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The use of the Wiener–Khinchin theorem in the reverberation chamber reveals the relationships between a number of important parameters—the coherence bandwidth and the Q-factor measured in the time domain, the coherence time and the Q-factor measured in the frequency domain, the K-factor and the Doppler spectrum, and the K-factor and the total scattering cross section. The lower bound of the average K-factor is also given. Different physical quantities, which share similar mathematical insights, are unified. Analytical derivations are given, and results are validated by measurements. [ABSTRACT FROM AUTHOR]

Abstract: We consider the spatial power spectra associated with fluctuations of quadratic operators in field theory, such as quantum stress tensor components. We show that the power spectrum can be negative, in contrast to most fluctuation phenomena where the Wiener–Khinchin theorem requires a positive power spectrum. We show why the usual argument for positivity fails in this case, and discuss the physical interpretation of negative power spectra. Possible applications to cosmology are discussed. [Copyright &y& Elsevier]

Abstract: The Wiener–Khinchin theorem states that the power spectrum is the Fourier transform of the autocovariance function. One form of the autocovariance function can be obtained through recurrence quantification. We show that the advantage of defining the autocorrelation function with recurrences can demonstrate higher dimensional dynamics. [Copyright &y& Elsevier]

• Quantum indeterminacy of the electromagnetic field results in 1 / f voltage noise. • Power spectrum of quantum fluctuations is well-defined despite quantum indeterminacy. • A lower bound on the power spectrum of voltage fluctuations exists. • 1 / f -noise in InGaAs quantum wells is near the quantum bound. An approach to the problem of 1 / f voltage noise in conductors is developed based on an uncertainty relation for the Fourier-transformed signal. It is shown that a lower bound on the power spectrum of voltage fluctuations exists. This bound is calculated explicitly in the case of unpolarized charge carriers with a parabolic dispersion, and is found to have a 1 / f low-frequency asymptotic. A comparison with the 1 / f -noise measurements in InGaAs quantum wells is made which shows that the observed noise levels are only a few times higher than the bound established. [ABSTRACT FROM AUTHOR]