modes about a magnetic field by periodic optical pumping. We
demonstrate ultrafast optical rotations of spins about arbitrary axes on a
picosecond time scale using laser pulses as control fields.Comment: 10 pages, 4 figure
Ultrafast optical control of entanglement between two quantum dot spins
The interaction between two quantum bits enables entanglement, the
two-particle correlations that are at the heart of quantum information science.
In semiconductor quantum dots much work has focused on demonstrating single
spin qubit control using optical techniques. However, optical control of
entanglement of two spin qubits remains a major challenge for scaling from a
single qubit to a full-fledged quantum information platform. Here, we combine
advances in vertically-stacked quantum dots with ultrafast laser techniques to
achieve optical control of the entangled state of two electron spins. Each
electron is in a separate InAs quantum dot, and the spins interact through
tunneling, where the tunneling rate determines how rapidly entangling
operations can be performed. The two-qubit gate speeds achieved here are over
an order of magnitude faster than in other systems. These results demonstrate
the viability and advantages of optically controlled quantum dot spins for
multi-qubit systems.Comment: 24 pages, 5 figure
Ultrafast control of donor-bound electron spins with single detuned optical pulses
The ability to control spins in semiconductors is important in a variety of
fields including spintronics and quantum information processing. Due to the
potentially fast dephasing times of spins in the solid state [1-3], spin
control operating on the picosecond or faster timescale may be necessary. Such
speeds, which are not possible to attain with standard electron spin resonance
(ESR) techniques based on microwave sources, can be attained with broadband
optical pulses. One promising ultrafast technique utilizes single broadband
pulses detuned from resonance in a three-level Lambda system [4]. This
attractive technique is robust against optical pulse imperfections and does not
require a fixed optical reference phase. Here we demonstrate the principle of
coherent manipulation of spins theoretically and experimentally. Using this
technique, donor-bound electron spin rotations with single-pulse areas
exceeding pi/4 and two-pulses areas exceeding pi/2 are demonstrated. We believe
the maximum pulse areas attained do not reflect a fundamental limit of the
technique and larger pulse areas could be achieved in other material systems.
This technique has applications from basic solid-state ESR spectroscopy to
arbitrary single-qubit rotations [4, 5] and bang-bang control[6] for quantum
computation.Comment: 15 pages, 4 figures, submitted 12/2008. Since the submission of this
work we have become aware of related work: J. Berezovsky, M. H. Mikkelsen, N.
G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 320: 349-352 (2008
CO2PipeHaz: Quantitative hazard assessment for next generation CO2 pipelines
Without a clear understanding of the hazards associated with the failure of CO2 pipelines, carbon capture and storage (CCS) cannot be considered as a viable proposition for tackling the effects of global warming. Given that CO2 is an asphyxiant at high concentrations, the development of reliable validated pipeline outflow and dispersion models are central to addressing this challenge. This information is pivotal to quantifying all the hazard consequences associated with the failure of CO2 transportation pipelines, which forms the basis for emergency response planning and determining minimum safe distances to populated areas. This paper presents an overview of the main findings of the recently completed CO2PipeHaz project [1] which focussed on the hazard assessment of CO2 pipelines to be employed as an integral part of CCS. Funded by the European Commission FP7 Energy programme, the project's main objective was to address this fundamentally important issue
Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots
In this chapter we review the use of spins in optically-active InAs quantum
dots as the key physical building block for constructing a quantum repeater,
with a particular focus on recent results demonstrating entanglement between a
quantum memory (electron spin qubit) and a flying qubit (polarization- or
frequency-encoded photonic qubit). This is a first step towards demonstrating
entanglement between distant quantum memories (realized with quantum dots),
which in turn is a milestone in the roadmap for building a functional quantum
repeater. We also place this experimental work in context by providing an
overview of quantum repeaters, their potential uses, and the challenges in
implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the
Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W.
Mitchell
Optical control of one and two hole spins in interacting quantum dots
A single hole spin in a semiconductor quantum dot has emerged as a quantum
bit that is potentially superior to an electron spin. A key feature of holes is
that they have a greatly reduced hyperfine interaction with nuclear spins,
which is one of the biggest difficulties in working with an electron spin. It
is now essential to show that holes are viable for quantum information
processing by demonstrating fast quantum gates and scalability. To this end we
have developed InAs/GaAs quantum dots coupled through coherent tunneling and
charged with controlled numbers of holes. We report fast, single qubit gates
using a sequence of short laser pulses. We then take the important next step
toward scalability of quantum information by optically controlling two
interacting hole spins in separate dots.Comment: 5 figure
Radiation-Induced c-Jun Activation Depends on MEK1-ERK1/2 Signaling Pathway in Microglial Cells
Radiation-induced normal brain injury is associated with acute and/or chronic inflammatory responses, and has been a major concern in radiotherapy. Recent studies suggest that microglial activation is a potential contributor to chronic inflammatory responses following irradiation; however, the molecular mechanism underlying the response of microglia to radiation is poorly understood. c-Jun, a component of AP-1 transcription factors, potentially regulates neural cell death and neuroinflammation. We observed a rapid increase in phosphorylation of N-terminal c-Jun (on serine 63 and 73) and MAPK kinases ERK1/2, but not JNKs, in irradiated murine microglial BV2 cells. Radiation-induced c-Jun phosphorylation is dependent on the canonical MEK-ERK signaling pathway and required for both ERK1 and ERK2 function. ERK1/2 directly interact with c-Jun in vitro and in cells; meanwhile, the JNK binding domain on c-Jun is not required for its interaction with ERK kinases. Radiation-induced reactive oxygen species (ROS) potentially contribute to c-Jun phosphorylation through activating the ERK pathway. Radiation stimulates c-Jun transcriptional activity and upregulates c-Jun-regulated proinflammatory genes, such as tumor necrosis factor-α, interleukin-1β, and cyclooxygenase-2. Pharmacologic blockade of the ERK signaling pathway interferes with c-Jun activity and inhibits radiation-stimulated expression of c-Jun target genes. Overall, our study reveals that the MEK-ERK1/2 signaling pathway, but not the JNK pathway, contributes to the c-Jun-dependent microglial inflammatory response following irradiation
Systematic meta-analyses and field synopsis of genetic and epigenetic studies in paediatric inflammatory bowel disease
We provide a comprehensive field synopsis of genetic and epigenetic associations for paediatric Inflammatory Bowel Disease (IBD). A systematic review was performed and included 84 genetic association studies reporting data for 183 polymorphisms in 71 genes. Meta-analyses were conducted for 20 SNPs in 10 genes of paediatric Crohn’s disease (CD) and for 8 SNPs in 5 genes of paediatric ulcerative colitis (UC). Five epigenetic studies were also included, but formal meta-analysis was not possible. Venice criteria and Bayesian false discovery probability test were applied to assess the credibility of associations. Nine SNPs in 4 genes were considered to have highly credible associations with paediatric CD, of which four variants (rs2066847, rs12521868, rs26313667, rs1800629) were not previously identified in paediatric GWAS. Differential DNA methylation in NOD2 and TNF-α, dysregulated expression in let-7 and miR-124 were associated with paediatric IBD, but not as yet replicated. Highly credible SNPs associated with paediatric IBD have also been implicated in adult IBD, with similar magnitudes of associations. Early onset and distinct phenotypic features of paediatric IBD might be due to distinct epigenetic changes, but these findings need to be replicated. Further progress identifying genetic and epigenetic susceptibility of paediatric IBD will require international collaboration, population diversity and harmonization of protocols
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