1,775 research outputs found
Correction: Patients affected with Fabry disease have an increased incidence of progressive hearing loss and sudden deafness: an investigation of twenty-two hemizygous male patients
Topics in Quantum Computers
I provide an introduction to quantum computers, describing how they might be
realized using language accessible to a solid state physicist. A listing of the
minimal requirements for creating a quantum computer is given. I also discuss
several recent developments in the area of quantum error correction, a subject
of importance not only to quantum computation, but also to some aspects of the
foundations of quantum theory.Comment: 22 pages, Latex, 1 eps figure, Paper to be published in "Mesoscopic
Electron Transport", edited by L. Kowenhoven, G. Schoen and L. Sohn, NATO ASI
Series E, Kluwer Ac. Publ., Dordrecht. v2: typos in refrences fixe
Quantum Computing with Very Noisy Devices
In theory, quantum computers can efficiently simulate quantum physics, factor
large numbers and estimate integrals, thus solving otherwise intractable
computational problems. In practice, quantum computers must operate with noisy
devices called ``gates'' that tend to destroy the fragile quantum states needed
for computation. The goal of fault-tolerant quantum computing is to compute
accurately even when gates have a high probability of error each time they are
used. Here we give evidence that accurate quantum computing is possible with
error probabilities above 3% per gate, which is significantly higher than what
was previously thought possible. However, the resources required for computing
at such high error probabilities are excessive. Fortunately, they decrease
rapidly with decreasing error probabilities. If we had quantum resources
comparable to the considerable resources available in today's digital
computers, we could implement non-trivial quantum computations at error
probabilities as high as 1% per gate.Comment: 47 page
Efficient fault-tolerant quantum computing
Fault tolerant quantum computing methods which work with efficient quantum
error correcting codes are discussed. Several new techniques are introduced to
restrict accumulation of errors before or during the recovery. Classes of
eligible quantum codes are obtained, and good candidates exhibited. This
permits a new analysis of the permissible error rates and minimum overheads for
robust quantum computing. It is found that, under the standard noise model of
ubiquitous stochastic, uncorrelated errors, a quantum computer need be only an
order of magnitude larger than the logical machine contained within it in order
to be reliable. For example, a scale-up by a factor of 22, with gate error rate
of order , is sufficient to permit large quantum algorithms such as
factorization of thousand-digit numbers.Comment: 21 pages plus 5 figures. Replaced with figures in new format to avoid
problem
Quantum information can be negative
Given an unknown quantum state distributed over two systems, we determine how
much quantum communication is needed to transfer the full state to one system.
This communication measures the "partial information" one system needs
conditioned on it's prior information. It turns out to be given by an extremely
simple formula, the conditional entropy. In the classical case, partial
information must always be positive, but we find that in the quantum world this
physical quantity can be negative. If the partial information is positive, its
sender needs to communicate this number of quantum bits to the receiver; if it
is negative, the sender and receiver instead gain the corresponding potential
for future quantum communication. We introduce a primitive "quantum state
merging" which optimally transfers partial information. We show how it enables
a systematic understanding of quantum network theory, and discuss several
important applications including distributed compression, multiple access
channels and multipartite assisted entanglement distillation (localizable
entanglement). Negative channel capacities also receive a natural
interpretation
Gravitational Microlensing Evidence for a Planet Orbiting a Binary Star System
The study of extra-solar planetary systems has emerged as a new discipline of
observational astronomy in the past few years with the discovery of a number of
extra-solar planets. The properties of most of these extra-solar planets were
not anticipated by theoretical work on the formation of planetary systems. Here
we report observations and light curve modeling of gravitational microlensing
event MACHO-97-BLG-41, which indicates that the lens system consists of a
planet orbiting a binary star system. According to this model, the mass ratio
of the binary star system is 3.8:1 and the stars are most likely to be a late K
dwarf and an M dwarf with a separation of about 1.8 AU. A planet of about 3
Jupiter masses orbits this system at a distance of about 7 AU. If our
interpretation of this light curve is correct, it represents the first
discovery of a planet orbiting a binary star system and the first detection of
a Jovian planet via the gravitational microlensing technique. It suggests that
giant planets may be common in short period binary star systems.Comment: 11 pages, with 1 color and 2 b/w Figures included (published version
The Uncertainty Principle in the Presence of Quantum Memory
The uncertainty principle, originally formulated by Heisenberg, dramatically
illustrates the difference between classical and quantum mechanics. The
principle bounds the uncertainties about the outcomes of two incompatible
measurements, such as position and momentum, on a particle. It implies that one
cannot predict the outcomes for both possible choices of measurement to
arbitrary precision, even if information about the preparation of the particle
is available in a classical memory. However, if the particle is prepared
entangled with a quantum memory, a device which is likely to soon be available,
it is possible to predict the outcomes for both measurement choices precisely.
In this work we strengthen the uncertainty principle to incorporate this case,
providing a lower bound on the uncertainties which depends on the amount of
entanglement between the particle and the quantum memory. We detail the
application of our result to witnessing entanglement and to quantum key
distribution.Comment: 5 pages plus 12 of supplementary information. Updated to match the
journal versio
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
Experimental investigation of classical and quantum correlations under decoherence
It is well known that many operations in quantum information processing
depend largely on a special kind of quantum correlation, that is, entanglement.
However, there are also quantum tasks that display the quantum advantage
without entanglement. Distinguishing classical and quantum correlations in
quantum systems is therefore of both fundamental and practical importance. In
consideration of the unavoidable interaction between correlated systems and the
environment, understanding the dynamics of correlations would stimulate great
interest. In this study, we investigate the dynamics of different kinds of
bipartite correlations in an all-optical experimental setup. The sudden change
in behaviour in the decay rates of correlations and their immunity against
certain decoherences are shown. Moreover, quantum correlation is observed to be
larger than classical correlation, which disproves the early conjecture that
classical correlation is always greater than quantum correlation. Our
observations may be important for quantum information processing.Comment: 7 pages, 4 figures, to appear in Nature Communication
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