Strong light-mediated coupling between a membrane oscillator and an atomic spin ensemble

This thesis presents theoretical and experimental work on light-mediated coupling between a collective atomic spin and a micromechanical membrane oscillator. With our work we address a fundamental question of quantum optics: Can a beam of light mediate coherent Hamiltonian interactions between two distant quantum systems? This is an intriguing question whose answer is not a priori clear, since the light carries away information about the systems and might be subject to losses, giving rise to intrinsic decoherence channels associated with the coupling. Our answer is affirmative and we derive a particularly simple sufficient condition for the interactions to be Hamiltonian: The light field needs to interact twice with the systems and the second interaction has to be the time reversal of the first. We demonstrate theoretically that, even in the presence of significant optical loss, coherent interactions can be realized and generate substantial amounts of entanglement between the systems.
In our experiments, we employ this approach to strongly couple a spin-polarized atomic ensemble and a micromechanical oscillator via a free-space laser beam across a distance of one meter in a room-temperature environment. The atomic ensemble consists of about ten million laser-cooled Rubidium atoms in an optical dipole trap that interact with the coupling laser via an off-resonant Faraday interaction. The mechanical oscillator is a silicon nitride membrane which is mounted in a single-sided optical cavity and couples to the laser field via radiation-pressure forces. In order to mediate a bidirectional Hamiltonian interaction between spin and membrane, the coupling beam is arranged in a loop such that it couples twice to the spin. This looped geometry enables destructive interference of quantum back-action by the light field on the spin.
Using this setup, we experimentally demonstrate for the first time strong Hamiltonian coupling between remote quantum systems and explore different dynamical regimes of cascaded light-mediated interactions: With the spin initialized in its ground state we observe normal-mode splitting and coherent energy exchange oscillations, both hallmarks of strong coupling. If we invert the spin to its highest energy state, we observe parametric-gain interactions, resulting in two-mode thermal noise squeezing. Furthermore, by shifting the phase of the light field between spin and membrane we can switch to non-Hamiltonian coupled dynamics, allowing us to observe level attraction and exceptional points. This high level of control in a strongly coupled modular system gives access to a unique toolbox for designing hybrid quantum systems and coherent optical feedback loops. Our approach to engineer coherent long-distance interactions with light makes it possible to couple very different systems in a modular way, opening up a range of new opportunities for quantum control.