The goal of this project is to study how light interacts with matter when that interaction becomes extraordinarily strong. Scientists have long understood how light interacts with bulk matter: for example, it can reflect off surfaces and bend when it enters glass. More recently, scientists have begun to explore the extremes of light-matter interaction. What happens when a single photon (particle of light) interacts with a single atom? The atom can either absorb or emit one photon, but if it is placed between two mirrors, it can absorb and re-emit the same photon many times. When this happens, the atom and photon form a new collective system, like a molecule between the atom and the photon. In this case, the interaction is said to be strong. In this project, the goal is to explore what happens when the interaction becomes still stronger, and more photons become involved. In particular, scientists have predicted that light itself will undergo a phase transition (like when water turns into ice). Here, patterned metal circuits, like those in a computer chip, will be used to study the interaction between microwave photons and artificial atoms. In particular, the chief aim is to observe a phase transition for these microwave photons and to study the new properties of light that emerge. This will give insight into how interactions lead to complex new behavior, a theme that runs throughout much of science and technology. Moreover, the specific circuits that will be used to study this are also used for building a new type of computer called a quantum computer; improvement in these circuits is an ancillary benefit.
The specific aim of this proposal is to study the emergence of many-body states of light in systems of interacting photons. Effective photon-photon interactions will be generated using superconducting microwave circuits, with effective interactions mediated by Josephson-junction qubits. Two different architectures will be used to study these phase transitions. First, arrays of microwave cavities, each coupled to a superconducting qubit, will be used to generate a lattice of interacting microwave photons. The lattice will be populated with a steady state drive and dissipation, which, under appropriate conditions, will lead to a steady-state, non-equilibrium phase transition. The nature of this phase transition and of the steady states will be explored. Second, superconducting qubits will be placed in a photonic bandgap medium. These qubits will result in the generation of bound single photon states; multiple qubits will result in overlapping bound photons, generating a highly tunable lattice to study these many-body quantum optics effects. This project provides a complementary approach to studying phase transitions with cold atoms, and can easily access non-equilibrium physics and transport properties of the system.
