Junghyun Kim

Emergent Harmonics in Josephson Tunnel Junctions Due to Series Inductance

  1. Junghyun Kim,
  2. Max Hays,
  3. Ilan T. Rosen,
  4. Junyoung An,
  5. Helin Zhang,
  6. Aranya Goswami,
  7. Kate Azar,
  8. Jeffrey M. Gertler,
  9. Bethany M. Niedzielski,
  10. Mollie E. Schwartz,
  11. Terry P. Orlando,
  12. Jeffrey A. Grover,
  13. Kyle Serniak,
  14. and William D. Oliver
Josephson tunnel junctions are essential elements of superconducting quantum circuits. The operability of these circuits presumes a 2π-periodic sinusoidal potential of a tunnel junction,
but higher-order corrections to this Josephson potential, often referred to as „harmonics,“ cause deviations from the expected circuit behavior. Two potential sources for these harmonics are the intrinsic current-phase relationship of the Josephson junction and the inductance of the metallic traces connecting the junction to other circuit elements. Here, we introduce a method to distinguish the origin of the observed harmonics using nearly-symmetric superconducting quantum interference devices (SQUIDs). Spectroscopic measurements of level transitions in multiple devices reveal features that cannot be explained by a standard cosine potential, but are accurately reproduced when accounting for a second-harmonic contribution to the model. The observed scaling of the second harmonic with Josephson-junction size indicates that it is due almost entirely to the trace inductance. These results inform the design of next-generation superconducting circuits for quantum information processing and the investigation of the supercurrent diode effect.
arxiv pdf

Non-degenerate noise-resilient superconducting qubit

  1. Max Hays,
  2. Junghyun Kim,
  3. and William D. Oliver
We propose a superconducting qubit based on engineering the first and second harmonics of the Josephson energy and phase relation EJ1cosφ and EJ2cos2φ. By constructing a circuit such
that EJ2 is negative and |EJ1|≪|EJ2|, we create a periodic potential with two non-degenerate minima. The qubit, which we dub „harmonium“, is formed from the lowest-energy states of each minimum. Bit-flip protection of the qubit arises due to the localization of each qubit state to their respective minima, while phase-flip protection can be understood by considering the circuit within the Born-Oppenheimer approximation. We demonstrate with time-domain simulations that single- and two-qubit gates can be performed in approximately one hundred nanoseconds. Finally, we compute the qubit coherence times using numerical diagonalization of the complete circuit in conjunction with state-of-the-art noise models. We estimate out-of-manifold heating times on the order of milliseconds, which can be treated as erasure errors using conventional dispersive readout. We estimate pure-dephasing times on the order of many tens of milliseconds, and bit-flip times on the order of seconds.
arxiv pdf