49,847 research outputs found
Fast Polarization for Processes with Memory
Fast polarization is crucial for the performance guarantees of polar codes.
In the memoryless setting, the rate of polarization is known to be exponential
in the square root of the block length. A complete characterization of the rate
of polarization for models with memory has been missing. Namely, previous works
have not addressed fast polarization of the high entropy set under memory. We
consider polar codes for processes with memory that are characterized by an
underlying ergodic finite-state Markov chain. We show that the rate of
polarization for these processes is the same as in the memoryless setting, both
for the high and for the low entropy sets.Comment: 17 pages, 3 figures. Submitted to IEEE Transactions on Information
Theor
Finite temperature effects on spin polarization of neutron matter in a strong magnetic field
Spin polarization of neutron matter at finite temperatures and strong
magnetic fields up to G is studied in the model with the Skyrme
effective interaction. It is shown that, together with the thermodynamically
stable branch of solutions for the spin polarization parameter corresponding to
the case when the majority of neutron spins are oriented opposite to the
direction of the magnetic field (negative spin polarization), the
self-consistent equations, beginning from some threshold density, have also two
other branches of solutions corresponding to positive spin polarization. The
influence of finite temperatures on spin polarization remains moderate in the
Skyrme model up to temperatures relevant for protoneutron stars, and, in
particular, the scenario with the metastable state characterized by positive
spin polarization, considered at zero temperature in Phys. Rev. C {\bf 80},
065801 (2009), is preserved at finite temperatures as well. It is shown that
above certain density the entropy for various branches of spin polarization in
neutron matter with the Skyrme interaction in a strong magnetic field
demonstrates the unusual behavior being larger than that of the nonpolarized
state. By providing the corresponding low-temperature analysis, it is clarified
that this unexpected behavior should be addressed to the dependence of the
entropy of a spin polarized state on the effective masses of neutrons with spin
up and spin down, and to a certain constraint on them which is violated in the
respective density range.Comment: Prepared with RevTeX4, 6pp., 4 figs; v2: accepted in JKA
Demonstration of an optical-coherence converter
Studying the coherence of an optical field is typically compartmentalized
with respect to its different optical degrees of freedom (DoFs) -- spatial,
temporal, and polarization. Although this traditional approach succeeds when
the DoFs are uncoupled, it fails at capturing key features of the field's
coherence if the DOFs are indeed correlated -- a situation that arises often.
By viewing coherence as a `resource' that can be shared among the DoFs, it
becomes possible to convert the entropy associated with the fluctuations in one
DoF to another DoF that is initially fluctuation-free. Here, we verify
experimentally that coherence can indeed be reversibly exchanged -- without
loss of energy -- between polarization and the spatial DoF of a partially
coherent field. Starting from a linearly polarized spatially incoherent field
-- one that produces no spatial interference fringes -- we obtain a spatially
coherent field that is unpolarized. By reallocating the entropy to
polarization, the field becomes invariant with regards to the action of a
polarization scrambler, thus suggesting a strategy for avoiding the deleterious
effects of a randomizing system on a DoF of the optical field.Comment: 7 pages; 6 figure
Determination of optimal reversed field with maximal electrocaloric cooling by a direct entropy analysis
Application of a negative field on a positively poled ferroelectric sample
can enhance the electrocaloric cooling and appears as a promising method to
optimize the electrocaloric cycle. Experimental measurements show that the
maximal cooling does not appear at the zero-polarization point, but around the
shoulder of the P-E loop. This phenomenon cannot be explained by the theory
based on the constant total entropy assumption under adiabatic condition. In
fact, adiabatic condition does not imply constant total entropy when
irreversibility is involved. A direct entropy analysis approach based on work
loss is proposed in this work, which takes the entropy contribution of the
irreversible process into account. The optimal reversed field determined by
this approach agrees with the experimental observations. This study signifies
the importance of considering the irreversible process in the electrocaloric
cycles
Numerical evidence of the double-Griffiths phase of the random quantum Ashkin-Teller chain
The random quantum Ashkin-Teller chain is studied numerically by means of
time-dependent Density-Matrix Renormalization Group. The critical lines are
estimated as the location of the peaks of the integrated autocorrelation times,
computed from spin-spin and polarization-polarization autocorrelation
functions. Disorder fluctuations of magnetization and polarization are observed
to be maximum on these critical lines. Entanglement entropy leads to the same
phase diagram, though with larger Finite-Size effects. The decay of spin-spin
and polarization-polarization autocorrelation functions provides numerical
evidence of the existence of a double Griffiths phase when taking into account
finite-size effects. The two associated dynamical exponents z increase rapidly
as the critical lines are approached, in agreement with the recent conjecture
of a divergence at the two transitions in the thermodynamic limit
Secure self-calibrating quantum random bit generator
Random bit generators (RBGs) are key components of a variety of information
processing applications ranging from simulations to cryptography. In
particular, cryptographic systems require "strong" RBGs that produce
high-entropy bit sequences, but traditional software pseudo-RBGs have very low
entropy content and therefore are relatively weak for cryptography. Hardware
RBGs yield entropy from chaotic or quantum physical systems and therefore are
expected to exhibit high entropy, but in current implementations their exact
entropy content is unknown. Here we report a quantum random bit generator
(QRBG) that harvests entropy by measuring single-photon and entangled
two-photon polarization states. We introduce and implement a quantum
tomographic method to measure a lower bound on the "min-entropy" of the system,
and we employ this value to distill a truly random bit sequence. This approach
is secure: even if an attacker takes control of the source of optical states, a
secure random sequence can be distilled.Comment: 5 pages, 2 figure
Quantum entanglement in strong-field ionization
We investigate the time-evolution of quantum entanglement between an
electron, liberated by a strong few-cycle laser pulse, and its parent ion-core.
Since the standard procedure is numerically prohibitive in this case, we
propose a novel way to quantify the quantum correlation in such a system: we
use the reduced density matrices of the directional subspaces along the
polarization of the laser pulse and along the transverse directions as building
blocks for an approximate entanglement entropy. We present our results, based
on accurate numerical simulations, in terms of several of these entropies, for
selected values of the peak electric field strength and the carrier-envelope
phase difference of the laser pulse. The time evolution of the mutual entropy
of the electron and the ion-core motion along the direction of the laser
polarization is similar to our earlier results based on a simple
one-dimensional model. However, taking into account also the dynamics
perpendicular to the laser polarization reveals a surprisingly different
entanglement dynamics above the laser intensity range corresponding to pure
tunneling: the quantum entanglement decreases with time in the over-the-barrier
ionization regime
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