Quantum noise | EPFL Graph Search

Quantum noise is noise arising from the indeterminate state of matter in accordance with fundamental principles of quantum mechanics, specifically the uncertainty principle and via zero-point energy fluctuations. Quantum noise is due to the apparently discrete nature of the small quantum constituents such as electrons, as well as the discrete nature of quantum effects, such as photocurrents. Quantified noise is similar to classical noise theory and will not always return an asymmetric spectral density. Shot noise as coined by J. Verdeyen is a form of quantum noise related to the statistics of photon counting, the discrete nature of electrons, and intrinsic noise generation in electronics. In contrast to shot noise, the quantum mechanical uncertainty principle sets a lower limit to a measurement. The uncertainty principle requires any amplifier or detector to have noise. Macroscopic manifestations of quantum phenomena are easily disturbed, so quantum nois

Physics of optically driven nanojunctions

Philippe Andreas Rölli

Recent years have seen spectacular developments in the domain of nano-optics. Alongside the well-known techniques of super-resolution microscopy progress in nanofabrication has enabled important improvements in the fields of optical imaging and spectroscopy. Nowadays enhancements of specific near-field interactions by nano-antennae/cavities allows the optical readout of single molecule properties in a number of different systems. The observation of normal modes of vibration for example carries essential information on the orientation, link to the interface and environment in which the molecule is embedded. This thesis develops firstly a novel theoretical frame for these vibration-cavity interactions by following the optomechanical formalism, which has proven to be an accurate treatment of a wide variety of mechanical systems interacting with a laser field. By leveraging this framework, we investigate on the one hand several quantum limits. On the other hand, for the parameter space experimentally available and for well-designed cavities, novel regimes of interaction between the collective mode of vibration of an ensemble of molecules and an optical tone seem within reach. In parallel, this thesis experimentally explores one specific realization of such a system. In collaboration with the group of Christophe Galland we constructed the optical setups necessary to interrogate our nanostructures. The class of structures constructed, known as self-assembled nanojunctions or nanoparticles on mirror, reaches confinement of electric fields via plasmonic modes close to its fundamental limit and enables the study of many different mechanical systems whose thickness consist in a single molecule. Along this research we have developed techniques to characterize mechanical, electronic and thermal observables of nanojunctions interacting with an optical field. Our experimental study has enabled the discovery of a novel effect related to the electronic confinement of these quasi-0-dimensional structures. The characteristic photoluminescence of metallic nanostructures is found to consistently blink in nanojunctions akin to other quantum emitters. The mechanisms explaining in detail this blinking remain debatable but the numerous experimental data collected highlight the key role of the interface in the specificities of the blinking observed. This finding opens novel ways to characterize the important and to date ill-described question of the interfaces in nanojunctions. The observed modifications of the nanojunctions optical properties might constitute an interesting step towards the development of a novel class of functional materials ('gap-materials') with enhanced and tailored electro-optical properties. The last part of this thesis considers one technological application for this class of material by conceptually developing a new kind of detector based on the upconversion of an infrared photon to the visible; exploiting thus in a unique way the optical properties of the molecular system. The study of the quantum noises of these detectors highlights two exciting and technologically relevant directions for such devices : at room temperature these systems operate with levels of noise below state of the art devices, at lower temperatures these systems open an unexplored path towards single photon detection in a spectral range where this technology remains unavailable to date.

EPFL2020

Quantum optomechanics at room temperature

Hendrik Schütz

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017