John Clarke

Suppressing relaxation in superconducting qubits by quasiparticle pumping

  1. Simon Gustavsson,
  2. Fei Yan,
  3. Gianluigi Catelani,
  4. Jonas Bylander,
  5. Archana Kamal,
  6. Jeffrey Birenbaum,
  7. David Hover,
  8. Danna Rosenberg,
  9. Gabriel Samach,
  10. Adam P. Sears,
  11. Steven J. Weber,
  12. Jonilyn L. Yoder,
  13. John Clarke,
  14. Andrew J. Kerman,
  15. Fumiki Yoshihara,
  16. Yasunobu Nakamura,
  17. Terry P. Orlando,
  18. and William D. Oliver
Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous
coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.
arxiv pdf

Asymmetric frequency conversion in nonlinear systems driven by a biharmonic pump

  1. Archana Kamal,
  2. Ananda Roy,
  3. John Clarke,
  4. and Michel H. Devoret
A novel mechanism of asymmetric frequency conversion is investigated in nonlinear dispersive devices driven parametrically with a biharmonic pump. When the relative phase between the
first and second harmonics combined in a two-tone pump is appropriately tuned, nonreciprocal frequency conversion, either upward or downward, can occur. Full directionality and efficiency of the conversion process is possible, provided that the distribution of pump power over the harmonics is set correctly. While this asymmetric conversion effect is generic, we describe its practical realization in a model system consisting of a current-biased, resistively-shunted Josephson junction. Here, the multiharmonic Josephson oscillations, generated internally from the static current bias, provide the pump drive.
arxiv pdf

Gain, directionality and noise in microwave SQUID amplifiers: Input-output approach

  1. Archana Kamal,
  2. John Clarke,
  3. and Michel Devoret
We present a new theoretical framework to analyze microwave amplifiers based on the dc SQUID. Our analysis applies input-output theory generalized for Josephson junction devices biased
in the running state. Using this approach we express the high frequency dynamics of the SQUID as a scattering between the participating modes. This enables us to elucidate the inherently nonreciprocal nature of gain as a function of bias current and input frequency. This method can, in principle, accommodate an arbitrary number of Josephson harmonics generated in the running state of the junction. We report detailed calculations taking into account the first few harmonics that provide simple semi-quantitative results showing a degradation of gain, directionality and noise of the device as a function of increasing signal frequency. We also discuss the fundamental limits on device performance and applications of this formalism to real devices.
arxiv pdf

Heralded state preparation in a superconducting qubit

  1. J. E. Johnson,
  2. C. Macklin,
  3. D. H. Slichter,
  4. R. Vijay,
  5. E. B. Weingarten,
  6. John Clarke,
  7. and I. Siddiqi
We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a superconducting flux qubit in which the pointer state distributions can be resolved to below one part in
1000. In the weak excitation regime, continuous measurement permits the use of heralding to ensure initialization to a fiducial state, such as the ground state. This procedure boosts readout fidelity to 93.9% by suppressing errors due to spurious thermal population. Furthermore, heralding potentially enables a simple, fast qubit reset protocol without changing the system parameters to induce Purcell relaxation.
arxiv pdf