Thermal Fluctuations and Electromagnetic Noise Spectra in Quantum Statistical Mechanics.

We derive the thermal noise spectrum of the of the longitudinal and transverse electric field operator of a given wave vector starting from the quantum-statistical definitions and relate it to the complex frequency and wave vector dependent complex conductivity in a homogeneous, isotropic system of electromagnetic interacting electrons. No additional assumptions were used in the proof. We analyze separately the longitudinal and transverse case with their peculiarities. The Nyquist formula for vanishing frequency and wave vector, as well as its modification for non-vanishing frequencies and wave vectors follow immediately. Furthermore we discuss also the noise of the photon occupation numbers.

Timing and spectral analysis of 2S 1417−624 during its 2018 outburst

We investigate timing and spectral characteristics of the transient X-ray pulsar 2S 1417−624 during its 2018 outburst with NICER follow up observations. We describe the spectra with high-energy cut-off and partial covering fraction absorption (PCFA) model and present flux-dependent spectral changes of the source during the 2018 outburst. Utilizing the correlation-mode switching of the spectral model parameters, we confirm the previously reported sub-critical to critical regime transitions and we argue that secondary transition from the gas-dominated to the radiation pressure-dominated disc do not lead to significant spectral changes below 12 keV. Using the existing accretion theories, we model the spin frequency evolution of 2S 1417−624 and investigate the noise processes of a transient X-ray pulsar for the first time using both polynomial and luminosity-dependent models for the spin frequency evolution. For the first model, the power density spectrum of the torque fluctuations indicate that the source exhibits red noise component (Γ ∼ −2) within the timescales of outburst duration which is typical for disc-fed systems. On the other hand, the noise spectrum tends to be white on longer timescales with high timing noise level that indicates an ongoing accretion process in between outburst episodes. For the second model, most of the red noise component is eliminated and the noise spectrum is found to be consistent with a white noise structure observed in wind-fed systems.

Electromagnetic Interference of Power Converter with Random Modulation on the Power Line Communication System

Random Pulse Width Modulation (RPWM) allows controlling the switching signal of power converters in order to reduce the harmonic peaks by spreading the noise spectrum. Currently, many manufacturers of power converters are deploying this modulation scheme in order to comply with Electromagnetic Compatibility (EMC) test requirements. However, when the converters coexist with Power Line Communication (PLC) systems, such as in Smart Grid (SG) applications, resorting to RPWM needs further investigations since it potentially affects the communication channel by increasing the bit error rate. This possible detrimental effect is investigated in this work, by considering a PLC system for automatic meter reading (AMR) implemented in a SG application. To this end, the model of a complete PLC system is implemented in SIMULINK, and Quadrature Phase Shift Keying (QPSK) modulation is used to model the PLC modems in the communication channel. Results show that, even if the deployment of RPWM techniques may lead to an appreciable reduction/spreading of the peaks in the noise spectrum, it may also lead to an increase of the bit error rate on the PLC system.

All-optical noise spectroscopy of a solid-state spin

Noise spectroscopy elucidates the fundamental noise sources in spin systems, which is essential for developing spin qubits with long coherence times for quantum information processing, communication, and sensing. But noise spectroscopy typically relies on microwave coherent spin control to extract the noise spectrum, which becomes infeasible when there are high-frequency noise components stronger than the available microwave power. Here, we demonstrate an alternative all-optical approach to performing noise spectroscopy. Our approach utilises coherent Raman rotations of the spin state with controlled timing and phase to implement Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. Analysing the spin dynamics under these sequences enables us to extract the noise spectrum of a dense ensemble of nuclear spins interacting with a single spin in a quantum dot, which has thus far only been modelled theoretically. By providing large spectral bandwidths of over 100 MHz, our Raman-based approach could serve as an important tool to study spin dynamics and decoherence mechanisms for a broad range of solid-state spin qubits.

Current Fluctuations in Hybrid-Superconductor Normal Structures with Quantum Dots

<p>Nanostructures with quantum dots in proximity to superconducting electrodes are an ideal tool to study superconducting correlations in systems with few degrees of freedom that exhibit strong Coulomb-interaction effects. Such hybrid superconductor-normal structures show rich physics due to the interplay of superconductivity, Coulomb interaction and non-equilibrium. Superconducting correlations are established on the quantum dot when it is coupled to a superconductor even in the presence of strong Coulomb repulsion and Cooper pairs can tunnel coherently between the quantum dot and the superconductor. In this thesis, we investigate theoretically electronic transport through an interacting quantum dot coupled to normal and superconducting leads. The presence of the proximity effect can be detected by the dot's current, namely the Andreev current. However, current fluctuations might reveal information on the electronic transport and the internal structure of the system which is not visible in the mean value of the current. For this reason, we study the current fluctuations through the proximized quantum dot to get access to the properties of such a hybrid quantum-dot system. In particular, we are interested in the finite-frequency fluctuations to unveil the coherent dynamics underlying the proximity effect in the quantum dot and its internal time scales. At first, we present a study of the frequency-dependent current noise for subgap transport through an interacting single-level quantum dot tunnel-coupled to normal and superconducting leads. For this purpose, we employ a non-equilibrium diagrammatic real-time approach to calculate the finite-frequency current noise. The finite-frequency noise spectrum shows a sharp dip at a frequency corresponding to the energy splitting of the Andreev bound states which is a signature of the coherent exchange of Cooper pairs between the quantum dot and the superconductor. Furthermore, in the high frequency regime, the so called quantum noise regime, the noise spectrum exhibits steps at frequencies equal to the excitation energies. These steps can be related to the effective coupling strength of the excitations. However, the statistical description of the electron transport does not stop with the noise. Current cumulants of arbitrary order can be obtained by means of full counting statistics (FCS). We set up a theory based on the diagrammatic real-time approach to calculate the finite-time FCS for quantum transport with a non-Markovian master equation that captures the initial correlations between system and reservoir. This allows us to fully describe the current fluctuations of the hybrid quantum-dot system, that is the noise and all higher order current cumulants.</p>

Current Fluctuations in Hybrid-Superconductor Normal Structures with Quantum Dots

<p>Nanostructures with quantum dots in proximity to superconducting electrodes are an ideal tool to study superconducting correlations in systems with few degrees of freedom that exhibit strong Coulomb-interaction effects. Such hybrid superconductor-normal structures show rich physics due to the interplay of superconductivity, Coulomb interaction and non-equilibrium. Superconducting correlations are established on the quantum dot when it is coupled to a superconductor even in the presence of strong Coulomb repulsion and Cooper pairs can tunnel coherently between the quantum dot and the superconductor. In this thesis, we investigate theoretically electronic transport through an interacting quantum dot coupled to normal and superconducting leads. The presence of the proximity effect can be detected by the dot's current, namely the Andreev current. However, current fluctuations might reveal information on the electronic transport and the internal structure of the system which is not visible in the mean value of the current. For this reason, we study the current fluctuations through the proximized quantum dot to get access to the properties of such a hybrid quantum-dot system. In particular, we are interested in the finite-frequency fluctuations to unveil the coherent dynamics underlying the proximity effect in the quantum dot and its internal time scales. At first, we present a study of the frequency-dependent current noise for subgap transport through an interacting single-level quantum dot tunnel-coupled to normal and superconducting leads. For this purpose, we employ a non-equilibrium diagrammatic real-time approach to calculate the finite-frequency current noise. The finite-frequency noise spectrum shows a sharp dip at a frequency corresponding to the energy splitting of the Andreev bound states which is a signature of the coherent exchange of Cooper pairs between the quantum dot and the superconductor. Furthermore, in the high frequency regime, the so called quantum noise regime, the noise spectrum exhibits steps at frequencies equal to the excitation energies. These steps can be related to the effective coupling strength of the excitations. However, the statistical description of the electron transport does not stop with the noise. Current cumulants of arbitrary order can be obtained by means of full counting statistics (FCS). We set up a theory based on the diagrammatic real-time approach to calculate the finite-time FCS for quantum transport with a non-Markovian master equation that captures the initial correlations between system and reservoir. This allows us to fully describe the current fluctuations of the hybrid quantum-dot system, that is the noise and all higher order current cumulants.</p>

Adder Box Used in the Heavy Trucks Transmission Noise Reduction

The paper aims to analyze the vibrations of a summing box, used in heavy vehicles. An experimental setup is proposed and used for the analysis of these vibrations transmitted by the box housing, based on the measurement of the mechanical impedance. It is then shown that a simpler measurement, namely that of the generated noise spectrum, can provide equally useful results, with less effort and in a shorter time. The experimental setup is much simpler, involving a sonometer arranged in the experimental assembly. The symmetry of the box allows us to facilitate the execution of the experimental assembly, and the use of the symmetrical structure to facilitate the calculation of vibrations. The results obtained using this method in the case of a redesigned adder box are presented in the paper.