Alexander L. Burin

Theory of nonlinear microwave absorption by interacting two-level systems

  1. Alexander L. Burin,
  2. and Andrii O. Maksymov
The microwave absorption and noise caused by quantum two-level systems (TLS) dramatically suppress the coherence in Josephson junction qubits that are promising candidates for a quantum
information applications. Microwave absorption by TLSs is not clearly understood yet because of the complexity of their interactions leading to the spectral diffusion. Here, the theory of the non-linear absorption in the presence of spectral diffusion is developed using the generalized master equation formalism. The theory predicts that the linear absorption regime holds while a TLS Rabi frequency is smaller than their phase decoherence rate. At higher external fields, a novel non-linear absorption regime is found with the loss tangent inversely proportional to the intensity of the field. The theory can be generalized to acoustic absorption and lower dimensions realized in superconducting qubits.
arxiv pdf

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

  1. Naftali Kirsh,
  2. Elisha Svetitsky,
  3. Alexander L. Burin,
  4. Moshe Schechter,
  5. and Nadav Katz
Atomic sized two-level systems (TLSs) in dielectrics are known as a major source of loss in superconducting devices, particularly due to frequency noise. However, the induced frequency
shifts on the devices, even by far off-resonance TLSs, is often suppressed by symmetry when standard single-tone spectroscopy is used. We introduce a two-tone spectroscopy on the normal modes of a pair of coupled superconducting coplanar waveguide resonators to uncover this effect by asymmetric saturation. Together with an appropriate generalized saturation model this enables us to extract the average single-photon Rabi frequency of dominant TLSs to be Ω0/2π≈79 kHz. At high photon numbers we observe an enhanced sensitivity to nonlinear kinetic inductance when using the two-tone method and estimate the value of the Kerr coefficient as K/2π≈−1×10−4 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.
arxiv pdf