Seminar: "Spectroscopy of Topological Josephson Junction Circuits" by Dr. Çağlar Girit, CNRS and CEA-Saclay, France. 

Topology, like symmetry, is a fundamental concept in understanding the spectra of physical systems.  Non-trivial topological properties of a system may enable applications in fields such as in metrology or quantum information.  The most remarkable of these applications is the resistance standard using the quantum Hall effect and the voltage standard using the AC Josephson effect.  We show that a superconducting circuit with three Josephson tunnel junctions, the BiSQUID, has protected degeneracies in the topologically non-trivial regime.  The spectrum, which is intrinsic to the device and not "simulated," is analogous to that of a Weyl semimetal.  We have fabricated BiSQUIDs and measured the microwave spectra of both topologically trivial circuits, which are shown to be gapped, and non-trivial circuits, in which the spectra are consistent with gap closing.  A more realistic model indicates that the degeneracy persists in the presence of geometric inductance.  Our prototype topological Josephson circuit motivates the classification of all such circuits and serves as a basis for applications. 

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