Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime

The study of light–matter interaction has led to important advances in quantum optics and enabled numerous technologies. Over recent decades, progress has been made in increasing the strength of this interaction at the single-photon level. More recently, a major achievement has been the demonstration of the so-called strong coupling regime, a key advancement enabling progress in quantum information science. Here, we demonstrate light–matter interaction over an order of magnitude stronger than previously reported, reaching the nonperturbative regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest coupling, the spontaneous emission rate of the atom exceeds its transition frequency. In this USC regime, the description of atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required. Beyond light–matter interaction itself, the tunability of our system makes it a promising tool to study a number of important physical systems, such as the well-known spin-boson and Kondo models.