299 research outputs found
Effects of system-bath entanglement on the performance of light-harvesting systems: A quantum heat engine perspective
We explore energy transfer in a generic three-level system, which is coupled
to three non-equilibrium baths. Built on the concept of quantum heat engine,
our three-level model describes non-equilibrium quantum processes including
light-harvesting energy transfer, nano-scale heat transfer, photo-induced
isomerization, and photovoltaics in double quantum-dots. In the context of
light-harvesting, the excitation energy is first pumped up by sunlight, then is
transferred via two excited states which are coupled to a phonon bath, and
finally decays to the ground state. The efficiency of this process is evaluated
by steady state analysis via a polaron-transformed master equation; thus a wide
range of the system-phonon coupling strength can be covered. We show that the
coupling with the phonon bath not only modifies the steady state, resulting in
population inversion, but also introduces a finite steady state coherence which
optimizes the energy transfer flux and efficiency. In the strong coupling
limit, the steady state coherence disappears and the efficiency approaches the
heat engine limit given by Scovil and Schultz-Dubois in Phys. Rew. Lett. 2, 262
(1959).Comment: 10 pages, 8 figures, all comments are welcom
Selection of a Forwarding Area for Contention-Based Geographic Forwarding in Wireless Multi-Hop Networks
a state-free forwarding technique. In this paper, we develop a general analytical framework to evaluate the performance of CGF with different forwarding areas in wireless multi-hop networks. In particular, we compare the performance of CGF for three typical forwarding areas, analytically and by extensive simulations. We further investigate the impact of several important assumptions on our analytical results. Our study provides guidelines regarding the selection of a specific forwarding area during the design phase of a CGF protocol. It also serves as a general performance evaluation framework for CGF protocols as well as traditional geographic forwarding protocols. Index Terms—Ad hoc and sensor networks, geographic forwarding, performance evaluation, void handling, wireless networks. I
The Position and Function of Macroscopic Analysis in the Failure Analysis of Railway Fasteners
Macroscopic analysis plays an important role in failure analysis, which cannot be replaced by other analyzing methods. In recent years, with the development of characterization techniques, more and more engineers and technicians rely on the advanced analytical testing methods in the process of failure analysis, ignoring the methods and means of macroscopic analysis. This can easily lead to some wrong judgments. Therefore, this chapter will combine with the cases to explain the position and role of macroanalysis in the failure analysis of rail fastening clips and to offer references for engineers and technicians in relevant fields
The Critical Behavior of Quantum Stirling Heat Engine
We investigate the performance of a Stirling cycle with a working substance
(WS) modeled as the quantum Rabi model (QRM), exploring the impact of
criticality on its efficiency. Our findings indicate that the criticality of
the QRM has a positive effect on improving the efficiency of the Stirling
cycle. Furthermore, we observe that the Carnot efficiency is asymptotically
achievable as the WS parameter approaches the critical point, even when both
the temperatures of the cold and hot reservoirs are finite. Additionally, we
derive the critical behavior for the efficiency of the Stirling cycle,
demonstrating how the efficiency asymptotically approaches the Carnot
efficiency as the WS parameter approaches the critical point. Our work deepens
the understanding of the impact of criticality on the performance of a Stirling
heat engine.Comment: 7 pages, 3 figure
Observation of the Density Minimum in Deeply Supercooled Confined Water
Small angle neutron scattering (SANS) is used to measure the density of heavy
water contained in 1-D cylindrical pores of mesoporous silica material
MCM-41-S-15, with pores of diameter of 15+-1 A. In these pores the homogenous
nucleation process of bulk water at 235 K does not occur and the liquid can be
supercooled down to at least 160 K. The analysis of SANS data allows us to
determine the absolute value of the density of D2O as a function of
temperature. We observe a density minimum at 210+-5 K with a value of
1.041+-0.003 g/cm3. We show that the results are consistent with the
predictions of molecular dynamics simulations of supercooled bulk water. This
is the first experimental report of the existence of the density minimum in
supercooled water
Detecting Abrupt Change of Channel Covariance Matrix in IRS-Assisted Communication
The knowledge of channel covariance matrices is crucial to the design of
intelligent reflecting surface (IRS) assisted communication. However, channel
covariance matrices may change suddenly in practice. This letter focuses on the
detection of the above change in IRS-assisted communication. Specifically, we
consider the uplink communication system consisting of a single-antenna user
(UE), an IRS, and a multi-antenna base station (BS). We first categorize two
types of channel covariance matrix changes based on their impact on system
design: Type I change, which denotes the change in the BS receive covariance
matrix, and Type II change, which denotes the change in the IRS
transmit/receive covariance matrix. Secondly, a powerful method is proposed to
detect whether a Type I change occurs, a Type II change occurs, or no change
occurs. The effectiveness of our proposed scheme is verified by numerical
results.Comment: accepted by IEEE Wireless Communications Letter
CDDM: Channel Denoising Diffusion Models for Wireless Semantic Communications
Diffusion models (DM) can gradually learn to remove noise, which have been
widely used in artificial intelligence generated content (AIGC) in recent
years. The property of DM for eliminating noise leads us to wonder whether DM
can be applied to wireless communications to help the receiver mitigate the
channel noise. To address this, we propose channel denoising diffusion models
(CDDM) for semantic communications over wireless channels in this paper. CDDM
can be applied as a new physical layer module after the channel equalization to
learn the distribution of the channel input signal, and then utilizes this
learned knowledge to remove the channel noise. We derive corresponding training
and sampling algorithms of CDDM according to the forward diffusion process
specially designed to adapt the channel models and theoretically prove that the
well-trained CDDM can effectively reduce the conditional entropy of the
received signal under small sampling steps. Moreover, we apply CDDM to a
semantic communications system based on joint source-channel coding (JSCC) for
image transmission. Extensive experimental results demonstrate that CDDM can
further reduce the mean square error (MSE) after minimum mean square error
(MMSE) equalizer, and the joint CDDM and JSCC system achieves better
performance than the JSCC system and the traditional JPEG2000 with low-density
parity-check (LDPC) code approach.Comment: submitted to IEEE Transactions on Wireless Communications. arXiv
admin note: substantial text overlap with arXiv:2305.0916
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