Bath engineering with parametric mixing in cQED
While loss and decay are inevitable in quantum systems, loss need not be our enemy. My PhD work focuses on engineering losses and effective ‘baths’ for quantum systems to, for instance, gain in-situ control of the effective temperature/populations and relaxation rates of a multi-level transmon qubit. This is achieved by dispersively coupling a single transmon qubit to a SNAIL mode. The SNAIL has two functions: first to be the source of three- (and sometimes four-) wave mixing that allows us to drive parametric operations to create and exchange photons among the qubit and SNAIL modes at a rate of our choosing, and second to be a source of loss that converts these coherent processes to controlled heating and cooling of the transmon. My talk will highlight two experimental use cases for such a multimode system. First, a fully controllable quantum bath where we can push the qubit towards states with positive, negative, and infinite temperature, and even make ‘unnatural’ relaxation states such as a transmon that relaxes equally from |e> to |g> and |f>. These transmons may have applications in problems like quantum simulators and cavity arrays. Second, we use parametric transmon heating to realize a single-, artificial-atom micro-maser as a very narrow cryogenic light source. This is an ideal platform for studying quantum optics in a microwave circuit setting, for instance realizing a recent proposal to build a maser far narrower than the Schawlow-Townes limit.