12,339 research outputs found
Simulations of closed timelike curves
Proposed models of closed timelike curves (CTCs) have been shown to enable
powerful information-processing protocols. We examine the simulation of models
of CTCs both by other models of CTCs and by physical systems without access to
CTCs. We prove that the recently proposed transition probability CTCs (T-CTCs)
are physically equivalent to postselection CTCs (P-CTCs), in the sense that one
model can simulate the other with reasonable overhead. As a consequence, their
information-processing capabilities are equivalent. We also describe a method
for quantum computers to simulate Deutschian CTCs (but with a reasonable
overhead only in some cases). In cases for which the overhead is reasonable, it
might be possible to perform the simulation in a table-top experiment. This
approach has the benefit of resolving some ambiguities associated with the
equivalent circuit model of Ralph et al. Furthermore, we provide an explicit
form for the state of the CTC system such that it is a maximum-entropy state,
as prescribed by Deutsch.Comment: 15 pages, 1 figure, accepted for publication in Foundations of
Physic
Extra Shared Entanglement Reduces Memory Demand in Quantum Convolutional Coding
We show how extra entanglement shared between sender and receiver reduces the
memory requirements for a general entanglement-assisted quantum convolutional
code. We construct quantum convolutional codes with good error-correcting
properties by exploiting the error-correcting properties of an arbitrary basic
set of Pauli generators. The main benefit of this particular construction is
that there is no need to increase the frame size of the code when extra shared
entanglement is available. Then there is no need to increase the memory
requirements or circuit complexity of the code because the frame size of the
code is directly related to these two code properties. Another benefit, similar
to results of previous work in entanglement-assisted convolutional coding, is
that we can import an arbitrary classical quaternary code for use as an
entanglement-assisted quantum convolutional code. The rate and error-correcting
properties of the imported classical code translate to the quantum code. We
provide an example that illustrates how to import a classical quaternary code
for use as an entanglement-assisted quantum convolutional code. We finally show
how to "piggyback" classical information to make use of the extra shared
entanglement in the code.Comment: 7 pages, 1 figure, accepted for publication in Physical Review
Quantum state cloning using Deutschian closed timelike curves
We show that it is possible to clone quantum states to arbitrary accuracy in
the presence of a Deutschian closed timelike curve (D-CTC), with a fidelity
converging to one in the limit as the dimension of the CTC system becomes
large---thus resolving an open conjecture from [Brun et al., Physical Review
Letters 102, 210402 (2009)]. This result follows from a D-CTC-assisted scheme
for producing perfect clones of a quantum state prepared in a known eigenbasis,
and the fact that one can reconstruct an approximation of a quantum state from
empirical estimates of the probabilities of an informationally-complete
measurement. Our results imply more generally that every continuous, but
otherwise arbitrarily non-linear map from states to states can be implemented
to arbitrary accuracy with D-CTCs. Furthermore, our results show that Deutsch's
model for CTCs is in fact a classical model, in the sense that two arbitrary,
distinct density operators are perfectly distinguishable (in the limit of a
large CTC system); hence, in this model quantum mechanics becomes a classical
theory in which each density operator is a distinct point in a classical phase
space.Comment: 6 pages, 1 figure; v2: modifications to the interpretation of our
results based on the insightful comments of the referees; v3: minor change,
accepted for publication in Physical Review Letter
Coherent Communication with Continuous Quantum Variables
The coherent bit (cobit) channel is a resource intermediate between classical
and quantum communication. It produces coherent versions of teleportation and
superdense coding. We extend the cobit channel to continuous variables by
providing a definition of the coherent nat (conat) channel. We construct
several coherent protocols that use both a position-quadrature and a
momentum-quadrature conat channel with finite squeezing. Finally, we show that
the quality of squeezing diminishes through successive compositions of coherent
teleportation and superdense coding.Comment: 4 pages, 3 figure
Is the solar convection zone in strict thermal wind balance?
Context: The solar rotation profile is conical rather than cylindrical as one
could expect from classical rotating fluid dynamics (e.g. Taylor-Proudman
theorem). Thermal coupling to the tachocline, baroclinic effects and
latitudinal transport of heat have been advocated to explain this peculiar
state of rotation. Aims: To test the validity of thermal wind balance in the
solar convection zone using helioseismic inversions for both the angular
velocity and fluctuations in entropy and temperature. Methods: Entropy and
temperature fluctuations obtained from 3-D hydrodynamical numerical simulations
of the solar convection zone are compared with solar profiles obtained from
helioseismic inversions. Results: The temperature and entropy fluctuations in
3-D numerical simulations have smaller amplitude in the bulk of the solar
convection zone than those found from seismic inversions. Seismic inversion
find variations of temperature from about 1 K at the surface up to 100 K at the
base of the convection zone while in 3-D simulations they are of order 10 K
throughout the convection zone up to 0.96 . In 3-D simulations,
baroclinic effects are found to be important to tilt the isocontours of
away from a cylindrical profile in most of the convection zone helped
by Reynolds and viscous stresses at some locations. By contrast the baroclinic
effect inverted by helioseismology are much larger than what is required to
yield the observed angular velocity profile. Conclusion: The solar convection
does not appear to be in strict thermal wind balance, Reynolds stresses must
play a dominant role in setting not only the equatorial acceleration but also
the observed conical angular velocity profile.Comment: 8 pages, 6 figures (low resolution), Accepted by Astronomy and
Astrophysics - Affiliation: (1) AIM, CEA/DSM-CNRS-Univ. Paris Diderot,
IRFU/SAp, France & (2) LUTH, Observatoire de Paris, CNRS-Univ. Paris Diderot,
France ; (3) Tata Institute of Fundamental Research, India; (4) Centre for
Basic Sciences, University of Mumbai, Indi
Polar cap magnetic field reversals during solar grand minima: could pores play a role?
We study the magnetic flux carried by pores located outside active regions
with sunspots and investigate their possible contribution to the reversal of
the global magnetic field of the Sun. We find that they contain a total flux of
comparable amplitude to the total magnetic flux contained in polar caps. The
pores located at distances of 40--100~Mm from the closest active region have
systematically the correct sign to contribute to the polar cap reversal. These
pores can predominantly be found in bipolar magnetic regions. We propose that
during grand minima of solar activity, such a systematic polarity trend, akin
to a weak magnetic (Babcock-Leighton-like) source term could still be operating
but was missed by the contemporary observers due to the limited resolving power
of their telescopes.Comment: 11 pages, 9 figures, accepted for publication in
Astronomy&Astrophysic
Achieving control of in-plane elastic waves
We derive the elastic properties of a cylindrical cloak for in-plane coupled
shear and pressure waves. The cloak is characterized by a rank 4 elasticity
tensor with 16 spatially varying entries which are deduced from a geometric
transform. Remarkably, the Navier equations retain their form under this
transform, which is generally untrue [Milton et al., New J. Phys. 8, 248
(2006)]. We numerically check that clamped and freely vibrating obstacles
located inside the neutral region are cloaked disrespectful of the frequency
and the polarization of an incoming elastic wave.Comment: 9 pages, 4 figure
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