5,831 research outputs found
Black holes and wormholes subject to conformal mappings
Solutions of the field equations of theories of gravity which admit distinct
conformal frame representations can look very different in these frames. We
show that Brans class IV solutions describe wormholes in the Jordan frame (in a
certain parameter range) but correspond to horizonless geometries in the
Einstein frame. The reasons for such a change of behaviour under conformal
mappings are elucidated in general, using Brans IV solutions as an example.Comment: 7 pages, 2 figure
The Quantum Frontier
The success of the abstract model of computation, in terms of bits, logical
operations, programming language constructs, and the like, makes it easy to
forget that computation is a physical process. Our cherished notions of
computation and information are grounded in classical mechanics, but the
physics underlying our world is quantum. In the early 80s researchers began to
ask how computation would change if we adopted a quantum mechanical, instead of
a classical mechanical, view of computation. Slowly, a new picture of
computation arose, one that gave rise to a variety of faster algorithms, novel
cryptographic mechanisms, and alternative methods of communication. Small
quantum information processing devices have been built, and efforts are
underway to build larger ones. Even apart from the existence of these devices,
the quantum view on information processing has provided significant insight
into the nature of computation and information, and a deeper understanding of
the physics of our universe and its connections with computation.
We start by describing aspects of quantum mechanics that are at the heart of
a quantum view of information processing. We give our own idiosyncratic view of
a number of these topics in the hopes of correcting common misconceptions and
highlighting aspects that are often overlooked. A number of the phenomena
described were initially viewed as oddities of quantum mechanics. It was
quantum information processing, first quantum cryptography and then, more
dramatically, quantum computing, that turned the tables and showed that these
oddities could be put to practical effect. It is these application we describe
next. We conclude with a section describing some of the many questions left for
future work, especially the mysteries surrounding where the power of quantum
information ultimately comes from.Comment: Invited book chapter for Computation for Humanity - Information
Technology to Advance Society to be published by CRC Press. Concepts
clarified and style made more uniform in version 2. Many thanks to the
referees for their suggestions for improvement
Are quantization rules for horizon areas universal?
Doubts have been expressed on the universality of holographic/string-inspired
quantization rules for the horizon areas of stationary black holes or the
products of their radii, already in simple 4-dimensional general relativity.
Realistic black holes are not stationary but time-dependent. Using two examples
of 4D general-relativistic spacetimes containing dynamical black holes for at
least part of the time, it is shown that the quantization rules (even counting
virtual horizons) cannot hold, except possibly at isolated instants of time,
and do not seem to be universal.Comment: One example and one figure added, two figures improved, bibliography
expanded and updated. Matches the version accepted for publication in Phys.
Rev.
Probing Quantum Optical Excitations with Fast Electrons
Probing optical excitations with nanometer resolution is important for
understanding their dynamics and interactions down to the atomic scale.
Electron microscopes currently offer the unparalleled ability of rendering
spatially-resolved electron spectra with combined meV and sub-nm resolution,
while the use of ultrafast optical pulses enables fs temporal resolution and
exposure of the electrons to ultraintense confined optical fields. Here, we
theoretically investigate fundamental aspects of the interaction of fast
electrons with localized optical modes that are made possible by these
advances. We use a quantum-optics description of the optical field to predict
that the resulting electron spectra strongly depend on the statistics of the
sample excitations (bosonic or fermionic) and their population (Fock, coherent,
or thermal), whose autocorrelation functions are directly retrieved from the
ratios of electron gain intensities. We further explore feasible experimental
scenarios to probe the quantum characteristics of the sampled excitations and
their populations.Comment: 13 pages, 6 figures, 56 reference
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