PhD Defense by Massimiliano Rossi

Title: Quantum Measurement and Control of a Mechanical Resonator

Abstract: Measuring the displacement of an object is a ubiquitous task, both in scientific applications and in the everyday life. The laws of classical physics and experience do not pose any limit to the achievable precision in such measurements, which can be carried out without perturbing the measured object. In contrast, quantum physics predicts that the larger the precision achieved in a displacement measurement, the larger the disturbance, or quantum backaction, affecting the object being measured.
    As of today, the most precise displacement measurements are done by reflecting a laser field off a mechanical object and interferometrically measuring the laser' phase. This interaction, stemming from radiation pressure forces, is at the heart of the field of optomechanics. In such a measurement, the precision and the quantum backaction arise from the quantum fluctuations of the laser’s phase and intensity, respectively. In addition, mechanical systems also experience fluctuating thermal forces, which introduce more disturbance and hinder the observation of quantum effects of the measurement. Nevertheless, an efficient quantum measurement can be realized whenever the information about the displacement is gathered at a rate close to the one at which the resonator is perturbed. When available, the result of this measurement can be used to refine the knowledge held by an observer about the mechanics. Based on this, the observer can tailor and exert a force to control the resonator and steer its state towards the desired one: a form of measurement-based quantum control.

    In this thesis, we measure the displacement of a vibrating membrane interacting with a laser. For the first time, we perform measurements in which the imprecision and the disturbance are kept at the minimum imposed by quantum physics, to within 33%. Consequently, we can track the random displacement of the membrane with zero-point-motion precision, corresponding to the intrinsic quantum uncertainty of position. Based on this, we exert a force to counteract this random motion and reduce the mechanical energy to the ground state, that is, the lowest value permitted by quantum mechanics. This extends the measurement-based quantum control to mechanical degrees of freedom, thus realizing a long-standing goal in the field.

    Quantum measurements are an important tool in several applications. We experimentally explore two of them, sensing beyond quantum limits and the generation of entanglement.

 Zoom: https://ucph-ku.zoom.us/j/67990641911?pwd=MnZBbi9TNDdZTEZ1dUJaeHRQNmc4UT09
(Zoom meeting ID: 679 9064 1911, PWD: 231506)