S. J. Weber

Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise

  1. D. H. Slichter,
  2. R. Vijay,
  3. S. J. Weber,
  4. S. Boutin,
  5. M. Boissonneault,
  6. J. M. Gambetta,
  7. A. Blais,
  8. and I. Siddiqi
We observe measurement-induced qubit state mixing in a transmon qubit dispersively coupled to a planar readout cavity. Our results indicate that dephasing noise at the qubit-readout
detuning frequency is up-converted by readout photons to cause spurious qubit state transitions, thus limiting the nondemolition character of the readout. Furthermore, we use the qubit transition rate as a tool to extract an equivalent flux noise spectral density at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^alpha$ fit to the measured flux noise spectral density below 1 Hz.
arxiv pdf

Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations

  1. R. Vijay,
  2. C. Macklin,
  3. D. H. Slichter,
  4. S. J. Weber,
  5. K. W. Murch,
  6. R. Naik,
  7. A. N. Korotkov,
  8. and I. Siddiqi
The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single, albeit probabilistic, outcome. The time-scale of this
„instantaneous“ process can be stretched using weak measurements so that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. We monitor the dynamics of a resonantly driven quantum two-level system — a superconducting quantum bit –using a near-noiseless parametric amplifier. The high-fidelity measurement output is used to actively stabilize the phase of Rabi oscillations, enabling them to persist indefinitely. This new functionality shows promise for fighting decoherence and defines a path for continuous quantum error correction.
arxiv pdf