This work focuses on the detection, manipulation, and use of cold gases of atoms trapped within a high-quality optical resonator. In such a system, the internal state of the atoms -- their quantum spin -- interacts strongly with light injected into the cavity. This well controlled interaction allows us to explore the quantum limits of how well the spin state of the atomic gas can be measured, to direct light-mediated interactions between the atomic spins, and to examine the behavior of a complex quantum system under constant measurement. Specific aims of the effort are to demonstrate a new range of dynamical phenomena predicted to occur due to feedback between the cavity field and the spin ensemble, and to employ magnetic resonance imaging (MRI) to study dynamical phenomena with the highest spatial resolution. Our scientific findings will clarify how complex non-equilibrium phenomena result from the hybridization of several simple quantum elements, and will shed light on magnetic and transport phenomena in solid-state materials. The work also provides excellent laboratory skills and scientific training to future scientific leaders.
," performed with NSF support at the University of California, Berkeley, explored the quantum mechanical consequences of optomechanical interactions, that is, of the interactions between light and a massive moveable object. In our experiment, that moveable object was a dilute gas of just a few thousand atoms, which was cooled to extremely low temperatures and then trapped within a laser tweezer trap. The advantage of this approach is that the mechanical object is already very cold, and hence already behaving in a quantum mechanical manner, by the time it begins interacting with light. Our experiments were the first that demonstrated several important quantum mechanical phenomena in this setting. One important demonstration was that the motion of the atoms in response to optical forces acted to suppress the quantum fluctuations of the light field, an effect known as ponderomotive squeezing. Another important observation was that light scattered by the mechanical element tended to be lower in energy than the light that struck the object, demonstrating that the object was predominantly found in its minimum energy state of motion. These quantum effects were harnessed in a final study that focused on the task of measuring forces that are applied to the mechanical element. We found that there was a smallest force that could be sensed, known as the standard quantum limit for force detection, defined by basic properties of the instrument doing the measurement. A new direction of research was launched where we study not one, but several mechanical objects inside an optical resonator, interacting commonly with a beam of light. In this situation, the mechanical objects begin influencing one another, exerting forces that can be construed as being conveyed by the exchange of photons of light between them. We observed distinctive quantum mechanical phenomena in this new setting as well, notably in the observation of fluctuations of the light-mediated force between these elements. Our research provides important insights on the limits that quantum mechanics sets for a number of sensing and information processing tasks that can be performed with such cavity optomechanical systems. For example, the force detection limits observed in our work apply also to force sensors used to study biological systems, to characterize surfaces at the atomic level, or to detect gravitational radiation. These connections to our work were highlighted in a number of popular science articles written about our work.