5,765 research outputs found
Selection from read-only memory with limited workspace
Given an unordered array of elements drawn from a totally ordered set and
an integer in the range from to , in the classic selection problem
the task is to find the -th smallest element in the array. We study the
complexity of this problem in the space-restricted random-access model: The
input array is stored on read-only memory, and the algorithm has access to a
limited amount of workspace. We prove that the linear-time prune-and-search
algorithm---presented in most textbooks on algorithms---can be modified to use
bits instead of words of extra space. Prior to our
work, the best known algorithm by Frederickson could perform the task with
bits of extra space in time. Our result separates
the space-restricted random-access model and the multi-pass streaming model,
since we can surpass the lower bound known for the latter
model. We also generalize our algorithm for the case when the size of the
workspace is bits, where . The running time
of our generalized algorithm is ,
slightly improving over the
bound of Frederickson's algorithm. To obtain the improvements mentioned above,
we developed a new data structure, called the wavelet stack, that we use for
repeated pruning. We expect the wavelet stack to be a useful tool in other
applications as well.Comment: 16 pages, 1 figure, Preliminary version appeared in COCOON-201
A high bandwidth quantum repeater
We present a physical- and link-level design for the creation of entangled
pairs to be used in quantum repeater applications where one can control the
noise level of the initially distributed pairs. The system can tune
dynamically, trading initial fidelity for success probability, from high
fidelity pairs (F=0.98 or above) to moderate fidelity pairs. The same physical
resources that create the long-distance entanglement are used to implement the
local gates required for entanglement purification and swapping, creating a
homogeneous repeater architecture. Optimizing the noise properties of the
initially distributed pairs significantly improves the rate of generating
long-distance Bell pairs. Finally, we discuss the performance trade-off between
spatial and temporal resources.Comment: 5 page
Efficient optical quantum information processing
Quantum information offers the promise of being able to perform certain
communication and computation tasks that cannot be done with conventional
information technology (IT). Optical Quantum Information Processing (QIP) holds
particular appeal, since it offers the prospect of communicating and computing
with the same type of qubit. Linear optical techniques have been shown to be
scalable, but the corresponding quantum computing circuits need many auxiliary
resources. Here we present an alternative approach to optical QIP, based on the
use of weak cross-Kerr nonlinearities and homodyne measurements. We show how
this approach provides the fundamental building blocks for highly efficient
non-absorbing single photon number resolving detectors, two qubit parity
detectors, Bell state measurements and finally near deterministic control-not
(CNOT) gates. These are essential QIP devicesComment: Accepted to the Journal of optics B special issue on optical quantum
computation; References update
Weak non-linearities and cluster states
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes have a low
initial success probability and low detector efficiencies cause a serious
blowup in resources. In contrast, our approach uses continuous variables and
highly efficient measurements. We present a two-qubit scheme, with a simple
homodyne measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling, showing how these can be vastly improved with access to
this new probability range.Comment: 5 pages, to appear in Phys. Rev.
Applications of Coherent Population Transfer to Quantum Information Processing
We develop a theoretical framework for the exploration of quantum mechanical
coherent population transfer phenomena, with the ultimate goal of constructing
faithful models of devices for classical and quantum information processing
applications. We begin by outlining a general formalism for weak-field quantum
optics in the Schr\"{o}dinger picture, and we include a general
phenomenological representation of Lindblad decoherence mechanisms. We use this
formalism to describe the interaction of a single stationary multilevel atom
with one or more propagating classical or quantum laser fields, and we describe
in detail several manifestations and applications of electromagnetically
induced transparency. In addition to providing a clear description of the
nonlinear optical characteristics of electromagnetically transparent systems
that lead to ``ultraslow light,'' we verify that -- in principle -- a
multi-particle atomic or molecular system could be used as either a low power
optical switch or a quantum phase shifter. However, we demonstrate that the
presence of significant dephasing effects destroys the induced transparency,
and that increasing the number of particles weakly interacting with the probe
field only reduces the nonlinearity further. Finally, a detailed calculation of
the relative quantum phase induced by a system of atoms on a superposition of
spatially distinct Fock states predicts that a significant quasi-Kerr
nonlinearity and a low entropy cannot be simultaneously achieved in the
presence of arbitrary spontaneous emission rates. Within our model, we identify
the constraints that need to be met for this system to act as a one-qubit and a
two-qubit conditional phase gate.Comment: 25 pages, 14 figure
Leading the evaluation of institutional online learning environments for quality enhancement in times of change
This paper reports on findings from a nationally funded project which aims to design and implement a quality management framework for online learning environments (OLEs). Evaluation is a key component of any quality management system and it is this aspect of the framework that is the focus of this paper. In developing the framework initial focus groups were conducted at the five participating institutions. These revealed that, although regarded as important, there did not appear to be a shared understanding of the nature and purpose of evaluation. A second series of focus groups revealed there were multiple perspectives arising from those with a vested interest in online learning. These perspectives will be outlined. Overall, how evaluation was undertaken was highly variable within and across the five institutions reflecting where they were at in relation to the development of their OLE
Sediment resuspension and erosion by vortex rings
Particle resuspension and erosion induced by a vortex ringinteracting with a sediment layer was investigated experimentally using flow visualization (particle image velocimetry), high-speed video, and a recently developed light attenuation method for measuring displacements in bed level. Near-spherical sediment particles were used throughout with relative densities of 1.2–7 and diameters (d)(d) ranging between 90 and 1600 μm1600 μm. Attention was focused on initially smooth, horizontal bedforms with the vortex ring aligned to approach the bed vertically. Interaction characteristics were investigated in terms of the dimensionless Shields parameter, defined using the vortex-ring propagation speed. The critical conditions for resuspension (whereby particles are only just resuspended) were determined as a function of particle Reynolds number (based on the particle settling velocity and dd). The effects of viscous damping were found to be significant for d/δ<15d/δ<15, where δδ denotes the viscous sublayer thickness. Measurements of bed deformation were obtained during the interaction period, for a range of impact conditions. The (azimuthal) mean crater profile is shown to be generally self-similar during the interaction period, except for the most energetic impacts and larger sediment types. Loss of similarity occurs when the local bed slope approaches the repose limit, leading to collapse. Erosion, deposition, and resuspension volumes are analyzed as a function interaction time, impact condition, and sediment size
Contiguous polarisation spectra of the Earth from 300 to 850 nm measured by GOME-2 onboard MetOp-A
In this paper we present the first contiguous high-resolution
spectra of the Earth's polarisation observed by a satellite
instrument. The measurements of the Stokes fraction <i>Q/I</i> are
performed by the spectrometer GOME-2 onboard the
MetOp-A satellite. Polarisation measurements by
GOME-2 are performed by onboard polarisation measurement
devices (PMDs) and the high-resolution measurements discussed in
this paper are taken in the special "PMD RAW" mode of
operation. The spectral resolution of these PMD RAW polarisation
measurements varies from 3 nm in the ultraviolet (UV) to
35 nm in the near-infrared wavelength range. We first
compare measurements of the polarisation from cloud-free scenes with
radiative transfer calculations for a number of cases. We find good
agreement but also a spectral discrepancy at 800 nm, which
we attribute to remaining imperfections in the calibration key
data. Secondly, we study the polarisation of scenes with special
scattering geometries that normally lead to near-zero <i>Q/I</i>. The
GOME-2 polarisation spectra indeed show this behaviour and
confirm the existence of the small discrepancy found
earlier. Thirdly, we study the Earth polarisation for a variety of
scenes. This provides a blueprint of <i>Q/I</i> over land and sea
surfaces for various degrees of cloud cover. Fourthly, we compare
the spectral dependence of measurements of <i>Q/I</i> in the UV with the
generalised distribution function proposed by Schutgens
and Stammes (2002) to describe the shape of the UV polarisation
spectrum. The GOME-2 data confirm that these functions match
the spectral behaviour captured by the GOME-2 PMD RAW mode
Secure self-calibrating quantum random bit generator
Random bit generators (RBGs) are key components of a variety of information
processing applications ranging from simulations to cryptography. In
particular, cryptographic systems require "strong" RBGs that produce
high-entropy bit sequences, but traditional software pseudo-RBGs have very low
entropy content and therefore are relatively weak for cryptography. Hardware
RBGs yield entropy from chaotic or quantum physical systems and therefore are
expected to exhibit high entropy, but in current implementations their exact
entropy content is unknown. Here we report a quantum random bit generator
(QRBG) that harvests entropy by measuring single-photon and entangled
two-photon polarization states. We introduce and implement a quantum
tomographic method to measure a lower bound on the "min-entropy" of the system,
and we employ this value to distill a truly random bit sequence. This approach
is secure: even if an attacker takes control of the source of optical states, a
secure random sequence can be distilled.Comment: 5 pages, 2 figure
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