Atom interferometry is a powerful metrological tool that has been developed over the last few decades. Large momentum transfer (LMT) methodsmanipulate atomic trajectories with tens or hundreds of photon momenta\nin order to increase sensitivity. This thesis furthers progress towards using\nLMT methods in next-generation atom interferometers. One main result establishes symmetric Bloch oscillations as a new, viable technique for LMT.\nTheory and numerics are used to show how the process is coherent and\nadiabatic, and experimentally we demonstrate coherence in an interferometer with up to 240¯hk, where ¯hk is the momentum of a single photon of\n852nm light. This was the second largest coherent momentum splitting\ndemonstrated at at the time of publication. The rest of the thesis focuses\non design and construction of a new atomic fountain to measure the fine\nstructure constant α. Discrepancies in recent measurements of α [67, 55]\nare currently limiting theory predictions for the electron gyromagnetic ratio\n[25] - an improved measurement of α is therefore highly motivated and would\nenable an improved test of the consistency of the Standard Model. Previously, our group published a measurement of α at the 0.2 ppb level in 2018\n[67]. We built a new experiment with a goal of improving the measurement\nby a factor of 3-10. Much of the thesis focuses on systematic effects related\nto spatial intensity inhomogeneities on the laser beam, which are some of\nthe hardest to characterize systematic effects looking forward. A large clear\naperture vacuum chamber accommodates larger waist laser beams without\nclipping on chamber walls. In addition, a high-speed, user-friendly Monte\nCarlo simulation package was made to predict experimental systematic shifts\nin the measured value of α due to laser beam inhomogeneities.
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, Heisenbergâ s 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
∼104 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 â variational 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.