Quantum simulation of an exotic quantum critical point in a two-site charge Kondo circuit

Tuning a material to the cusp between two distinct ground states can produce exotic physical properties, unlike those in either of the neighboring phases. The prospect of designing a model experimental system to capture such behavior is tantalizing. An array of tunnel-coupled quantum dots, each hosting a local spin, should have an appropriately complex phase diagram, but scaling up from individual dots to uniform clusters or lattices has proven difficult: though each site can be tuned to the same occupancy, each has a different set of localized wavefunctions whose couplings to neighboring sites cannot be made fully uniform. An array of metal nanostructures has complementary strengths and weaknesses: simple electrostatic tuning can make each element behave essentially identically, but intersite coupling is not tunable. In this work, we study a tunable nanoelectronic circuit comprising two coupled hybrid metallic-semiconductor islands, combining the strengths of the two types of materials, and demonstrating the potential for scalability. With two charge states of an island acting as an effective spin-1/2, the new architecture also offers a rich range of coupling interactions, and we exploit this to demonstrate a novel quantum critical point. Experimental results in the vicinity of the critical point match striking theoretical predictions.