Level attraction in a microwave optomechanical circuit

Level repulsion---the opening of a gap between two degenerate modes due to coupling---is ubiquitous anywhere from solid-state theory to quantum chemistry. In contrast, if one mode has negative energy, the mode frequencies attract instead. They converge and develop imaginary components, leading to an instability; an exceptional point marks the transition. This only occurs if the dissipation rates of the two modes are comparable. Here we expose a...