Noise in mesoscopic quantum systems

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Noise, i.e., fluctuations of an observable due to external factors, is a phenomenon that affects all measurements. Studying its effects is particularly important for quantum systems because of the fragile nature of quantum correlations. Usually noise is considered detrimental because it may reduce the quality of experimental results. However, it can also play a vital role in triggering processes such as chemical reactions. Furthermore, noise can be manipulated by changing the temperature of a system's environment, or by more advanced techniques. One example of such a technique is the improved resolution of gravitational wave detectors by using squeezed states of light instead of ordinary laser beams.******In this project we will study noise and its implications for three rather different systems: surface-enhanced stimulated Raman spectroscopy of molecules (SESRS), spin relaxation in single-molecule magnets (SMM), and the Purcell filter in circuit Quantum Electrodynamics (cQED). The SESRS system consists of a molecule attached to a gold nano-particle, which interacts with two laser pulses of different frequency. A SMM consists of a single molecular metal compound that displays magnetic hysteresis, and the cQED system of the third project consists of a micrometer-sized electric circuit that includes superconducting elements. The overarching element of all three systems is the noise mechanism, which follows surprisingly similar principles. In all three systems, noise is introduced through a finite number of damped oscillators. In SESRS and SSM, these oscillations are formed by nuclear vibrations in a molecule, while, in cQED, they correspond to oscillating microwave fields in a resonator. Working on such different systems will enable us to transfer ideas and methods between all systems.******This project has two goals: (i) manipulating noise to improve a system, and (ii) characterizing the noise dependence on the environment of the system. Goal (i) aims at improving the desired signal of a system by changing the way it interacts with the noise. The Purcell filter is an example of such a change: by adding additional microwave resonators, noise can be suppressed in a particular frequency range. In a similar way, molecular noise could be manipulated to increase the relaxation time in SMM or the radiation intensity in SESRS. Goal (ii) aims at extracting information about the environment of a molecule from its spectrum. In this way, radiation emitted by a molecule could be used to probe the general structure of the material surrounding it.