  The study of light-matter interaction has led to many fundamental discoveries
as well as numerous important technologies. Over the last decades, great
strides have been made in increasing the strength of this interaction at the
single-photon level, leading to a continual exploration of new physics and
applications. Recently, a major achievement has been the demonstration of the
so-called strong coupling regime, a key advancement enabling great 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. Our results open
the door to a wealth of new physics and applications. 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.
