Can quantum regression theorem be reconciled with quantum fluctuation dissipation theorem ?

In the attempt to derive the regression theorem from the fluctuation dissipation theorem several authors claim the violation of the former theorem in the quantum case. Here we pose the question: does it exists a quantum fluctuation dissipation theorem (QFDT) in its conventional interpretation? It is shown that the relation usually called as the QFDT is the condition of detailed macroscopic energetic balance. Following this interpretation the existing conflict between the two theorems in the quantum case is removed.Comment: 13 pages, 3rd Int. Conf. on Unsolved Problems on Noise (will be published in Proceedings

Voltage controlled terahertz transmission through GaN quantum wells

We report measurements of radiation transmission in the 0.220--0.325 THz frequency domain through GaN quantum wells grown on sapphire substrates at room and low temperatures. A significant enhancement of the transmitted beam intensity with the applied voltage on the devices under test is found. For a deeper understanding of the physical phenomena involved, these results are compared with a phenomenological theory of light transmission under electric bias relating the transmission enhancement to changes in the differential mobility of the two-dimensional electron gas

Noise influence on electron dynamics in semiconductors driven by a periodic electric field

Studies about the constructive aspects of noise and fluctuations in different non-linear systems have shown that the addition of external noise to systems with an intrinsic noise may result in a less noisy response. Recently, the possibility to reduce the diffusion noise in semiconductor bulk materials by adding a random fluctuating contribution to the driving static electric field has been tested. The present work extends the previous theories by considering the noise-induced effects on the electron transport dynamics in low-doped n-type GaAs samples driven by a high-frequency periodic electric field (cyclostationary conditions). By means of Monte Carlo simulations, we calculate the changes in the spectral density of the electron velocity fluctuations caused by the addition of an external correlated noise source. The results reported in this paper confirm that, under specific conditions, the presence of a fluctuating component added to an oscillating electric field can reduce the total noise power. Furthermore, we find a nonlinear behaviour of the spectral density with the noise intensity. Our study reveals that, critically depending on the external noise correlation time, the dynamical response of electrons driven by a periodic electric field receives a benefit by the constructive interplay between the fluctuating field and the intrinsic noise of the system.Comment: 9 pages, 4 figures, to appear in J. Stat. Mechanics: Theory and Experim., 200

Coherent TeraHertz Radiation Generation Due to Carrier Interaction with Low-Temperature Optical Phonons in Semiconductors: Achievements and Perspectives

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Modelling of small-signal response and electronic noise in semiconductor high-field transport

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