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

Abstract

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.

6 pages, 4 figures + supplementary (with 18 pages and 5 figures)

References in corpus (5)

• Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators
• Coplanar Waveguide Resonators for Circuit Quantum Electrodynamics
• Efficient and robust analysis of complex scattering data under noise in microwave resonators
• Quantum limited amplification and entanglement in coupled nonlinear resonators
• Contribution of dielectrics to frequency and noise of NbTiN superconducting resonators

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