232 research outputs found
Efficiency at Maximum Power of Laser Quantum Heat Engine Enhanced by Noise-Induced Coherence
Quantum coherence has been demonstrated in various systems including organic
solar cells and solid state devices. In this letter, we report the lower and
upper bounds for the performance of quantum heat engines determined by the
efficiency at maximum power. Our prediction based on the canonical 3-level
Scovil and Schulz-Dubois maser model strongly depends on the ratio of
system-bath couplings for the hot and cold baths and recovers the theoretical
bounds established previously for the Carnot engine. Further, introducing a
4-th level to the maser model can enhance the maximal power and its efficiency,
thus demonstrating the importance of quantum coherence in the thermodynamics
and operation of the heat engines beyond the classical limit
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
Non-canonical distribution and non-equilibrium transport beyond weak system-bath coupling regime: A polaron transformation approach
The concept of polaron, emerged from condense matter physics, describes the dynamical interaction of moving particle with its surrounding bosonic modes. This concept has been developed into a useful method to treat open quantum systems with a complete range of system-bath coupling strength. Especially, the polaron transformation approach shows its validity in the intermediate coupling regime, in which the Redfield equation or Fermi’s golden rule will fail. In the polaron frame, the equilibrium distribution carried out by perturbative expansion presents a deviation from the canonical distribution, which is beyond the usual weak coupling assumption in thermodynamics. A polaron transformed Redfield equation (PTRE) not only reproduces the dissipative quantum dynamics but also provides an accurate and efficient way to calculate the non-equilibrium steady states. Applications of the PTRE approach to problems such as exciton diffusion, heat transport and light-harvesting energy transfer are presented.National Science Foundation (U.S.) (Grant CHE- 1112825
Duality Symmetry of Quantum Electrodynamics
The duality symmetry between electricity and magnetism hidden in classical
Maxwell equations suggests the existence of dual charges, which have usually
been interpreted as magnetic charges and have not been observed in experiments.
In quantum electrodynamics (QED), both the electric and magnetic fields have
been unified into one gauge field , which makes this symmetry
inconspicuous. Here, we recheck the duality symmetry of QED by introducing a
dual gauge field. Within the framework of gauge-field theory, we show that the
electric-magnetic duality symmetry cannot give any new conservation law. By
checking charge-charge interaction and specifically the quantum Lorentz force
equation, we find that the dual charges are electric charges, not magnetic
charges. More importantly, we show that true magnetic charges are not
compatible with the gauge-field theory of QED, because the interaction between
a magnetic charge and an electric charge can not be mediated by gauge photons.Comment: 5 pages with 4 appendixes, 1 figur
A Real-time SAR Echo Simulator Based on FPGA and Parallel Computing
This paper designs and implements a SAR (Synthetic Aperture Radar) real-time echo simulator based on multi-FPGA parallel computing. The one-dimensional frequency-domain Fourier transform algorithm is used in the simulator, and the echo signal model and the rapid calculation algorithm of impulse response function are introduced. The pipeline compute structure, multichannel parallel computing and procedure flow design are the key technologies of the simulator, which are also presented in details. And finally, the validity and correctness of the SAR echo simulator are verified through the imaging results of the point-array target and the nature scene target
Evidence of the side jump mechanism in the anomalous Hall effect in paramagnets
Persistent confusion has existed between the intrinsic (Berry curvature) and
the side jump mechanisms of anomalous Hall effect (AHE) in ferromagnets. We
provide unambiguous identification of the side jump mechanism, in addition to
the skew scattering contribution in epitaxial paramagnetic NiCu
thin films, in which the intrinsic contribution is by definition excluded.
Furthermore, the temperature dependence of the AHE further reveals that the
side jump mechanism is dominated by the elastic scattering
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