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The study of light-matter interaction has led to many fundamental discoveriesas well as numerous important technologies. Over the last decades, greatstrides have been made in increasing the strength of this interaction at thesingle-photon level, leading to a continual exploration of new physics andapplications. Recently, a major achievement has been the demonstration of theso-called strong coupling regime, a key advancement enabling great progress inquantum information science. Here, we demonstrate light-matter interaction overan order of magnitude stronger than previously reported, reaching thenonperturbative regime of ultrastrong coupling (USC). We achieve this using asuperconducting artificial atom tunably coupled to the electromagneticcontinuum of a one-dimensional waveguide. For the largest coupling, thespontaneous emission rate of the atom exceeds its transition frequency. In thisUSC regime, the description of atom and light as distinct entities breaks down,and a new description in terms of hybrid states is required. Our results openthe door to a wealth of new physics and applications. Beyond light-matterinteraction itself, the tunability of our system makes it a promising tool tostudy a number of important physical systems such as the well-known spin-bosonand Kondo models.

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