Seminar: "Spectroscopy of Topological Josephson Junction Circuits" by Dr. Çağlar Girit


Monday, April 24, 2023 - 14:00 to 15:00




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. 




Çağlar Girit obtained his B.Sc. from MIT (2003) and his Ph.D. from UC Berkeley in 2010 under advisor Prof. Alex Zettl.
During his Ph.D. (2004-2010) he studied electronic and structural properties of graphene and hybrid graphene-superconductor devices. As a postdoctoral researcher (2010-2014), he measured Andreev bound states in superconducting atomic contacts.
In September 2014, he started the Flux Quantum Lab at Collège de France, a group focusing on novel applications of Josephson junctions.  In October 2014, he became a French National Research Agency (CNRS) permanent researcher (CR1).  In May 2022, he transferred his lab to the Quantronics Group, at SPEC, CEA-Saclay (CNRS UMR 3680) (
He received a National Science Foundation Graduate Research Fellowship (2004-2007), the UC Berkeley Lars Commins Memorial Award (2009), and a Marie Curie Postdoctoral Fellowship
(2012-2014).  His research is supported by IDEX grant ANR-10-IDEX-0001-02 PSL (2014-2016), Paris Programme Emergence(s) Grant (2014-2018), ERC Starting Grant (2015-2020), and ANR Grants TRIANGLE (2020-) and StrongQEDmpc (2023-).
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