165,218 research outputs found
Quantum Information Processing and Relativistic Quantum Fields
It is shown that an ideal measurement of a one-particle wave packet state of
a relativistic quantum field in Minkowski spacetime enables superluminal
signalling. The result holds for a measurement that takes place over an
intervention region in spacetime whose extent in time in some frame is longer
than the light-crossing time of the packet in that frame. Moreover, these
results are shown to apply not only to ideal measurements but also to unitary
transformations that rotate two orthogonal one-particle states into each other.
In light of these observations, possible restrictions on the allowed types of
intervention are considered. A more physical approach to such questions is to
construct explicit models of the interventions as interactions between the
field and other quantum systems such as detectors. The prototypical
Unruh-DeWitt detector couples to the field operator itself and so most likely
respects relativistic causality. On the other hand, detector models which
couple to a finite set of frequencies of field modes are shown to lead to
superluminal signalling. Such detectors do, however, provide successful
phenomenological models of atom-qubits interacting with quantum fields in a
cavity but are valid only on time scales many orders of magnitude larger than
the light-crossing time of the cavity.Comment: 16 pages, 2 figures. Improved abstract and discussion of 'ideal'
measurements. References to previous work adde
Computational capacity of the universe
Merely by existing, all physical systems register information. And by
evolving dynamically in time, they transform and process that information. The
laws of physics determine the amount of information that a physical system can
register (number of bits) and the number of elementary logic operations that a
system can perform (number of ops). The universe is a physical system. This
paper quantifies the amount of information that the universe can register and
the number of elementary operations that it can have performed over its
history. The universe can have performed no more than ops on
bits.Comment: 17 pages, TeX. submitted to Natur
Information, information processing and gravity
I discuss fundamental limits placed on information and information processing
by gravity. Such limits arise because both information and its processing
require energy, while gravitational collapse (formation of a horizon or black
hole) restricts the amount of energy allowed in a finite region. Specifically,
I use a criterion for gravitational collapse called the hoop conjecture. Once
the hoop conjecture is assumed a number of results can be obtained directly:
the existence of a fundamental uncertainty in spatial distance of order the
Planck length, bounds on information (entropy) in a finite region, and a bound
on the rate of information processing in a finite region. In the final section
I discuss some cosmological issues related to the total amount of information
in the universe, and note that almost all detailed aspects of the late universe
are determined by the randomness of quantum outcomes. This paper is based on a
talk presented at a 2007 Bellairs Research Institute (McGill University)
workshop on black holes and quantum information.Comment: 7 pages, 5 figures, revte
Quantum cryptography: key distribution and beyond
Uniquely among the sciences, quantum cryptography has driven both
foundational research as well as practical real-life applications. We review
the progress of quantum cryptography in the last decade, covering quantum key
distribution and other applications.Comment: It's a review on quantum cryptography and it is not restricted to QK
Generation and manipulation of nonclassical light using photonic crystals
Photonic crystal cavities can localize light into nanoscale volumes with high
quality factors. This permits a strong interaction between light and matter,
which is important for the construction of classical light sources with
improved properties (e.g., low threshold lasers) and of nonclassical light
sources (such as single and entangled photon sources) that are crucial pieces
of hardware of quantum information processing systems. This article will review
some of our recent experimental and theoretical results on the interaction
between single quantum dots and photonic crystal cavity fields, and on the
integration of multiple photonic crystal devices into functional circuits for
quantum information processing.Comment: 6 pages, 6 figures; replaced with revised versio
Physics and computer science: quantum computation and other approaches
This is a position paper written as an introduction to the special volume on
quantum algorithms I edited for the journal Mathematical Structures in Computer
Science (Volume 20 - Special Issue 06 (Quantum Algorithms), 2010)
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