10,667 research outputs found
Entropy exchange and entanglement in the Jaynes-Cummings model
The Jaynes-Cummings model is the simplest fully quantum model that describes
the interaction between light and matter. We extend a previous analysis by
Phoenix and Knight (S. J. D. Phoenix, P. L. Knight, Annals of Physics 186,
381). of the JCM by considering mixed states of both the light and matter. We
present examples of qualitatively different entropic correlations. In
particular, we explore the regime of entropy exchange between light and matter,
i.e. where the rate of change of the two are anti-correlated. This behavior
contrasts with the case of pure light-matter states in which the rate of change
of the two entropies are positively correlated and in fact identical. We give
an analytical derivation of the anti-correlation phenomenon and discuss the
regime of its validity. Finally, we show a strong correlation between the
region of the Bloch sphere characterized by entropy exchange and that
characterized by minimal entanglement as measured by the negative eigenvalues
of the partially transposed density matrix.Comment: 8 pages, 5 figure
Thermodynamic cost of reversible computing
Since reversible computing requires preservation of all information
throughout the entire computational process, this implies that all errors that
appear as a result of the interaction of the information-carrying system with
uncontrolled degrees of freedom must be corrected. But this can only be done at
the expense of an increase in the entropy of the environment corresponding to
the dissipation, in the form of heat, of the ``noisy'' part of the system's
energy.
This paper gives an expression of that energy in terms of the effective noise
temperature, and analyzes the relationship between the energy dissipation rate
and the rate of computation. Finally, a generalized Clausius principle based on
the concept of effective temperature is presented.Comment: 5 pages; added two paragraphs and fixed a number of typo
Deterministic Quantum Key Distribution Using Gaussian-Modulated Squeezed States
A continuous variable ping-pong scheme, which is utilized to generate
deterministically private key, is proposed. The proposed scheme is implemented
physically by using Gaussian-modulated squeezed states. The deterministic way,
i.e., no basis reconciliation between two parties, leads a two-times efficiency
comparing to the standard quantum key distribution schemes. Especially, the
separate control mode does not need in the proposed scheme so that it is
simpler and more available than previous ping-pong schemes. The attacker may be
detected easily through the fidelity of the transmitted signal, and may not be
successful in the beam splitter attack strategy.Comment: 7 pages, 4figure
Quantum Cryptography Approaching the Classical Limit
We consider the security of continuous-variable quantum cryptography as we
approach the classical-limit, i.e., when the unknown preparation noise at the
sender's station becomes significantly noisy or thermal (even by as much as
10,000 times the variance of the vacuum mode). We show that, provided the
channel transmission losses do not exceed 50%, the security of quantum
cryptography is not dependent on the channel transmission, and is therefore,
incredibly robust against significant amounts of excess preparation noise. We
extend these results to consider for the first time quantum cryptography at
wavelengths considerably longer than optical and find that regions of security
still exist all the way down to the microwave.Comment: Letter (4 pages) followed by appendix (4 pages). Updated from
published version with some minor correction
Structures and Electromagnetic Properties of New Metal-Ordered Manganites; RBaMn_{2}O_{6} (R = Y and Rare Earth Elements)
New metal-ordered manganites RBaMn_{2}O_{6} have been synthesized and
investigated in the structures and electromagnetic properties. RBaMn_{2}O_{6}
can be classified into three groups from the structural and electromagnetic
properties. The first group (R = La, Pr and Nd) has a metallic ferromagnetic
transition, followed by an A-type antiferromagnetic transition in
PrBaMn_{2}O_{6}. The second group (R = Sm, Eu and Gd) exhibits a charge-order
transition, followed by an antiferromagnetic long range ordering. The third
group (R = Tb, Dy and Ho) shows successive three phase transitions, the
structural, charge/orbital-order and magnetic transitions, as observed in
YBaMn_{2}O_{6}. Comparing to the metal-disordered manganites
(R^{3+}_{0.5}A^{2+}_{0.5})MnO_{3}, two remarkable features can be recognized in
RBaMn_{2}O_{6}; (1) relatively high charge-order transition temperature and (2)
the presence of structural transition above the charge-order temperature in the
third group. We propose a possible orbital ordering at the structural
transition, that is a possible freezing of the orbital, charge and spin degrees
of freedom at the independent temperatures in the third group. These features
are closely related to the peculiar structure that the MnO_{2} square-lattice
is sandwiched by the rock-salt layers of two kinds, RO and BaO with extremely
different lattice-sizes.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Continuous-Variable Quantum Key Distribution using Thermal States
We consider the security of continuous-variable quantum key distribution
using thermal (or noisy) Gaussian resource states. Specifically, we analyze
this against collective Gaussian attacks using direct and reverse
reconciliation where both protocols use either homodyne or heterodyne
detection. We show that in the case of direct reconciliation with heterodyne
detection, an improved robustness to channel noise is achieved when large
amounts of preparation noise is added, as compared to the case when no
preparation noise is added. We also consider the theoretical limit of infinite
preparation noise and show a secure key can still be achieved in this limit
provided the channel noise is less than the preparation noise. Finally, we
consider the security of quantum key distribution at various electromagnetic
wavelengths and derive an upper bound related to an entanglement-breaking
eavesdropping attack and discuss the feasibility of microwave quantum key
distribution.Comment: 12 pages, 11 figures. Updated from published version with some minor
correction
Security of coherent state quantum cryptography in the presence of Gaussian noise
We investigate the security against collective attacks of a continuous
variable quantum key distribution scheme in the asymptotic key limit for a
realistic setting. The quantum channel connecting the two honest parties is
assumed to be lossy and imposes Gaussian noise on the observed quadrature
distributions. Secret key rates are given for direct and reverse reconciliation
schemes including postselection in the collective attack scenario. The effect
of a non-ideal error correction and two-way communication in the classical
post-processing step is also taken into account.Comment: 12 pages, 5 figures updated version including two-way communication;
changed the definition of the excess noise to match the definition given
earlier (Phys. Rev. Lett. 92, 117901); submitted to PRA; presented at the 8th
International Conference on Quantum Communication, Measurement and Computing,
Tsukub
Nearest neighbor embedding with different time delays
A nearest neighbor based selection of time delays for phase space
reconstruction is proposed and compared to the standard use of time delayed
mutual information. The possibility of using different time delays for
consecutive dimensions is considered. A case study of numerically generated
solutions of the Lorenz system is used for illustration. The effect of
contamination with various levels of additive Gaussian white noise is
discussed.Comment: 4 pages, 5 figures, updated to final versio
Thermodynamic time asymmetry in nonequilibrium fluctuations
We here present the complete analysis of experiments on driven Brownian
motion and electric noise in a circuit, showing that thermodynamic entropy
production can be related to the breaking of time-reversal symmetry in the
statistical description of these nonequilibrium systems. The symmetry breaking
can be expressed in terms of dynamical entropies per unit time, one for the
forward process and the other for the time-reversed process. These entropies
per unit time characterize dynamical randomness, i.e., temporal disorder, in
time series of the nonequilibrium fluctuations. Their difference gives the
well-known thermodynamic entropy production, which thus finds its origin in the
time asymmetry of dynamical randomness, alias temporal disorder, in systems
driven out of equilibrium.Comment: to be published in : Journal of Statistical Mechanics: theory and
experimen
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