Revealing the nonlinear response of a tunneling two-level system ensemble using coupled modes

• Published in: arXiv.org 2017-07
• Main Authors: Kirsh, Naftali, Svetitsky, Elisha, Burin, Alexander L, Schechter, Moshe, Katz, Nadav
• Format: Article
• Language: English
• Subjects: Coplanar waveguides; Coupled modes; Frequency shift; Inductance; Nonlinear response; Rabi frequency; Spectroscopy; Spectrum analysis; Superconducting devices; Superconductivity
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1702.08240

Atomic sized two-level systems (TLSs) in amorphous dielectrics are known as a major source of loss in superconducting devices. In addition, individual TLS are known to induce large frequency shifts due to strong coupling to the devices. However, in the presence of a broad ensemble of TLSs these shifts are symmetrically canceled out and not observed in a typical single-tone spectroscopy experiment. We introduce a two-tone spectroscopy on the normal modes of a pair of coupled superconducting coplanar waveguide resonators to reveal this effect. Together with an appropriate saturation model this enables us to extract the average single-photon Rabi frequency of dominant TLSs to be \(\Omega_0/2\pi \approx 79 \) kHz. At high photon numbers we observe an enhanced frequency shift due to nonlinear kinetic inductance when using the two-tone method and estimate the value of the nonlinear coefficient as \(K/2\pi \approx -1\times 10^{-4}\) Hz/photon. Furthermore, the life-time of each resonance can be controlled (increased) by pumping of the other mode as demonstrated both experimentally and theoretically.