Theoretical study of reflection spectroscopy for superconducting quantum parametrons

  1. S. Masuda,
  2. A. Yamaguchi,
  3. T. Yamaji,
  4. T. Yamamoto,
  5. T. Ishikawa,
  6. Y. Matsuzaki,
  7. and S. Kawabata
Superconducting parametrons in the single-photon Kerr regime, also called KPOs, have been attracting increasing attention in terms of their applications to quantum annealing and universal
quantum computation. It is of practical importance to obtain information of superconducting parametrons operating under an oscillating pump field. Spectroscopy can provide information of a superconducting parametron under examination, such as energy level structure and occupation of energy levels, and also useful information for calibration of the pump field. We theoretically study the reflection spectroscopy of superconducting parametrons, and develop a method to obtain the reflection coefficient. We present formulae of the reflection coefficient, the nominal external and the internal decay rates, and examine the obtained spectra.

Nonclassical photon number distribution in a superconducting cavity under a squeezed drive

  1. S. Kono,
  2. Y. Masuyama,
  3. T. Ishikawa,
  4. Y. Tabuchi,
  5. R. Yamazaki,
  6. K. Usami,
  7. K. Koshino,
  8. and Y. Nakamura
A superconducting qubit in the strong dispersive regime of a circuit quantum electrodynamics system is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit
spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in the cavity that is driven continuously with a squeezed vacuum field generated by a Josephson parametric amplifier. By fitting the qubit spectrum with a model which takes into account the finite qubit excitation power, the photon number distribution, which is dissimilar from the apparent peak area ratio in the spectrum, is determined. The photon number distribution shows the even-odd photon number oscillation and quantitatively fulfills Klyshko’s criterion for the nonclassicality.