4,093 research outputs found
Investigating Bimodal Clustering in Human Mobility
We apply a simple clustering algorithm to a large dataset of cellular
telecommunication records, reducing the complexity of mobile phone users' full
trajectories and allowing for simple statistics to characterize their
properties. For the case of two clusters, we quantify how clustered human
mobility is, how much of a user's spatial dispersion is due to motion between
clusters, and how spatially and temporally separated clusters are from one
another.Comment: 4 pages, 2 figure
Algorithms on ensemble quantum computers.
In ensemble (or bulk) quantum computation, all computations are performed on an ensemble of computers rather than on a single computer. Measurements of qubits in an individual computer cannot be performed; instead, only expectation values (over the complete ensemble of computers) can be measured. As a result of this limitation on the model of computation, many algorithms cannot be processed directly on such computers, and must be modified, as the common strategy of delaying the measurements usually does not resolve this ensemble-measurement problem. Here we present several new strategies for resolving this problem. Based on these strategies we provide new versions of some of the most important quantum algorithms, versions that are suitable for implementing on ensemble quantum computers, e.g., on liquid NMR quantum computers. These algorithms are Shor's factorization algorithm, Grover's search algorithm (with several marked items), and an algorithm for quantum fault-tolerant computation. The first two algorithms are simply modified using a randomizing and a sorting strategies. For the last algorithm, we develop a classical-quantum hybrid strategy for removing measurements. We use it to present a novel quantum fault-tolerant scheme. More explicitly, we present schemes for fault-tolerant measurement-free implementation of Toffoli and σ(z)(¼) as these operations cannot be implemented "bitwise", and their standard fault-tolerant implementations require measurement
A Chebychev propagator for inhomogeneous Schr\"odinger equations
We present a propagation scheme for time-dependent inhomogeneous
Schr\"odinger equations which occur for example in optimal control theory or in
reactive scattering calculations. A formal solution based on a polynomial
expansion of the inhomogeneous term is derived. It is subjected to an
approximation in terms of Chebychev polynomials. Different variants for the
inhomogeneous propagator are demonstrated and applied to two examples from
optimal control theory. Convergence behavior and numerical efficiency are
analyzed.Comment: explicit description of algorithm and two appendices added version
accepted by J Chem Phy
Wavelength-independent coupler from fiber to an on-chip cavity, demonstrated over an 850nm span
A robust wide band (850 nm) fiber coupler to a whispering-gallery cavity with ultra-high quality factor is experimentally demonstrated. The device trades off ideality for broad-band, efficient input coupling. Output coupling efficiency can remain high enough for practical applications wherein pumping and power extraction must occur over very broad wavelength spans
Algorithmic Cooling and Scalable NMR Quantum Computers
We present here algorithmic cooling (via polarization-heat-bath)- a powerful
method for obtaining a large number of highly polarized spins in liquid
nuclear-spin systems at finite temperature. Given that spin-half states
represent (quantum) bits, algorithmic cooling cleans dirty bits beyond the
Shannon's bound on data compression, by employing a set of rapidly
thermal-relaxing bits. Such auxiliary bits could be implemented using spins
that rapidly get into thermal equilibrium with the environment, e.g., electron
spins.
Cooling spins to a very low temperature without cooling the environment could
lead to a breakthrough in nuclear magnetic resonance experiments, and our
``spin-refrigerating'' method suggests that this is possible.
The scaling of NMR ensemble computers is probably the main obstacle to
building useful quantum computing devices, and our spin-refrigerating method
suggests that this problem can be resolved.Comment: 21 pages, 3 figure
Transcription factor search for a DNA promoter in a three-states model
To ensure fast gene activation, Transcription Factors (TF) use a mechanism
known as facilitated diffusion to find their DNA promoter site. Here we analyze
such a process where a TF alternates between 3D and 1D diffusion. In the latter
(TF bound to the DNA), the TF further switches between a fast translocation
state dominated by interaction with the DNA backbone, and a slow examination
state where interaction with DNA base pairs is predominant. We derive a new
formula for the mean search time, and show that it is faster and less sensitive
to the binding energy fluctuations compared to the case of a single sliding
state. We find that for an optimal search, the time spent bound to the DNA is
larger compared to the 3D time in the nucleus, in agreement with recent
experimental data. Our results further suggest that modifying switching via
phosphorylation or methylation of the TF or the DNA can efficiently regulate
transcription.Comment: 4 pages, 3 figure
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