284 research outputs found
Computation with narrow CTCs
We examine some variants of computation with closed timelike curves (CTCs),
where various restrictions are imposed on the memory of the computer, and the
information carrying capacity and range of the CTC. We give full
characterizations of the classes of languages recognized by polynomial time
probabilistic and quantum computers that can send a single classical bit to
their own past. Such narrow CTCs are demonstrated to add the power of limited
nondeterminism to deterministic computers, and lead to exponential speedup in
constant-space probabilistic and quantum computation. We show that, given a
time machine with constant negative delay, one can implement CTC-based
computations without the need to know about the runtime beforehand.Comment: 16 pages. A few typo was correcte
Field Theory And Second Renormalization Group For Multifractals In Percolation
The field-theory for multifractals in percolation is reformulated in such a
way that multifractal exponents clearly appear as eigenvalues of a second
renormalization group. The first renormalization group describes geometrical
properties of percolation clusters, while the second-one describes electrical
properties, including noise cumulants. In this context, multifractal exponents
are associated with symmetry-breaking fields in replica space. This provides an
explanation for their observability. It is suggested that multifractal
exponents are ''dominant'' instead of ''relevant'' since there exists an
arbitrary scale factor which can change their sign from positive to negative
without changing the Physics of the problem.Comment: RevTex, 10 page
Revisiting consistency conditions for quantum states of systems on closed timelike curves: an epistemic perspective
There has been considerable recent interest in the consequences of closed
timelike curves (CTCs) for the dynamics of quantum mechanical systems. A vast
majority of research into this area makes use of the dynamical equations
developed by Deutsch, which were developed from a consistency condition that
assumes that mixed quantum states uniquely describe the physical state of a
system. We criticise this choice of consistency condition from an epistemic
perspective, i.e., a perspective in which the quantum state represents a state
of knowledge about a system. We demonstrate that directly applying Deutsch's
condition when mixed states are treated as representing an observer's knowledge
of a system can conceal time travel paradoxes from the observer, rather than
resolving them. To shed further light on the appropriate dynamics for quantum
systems traversing CTCs, we make use of a toy epistemic theory with a strictly
classical ontology due to Spekkens and show that, in contrast to the results of
Deutsch, many of the traditional paradoxical effects of time travel are
present.Comment: 10 pages, 6 figures, comments welcome; v2 added references and
clarified some points; v3 published versio
On the universality of anomalous one-dimensional heat conductivity
In one and two dimensions, transport coefficients may diverge in the
thermodynamic limit due to long--time correlation of the corresponding
currents. The effective asymptotic behaviour is addressed with reference to the
problem of heat transport in 1d crystals, modeled by chains of classical
nonlinear oscillators. Extensive accurate equilibrium and nonequilibrium
numerical simulations confirm that the finite-size thermal conductivity
diverges with the system size as . However, the
exponent deviates systematically from the theoretical prediction
proposed in a recent paper [O. Narayan, S. Ramaswamy, Phys. Rev.
Lett. {\bf 89}, 200601 (2002)].Comment: 4 pages, submitted to Phys.Rev.
Universal control of quantum subspaces and subsystems
We describe a broad dynamical-algebraic framework for analyzing the quantum
control properties of a set of naturally available interactions. General
conditions under which universal control is achieved over a set of
subspaces/subsystems are found. All known physical examples of universal
control on subspaces/systems are related to the framework developed here.Comment: 4 Pages RevTeX, Some typos fixed, references adde
Perfect state distinguishability and computational speedups with postselected closed timelike curves
Bennett and Schumacher's postselected quantum teleportation is a model of
closed timelike curves (CTCs) that leads to results physically different from
Deutsch's model. We show that even a single qubit passing through a
postselected CTC (P-CTC) is sufficient to do any postselected quantum
measurement, and we discuss an important difference between "Deutschian" CTCs
(D-CTCs) and P-CTCs in which the future existence of a P-CTC might affect the
present outcome of an experiment. Then, based on a suggestion of Bennett and
Smith, we explicitly show how a party assisted by P-CTCs can distinguish a set
of linearly independent quantum states, and we prove that it is not possible
for such a party to distinguish a set of linearly dependent states. The power
of P-CTCs is thus weaker than that of D-CTCs because the Holevo bound still
applies to circuits using them regardless of their ability to conspire in
violating the uncertainty principle. We then discuss how different notions of a
quantum mixture that are indistinguishable in linear quantum mechanics lead to
dramatically differing conclusions in a nonlinear quantum mechanics involving
P-CTCs. Finally, we give explicit circuit constructions that can efficiently
factor integers, efficiently solve any decision problem in the intersection of
NP and coNP, and probabilistically solve any decision problem in NP. These
circuits accomplish these tasks with just one qubit traveling back in time, and
they exploit the ability of postselected closed timelike curves to create
grandfather paradoxes for invalid answers.Comment: 15 pages, 4 figures; Foundations of Physics (2011
Entanglement preparation using symmetric multiports
We investigate the entanglement produced by a multi-path interferometer that
is composed of two symmetric multiports, with phase shifts applied to the
output of the first multiport. Particular attention is paid to the case when we
have a single photon entering the interferometer. For this situation we derive
a simple condition that characterize the types of entanglement that one can
generate. We then show how one can use the results from the single photon case
to determine what kinds of multi-photon entangled states one can prepare using
the interferometer.Comment: 6 pages, 2 figures, accepted for publication in European Journal of
Physics
Universal quantum control in irreducible state-space sectors: application to bosonic and spin-boson systems
We analyze the dynamical-algebraic approach to universal quantum control
introduced in P. Zanardi, S. Lloyd, quant-ph/0305013. The quantum state-space
encoding information decomposes into irreducible sectors and
subsystems associated to the group of available evolutions. If this group
coincides with the unitary part of the group-algebra \CC{\cal K} of some
group then universal control is achievable over the -irreducible components of . This general strategy is applied to
different kind of bosonic systems. We first consider massive bosons in a
double-well and show how to achieve universal control over all
finite-dimensional
Fock sectors. We then discuss a multi-mode massless case giving the
conditions for generating the whole infinite-dimensional multi-mode
Heisenberg-Weyl enveloping-algebra. Finally we show how to use an auxiliary
bosonic mode coupled to finite-dimensional systems to generate high-order
non-linearities needed for universal control.Comment: 10 pages, LaTeX, no figure
Entanglement preparation using symmetric multiports
We investigate the entanglement produced by a multi-path interferometer that
is composed of two symmetric multiports, with phase shifts applied to the
output of the first multiport. Particular attention is paid to the case when we
have a single photon entering the interferometer. For this situation we derive
a simple condition that characterize the types of entanglement that one can
generate. We then show how one can use the results from the single photon case
to determine what kinds of multi-photon entangled states one can prepare using
the interferometer.Comment: 6 pages, 2 figures, accepted for publication in European Journal of
Physics
Electromagnetic Fields and Charged Particle Motion Around Magnetized Wormholes
We perform a study to describe motion of charged particles under the
influence of electromagnetic and gravitational fields of a slowly rotating
wormhole with nonvanishing magnetic moment. We present analytic expression for
potentials of electromagnetic field for an axially symmetric slowly rotating
magnetized wormholes. While addressing important issues regarding the subject,
we compare our results of motion around black holes and wormholes in terms of
the ratio of radii of event horizons of a black hole and of the throat of a
wormhole. It is shown that both radial and circular motions of test bodies in
the vicinity of a magnetized wormhole could give rise to a peculiar
observational astrophysical phenomenon.Comment: 9 pages, 4 figures, 2 tables, accepted for publication in
Astrophysics & Space Scienc
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