2,111 research outputs found
A spin-boson thermal rectifier
Rectification of heat transfer in nanodevices can be realized by combining
the system inherent anharmonicity with structural asymmetry. we analyze this
phenomenon within the simplest anharmonic system -a spin-boson nanojunction
model. We consider two variants of the model that yield, for the first time,
analytical solutions: a linear separable model in which the heat reservoirs
contribute additively, and a non-separable model suitable for a stronger
system-bath interaction. Both models show asymmetric (rectifying) heat
conduction when the couplings to the heat reservoirs are different.Comment: 5 pages, 3 figures, RevTeX
Cavity QED in superconducting circuits: susceptibility at elevated temperatures
We study the properties of superconducting electrical circuits, realizing
cavity QED. In particular we explore the limit of strong coupling, low
dissipation, and elevated temperatures relevant for current and future
experiments. We concentrate on the cavity susceptibility as it can be directly
experimentally addressed, i.e., as the impedance or the reflection coefficient
of the cavity. To this end we investigate the dissipative Jaynes-Cummings model
in the strong coupling regime at high temperatures. The dynamics is
investigated within the Bloch-Redfield formalism. At low temperatures, when
only the few lowest levels are occupied the susceptibility can be presented as
a sum of contributions from independent level-to-level transitions. This
corresponds to the secular (random phase) approximation in the Bloch-Redfield
formalism. At temperatures comparable to and higher than the oscillator
frequency, many transitions become important and a multiple-peak structure
appears. We show that in this regime the secular approximation breaks down, as
soon as the peaks start to overlap. In other words, the susceptibility is no
longer a sum of contributions from independent transitions. We treat the
dynamics of the system numerically by exact diagonalization of the Hamiltonian
of the qubit plus up to 200 states of the oscillator. We compare the results
obtained with and without the secular approximation and find a qualitative
discrepancy already at moderate temperatures.Comment: 7 pages, 6 figure
Time-convolutionless master equation for quantum dots: Perturbative expansion to arbitrary order
The master equation describing the non-equilibrium dynamics of a quantum dot
coupled to metallic leads is considered. Employing a superoperator approach, we
derive an exact time-convolutionless master equation for the probabilities of
dot states, i.e., a time-convolutionless Pauli master equation. The generator
of this master equation is derived order by order in the hybridization between
dot and leads. Although the generator turns out to be closely related to the
T-matrix expressions for the transition rates, which are plagued by
divergences, in the time-convolutionless generator all divergences cancel order
by order. The time-convolutionless and T-matrix master equations are contrasted
to the Nakajima-Zwanzig version. The absence of divergences in the
Nakajima-Zwanzig master equation due to the nonexistence of secular reducible
contributions becomes rather transparent in our approach, which explicitly
projects out these contributions. We also show that the time-convolutionless
generator contains the generator of the Nakajima-Zwanzig master equation in the
Markov approximation plus corrections, which we make explicit. Furthermore, it
is shown that the stationary solutions of the time-convolutionless and the
Nakajima-Zwanzig master equations are identical. However, this identity neither
extends to perturbative expansions truncated at finite order nor to dynamical
solutions. We discuss the conditions under which the Nakajima-Zwanzig-Markov
master equation nevertheless yields good results.Comment: 13 pages + appendice
Excitation energy transfer: Study with non-Markovian dynamics
In this paper, we investigate the non-Markovian dynamics of a model to mimic
the excitation energy transfer (EET) between chromophores in photosynthesis
systems. The numerical path integral method is used. This method includes the
non-Markovian effects of the environmental affects and it does not need the
perturbation approximation in solving the dynamics of systems of interest. It
implies that the coherence helps the EET between chromophores through lasting
the transfer time rather than enhances the transfer rate of the EET. In
particular, the non-Markovian environment greatly increase the efficiency of
the EET in the photosynthesis systems.Comment: 5 pages, 5 figure
Multiple-time correlation functions for non-Markovian interaction: Beyond the Quantum Regression Theorem
Multiple time correlation functions are found in the dynamical description of
different phenomena. They encode and describe the fluctuations of the dynamical
variables of a system. In this paper we formulate a theory of non-Markovian
multiple-time correlation functions (MTCF) for a wide class of systems. We
derive the dynamical equation of the {\it reduced propagator}, an object that
evolve state vectors of the system conditioned to the dynamics of its
environment, which is not necessarily at the vacuum state at the initial time.
Such reduced propagator is the essential piece to obtain multiple-time
correlation functions. An average over the different environmental histories of
the reduced propagator permits us to obtain the evolution equations of the
multiple-time correlation functions. We also study the evolution of MTCF within
the weak coupling limit and it is shown that the multiple-time correlation
function of some observables satisfy the Quantum Regression Theorem (QRT),
whereas other correlations do not. We set the conditions under which the
correlations satisfy the QRT. We illustrate the theory in two different cases;
first, solving an exact model for which the MTCF are explicitly given, and
second, presenting the results of a numerical integration for a system coupled
with a dissipative environment through a non-diagonal interaction.Comment: Submitted (04 Jul 04
Quantum theory of the low-frequency linear susceptibility of interferometer-type superconducting qubits
We use the density matrix formalism to analyze the interaction of
interferometer-type superconducting qubits with a high quality tank circuit,
which frequency is well below the gap frequency of a qubit. We start with the
ground state characterization of the superconducting flux and charge qubits.
Then, by making use of a dressed state approach we describe the qubits'
spectroscopy when the qubit is irradiated by a microwave field which is tuned
to the gap frequency. The last section of the paper is devoted to continuous
monitoring of qubit states by using a DC SQUID in the inductive mode.Comment: 11 pages, 5 figures; the title and abstract are slightly changed;
several typos are corrected; in order to make our argumentation more clear we
added some comments in the introduction and other section
Decoherence in a superconducting flux qubit with a pi-junction
We consider the use of a pi-junction for flux qubits to realize degenerate
quantum levels without external magnetic field. On the basis of the
Caldeira-Leggett model, we derive an effective spin-Boson model, and study
decoherece of this type of qubits. We estimate the dephasing time by using
parameters from recent experiments of SIFS junctions, and show that high
critical current and large subgap resistance are required for the pi-junction
to realize a long coherent time.Comment: 5 pages, 2 figure
The Effects of Source and Amount of Nitrite on Quality Characteristics of All-Beef Frankfurters
In an effort to meet consumers’ demand for foods with more natural ingredients, processors have begun manufacturing meat products cured with natural nitrite sources. The objective of this study was to evaluate the quality characteristics of all-beef frankfurters cured with traditional or alternative sources of nitrite and using equivalent amounts of nitrite. Frankfurters cured with alternative sources of nitrite had a slightly darker, less red exterior and slightly more yellow interior than those containing sodium nitrite. No differences were observed for pH or water activity. Both curing methods can be used to manufacture all-beef frankfurters with similar characteristics when using equivalent amounts of nitrite
The Effects of Source and Amount of Nitrite on Quality Characteristics of All-Beef Frankfurters
In an effort to meet consumers’ demand for foods with more natural ingredients, processors have begun manufacturing meat products cured with natural nitrite sources. The objective of this study was to evaluate the quality characteristics of all-beef frankfurters cured with traditional or alternative sources of nitrite and using equivalent amounts of nitrite. Frankfurters cured with alternative sources of nitrite had a slightly darker, less red exterior and slightly more yellow interior than those containing sodium nitrite. No differences were observed for pH or water activity. Both curing methods can be used to manufacture all-beef frankfurters with similar characteristics when using equivalent amounts of nitrite
Circuit theory for decoherence in superconducting charge qubits
Based on a network graph analysis of the underlying circuit, a quantum theory
of arbitrary superconducting charge qubits is derived. Describing the
dissipative elements of the circuit with a Caldeira-Leggett model, we calculate
the decoherence and leakage rates of a charge qubit. The analysis includes
decoherence due to a dissipative circuit element such as a voltage source or
the quasiparticle resistances of the Josephson junctions in the circuit. The
theory presented here is dual to the quantum circuit theory for superconducting
flux qubits. In contrast to spin-boson models, the full Hilbert space structure
of the qubit and its coupling to the dissipative environment is taken into
account. Moreover, both self and mutual inductances of the circuit are fully
included.Comment: 8 pages, 3 figures; v2: published version; typo in Eq.(30) corrected,
minor changes, reference adde
- …