A. P. Sears

The Flux Qubit Revisited

  1. F. Yan,
  2. S. Gustavsson,
  3. A. Kamal,
  4. J. Birenbaum,
  5. A. P. Sears,
  6. D. Hover,
  7. T.J. Gudmundsen,
  8. J.L. Yoder,
  9. T. P. Orlando,
  10. J. Clarke,
  11. A.J. Kerman,
  12. and W. D. Oliver
The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). In this work, we revisit the design and fabrication
of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and coherence times in excess of 40 us at its flux-insensitive point. Qubit relaxation times across 21 qubits of widely varying designs are consistently matched with a single model involving ohmic charge noise, quasiparticle fluctuations, resonator loss, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, reaching T2 ~ 80 us , approximately the 2T1 limit. In addition to realizing a dramatically improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary state-of-art qubits based on transverse qubit-resonator interaction.
arxiv pdf

Thermal and Residual Excited-State Population in a 3D Transmon Qubit

  1. X. Y. Jin,
  2. A. Kamal,
  3. A. P. Sears,
  4. T. Gudmundsen,
  5. D. Hover,
  6. J. Miloxi,
  7. R. Slattery,
  8. F. Yan,
  9. J. Yoder,
  10. T. P. Orlando,
  11. S. Gustavsson,
  12. and W. D. Oliver
We present a systematic study of the first excited-state population in a 3D transmon qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of
the protocol developed by Geerlings et al. [1], we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates to 0.1%, near the resolution of our measurement. We verified this result using a flux qubit with ten-times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T_{eff} = 35 mK. Assuming T_{eff} is due solely to hot quasiparticles, the inferred qubit lifetime is 108 us and in plausible agreement with the measured 80 us.
arxiv pdf

Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED

  1. A. P. Sears,
  2. A. Petrenko,
  3. G. Catelani,
  4. L. Sun,
  5. Hanhee Paik,
  6. G. Kirchmair,
  7. L. Frunzio,
  8. L. I. Glazman,
  9. S. M. Girvin,
  10. and R. J. Schoelkopf
We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival
or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying textit{in situ} the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We investigate photon dynamics in this limit and observe large increases in coherence times as the cavity is decoupled from the environment. Our experiments suggest that the intrinsic coherence of small Josephson junctions, when corrected with a single Hahn echo, is greater than several hundred microseconds.
arxiv pdf