Eunseong Kim

Measurement-induced bistability in the excited states of a transmon

  1. Jeakyung Choi,
  2. Hyeok Hwang,
  3. and Eunseong Kim
High power measurement-induced cavity response is investigated in the |g>, |e>, and |f> states of a transmon. All the states exhibit photon blockades above a certain critical value,
a phenomenon that has previously been understood based on the bistability of semiclassical Duffing oscillators. The measurement-induced state transition (MIST) to high-level transmon states is expected to be one contributor to the bistability; however, the critical values measured in the |e> and |f> states are not coincident with the MIST. To understand this discrepancy, we utilize the recently developed semiclassical dynamics model of a cavity photon state. The appearance of dim and bright cavity states obtained from the model’s steady-state solution leads to the photon blockades at lower critical photon numbers, and this can explain the response of the bistable region in the |e> and |f> states.
arxiv pdf

Optimization of infrared and magnetic shielding of superconducting TiN and Al coplanar microwave resonators

  1. John Mark Kreikebaum,
  2. Allison Dove,
  3. William Livingston,
  4. Eunseong Kim,
  5. and Irfan Siddiqi
We present a systematic study of the effects of shielding on the internal quality factors (Qi) of Al and TiN microwave resonators designed for use in quantum coherent circuits. Measurements
were performed in an adiabatic demagnetization refrigerator, where typical magnetic fields of 200 {\mu}T are present at the unshielded sample stage. Radiation shielding consisted of 100 mK and 500 mK Cu cans coated with infrared absorbing epoxy. Magnetic shields consisted of Cryoperm 10 and Sn plating of the Cu cans. A 2.7 K radiation can and coaxial thermalization filters were present in all measurements. TiN samples with Qi = 1.3∗106 at 100 mK exhibited no significant variation in quality factor when tested with limited shielding. In contrast, Al resonators showed improved Qi with successive shielding, with the largest gains obtained from the addition of the first radiation and magnetic shields and saturating before the addition of Sn plating infrared absorbing epoxy.
arxiv pdf