3,063 research outputs found
Prevalence of Helicobacter pylori in patients with gastro-oesophageal reflux disease : systematic review.
Objectives: To ascertain the prevalence of Helicobacter pylori in patients with gastro-oesophageal reflux disease and its association with the disease.
Design: Systematic review of studies reporting the prevalence of H pylori in patients with and without gastro-oesophageal reflux disease.
Data sources: Four electronic databases, searched to November 2001, experts, pharmaceutical companies, and journals.
Main outcome measure: Odds ratio for prevalence of H pylori in patients with gastro-oesophageal reflux disease.
Results: 20 studies were included. The pooled estimate of the odds ratio for prevalence of H pylori was 0.60 (95% confidence interval 0.47 to 0.78), indicating a lower prevalence in patients with gastro-oesophageal reflux disease. Substantial heterogeneity was observed between studies. Location seemed to be an important factor, with a much lower prevalence of H pylori in patients with gastro-oesophageal reflux disease in studies from the Far East, despite a higher overall prevalence of infection than western Europe and North America. Year of study was not a source of heterogeneity.
Conclusion: The prevalence of H pylori infection was significantly lower in patients with than without gastro-oesophageal reflux, with geographical location being a strong contributor to the heterogeneity between studies. Patients from the Far East with reflux disease had a lower prevalence of H pylori infection than patients from western Europe and North America, despite a higher prevalence in the general population
Quantum rejection sampling
Rejection sampling is a well-known method to sample from a target
distribution, given the ability to sample from a given distribution. The method
has been first formalized by von Neumann (1951) and has many applications in
classical computing. We define a quantum analogue of rejection sampling: given
a black box producing a coherent superposition of (possibly unknown) quantum
states with some amplitudes, the problem is to prepare a coherent superposition
of the same states, albeit with different target amplitudes. The main result of
this paper is a tight characterization of the query complexity of this quantum
state generation problem. We exhibit an algorithm, which we call quantum
rejection sampling, and analyze its cost using semidefinite programming. Our
proof of a matching lower bound is based on the automorphism principle which
allows to symmetrize any algorithm over the automorphism group of the problem.
Our main technical innovation is an extension of the automorphism principle to
continuous groups that arise for quantum state generation problems where the
oracle encodes unknown quantum states, instead of just classical data.
Furthermore, we illustrate how quantum rejection sampling may be used as a
primitive in designing quantum algorithms, by providing three different
applications. We first show that it was implicitly used in the quantum
algorithm for linear systems of equations by Harrow, Hassidim and Lloyd.
Secondly, we show that it can be used to speed up the main step in the quantum
Metropolis sampling algorithm by Temme et al.. Finally, we derive a new quantum
algorithm for the hidden shift problem of an arbitrary Boolean function and
relate its query complexity to "water-filling" of the Fourier spectrum.Comment: 19 pages, 5 figures, minor changes and a more compact style (to
appear in proceedings of ITCS 2012
Distinguishing n Hamiltonians on C^n by a single measurement
If an experimentalist wants to decide which one of n possible Hamiltonians
acting on an n dimensional Hilbert space is present, he can conjugate the time
evolution by an appropriate sequence of known unitary transformations in such a
way that the different Hamiltonians result in mutual orthogonal final states.
We present a general scheme providing such a sequence.Comment: 4 pages, Revte
Effects of Noise, Correlations and errors in the preparation of initial states in Quantum Simulations
In principle a quantum system could be used to simulate another quantum
system. The purpose of such a simulation would be to obtain information about
problems which cannot be simulated with a classical computer due to the
exponential increase of the Hilbert space with the size of the system and which
cannot be measured or controlled in an actual experiment. The system will
interact with the surrounding environment, with the other particles in the
system and be implemented using imperfect controls making it subject to noise.
It has been suggested that noise does not need to be controlled to the same
extent as it must be for general quantum computing. However the effects of
noise in quantum simulations and how to treat them are not completely
understood. In this paper we study an existing quantum algorithm for the
one-dimensional Fano-Anderson model to be simulated using a liquid-state NMR
device. We calculate the evolution of different initial states in the original
model, and then we add interacting spins to simulate a more realistic
situation. We find that states which are entangled with their environment, and
sometimes correlated but not necessarily entangled have an evolution which is
described by maps which are not completely positive. We discuss the conditions
for this to occur and also the implications.Comment: Revtex 4-1, 14 pages, 21 figures, version 2 has typos corrected and
acknowledgement adde
Spatial search by quantum walk
Grover's quantum search algorithm provides a way to speed up combinatorial
search, but is not directly applicable to searching a physical database.
Nevertheless, Aaronson and Ambainis showed that a database of N items laid out
in d spatial dimensions can be searched in time of order sqrt(N) for d>2, and
in time of order sqrt(N) poly(log N) for d=2. We consider an alternative search
algorithm based on a continuous time quantum walk on a graph. The case of the
complete graph gives the continuous time search algorithm of Farhi and Gutmann,
and other previously known results can be used to show that sqrt(N) speedup can
also be achieved on the hypercube. We show that full sqrt(N) speedup can be
achieved on a d-dimensional periodic lattice for d>4. In d=4, the quantum walk
search algorithm takes time of order sqrt(N) poly(log N), and in d<4, the
algorithm does not provide substantial speedup.Comment: v2: 12 pages, 4 figures; published version, with improved arguments
for the cases where the algorithm fail
Quantum walk on a line for a trapped ion
We show that a multi-step quantum walk can be realized for a single trapped
ion with interpolation between quantum and random walk achieved by randomizing
the generalized Hadamard coin flip phase. The signature of the quantum walk is
manifested not only in the ion's position but also its phonon number, which
makes an ion trap implementation of the quantum walk feasible.Comment: 5 pages, 3 figure
Distribution of chirality in the quantum walk: Markov process and entanglement
The asymptotic behavior of the quantum walk on the line is investigated
focusing on the probability distribution of chirality independently of
position. The long-time limit of this distribution is shown to exist and to
depend on the initial conditions, and it also determines the asymptotic value
of the entanglement between the coin and the position. It is shown that for
given asymptotic values of both the entanglement and the chirality distribution
it is possible to find the corresponding initial conditions within a particular
class of spatially extended Gaussian distributions. Moreover it is shown that
the entanglement also measures the degree of Markovian randomness of the
distribution of chirality.Comment: 5 pages, 3 figures, It was accepted in Physcial Review
Noise resistance of adiabatic quantum computation using random matrix theory
Besides the traditional circuit-based model of quantum computation, several
quantum algorithms based on a continuous-time Hamiltonian evolution have
recently been introduced, including for instance continuous-time quantum walk
algorithms as well as adiabatic quantum algorithms. Unfortunately, very little
is known today on the behavior of these Hamiltonian algorithms in the presence
of noise. Here, we perform a fully analytical study of the resistance to noise
of these algorithms using perturbation theory combined with a theoretical noise
model based on random matrices drawn from the Gaussian Orthogonal Ensemble,
whose elements vary in time and form a stationary random process.Comment: 9 pages, 3 figure
Distribution of Interference in the Presence of Decoherence
We study the statistics of quantum interference for completely positive maps.
We calculate analytically the mean interference and its second moment for
finite dimensional quantum systems interacting with a simple environment
consisting of one or several spins (qudits). The joint propagation of the
entire system is taken as unitary with an evolution operator drawn from the
Circular Unitary Ensemble (CUE). We show that the mean interference decays with
a power law as function of the dimension of the Hilbert space of the
environment, with a power that depends on the temperature of the environment.Comment: 28 pages of pd
Scaling of running time of quantum adiabatic algorithm for propositional satisfiability
We numerically study quantum adiabatic algorithm for the propositional
satisfiability. A new class of previously unknown hard instances is identified
among random problems. We numerically find that the running time for such
instances grows exponentially with their size. Worst case complexity of quantum
adiabatic algorithm therefore seems to be exponential.Comment: 7 page
- …