Maxime Boissonneault

Superconducting qubit as a probe of quantum fluctuations in a nonlinear resonator

  1. Maxime Boissonneault,
  2. A. C. Doherty,
  3. F. R. Ong,
  4. P. Bertet,
  5. D. Vion,
  6. D. Esteve,
  7. and A. Blais
In addition to their central role in quantum information processing, qubits have proven to be useful tools in a range of other applications such as enhanced quantum sensing and as spectrometers
of quantum noise. Here we show that a superconducting qubit strongly coupled to a nonlinear resonator can act as a probe of quantum fluctuations of the intra-resonator field. Building on previous work [M. Boissoneault et al. Phys. Rev. A 85, 022305 (2012)], we derive an effective master equation for the qubit which takes into account squeezing of the resonator field. We show how sidebands in the qubit excitation spectrum that are predicted by this model can reveal information about squeezing and quantum heating. The main results of this paper have already been successfully compared to experimental data [F. R. Ong et al. Phys. Rev. Lett. 110, 047001 (2013)] and we present here the details of the derivations.
arxiv pdf

Improved qubit bifurcation readout in the straddling regime of circuit QED

  1. Maxime Boissonneault,
  2. J. M. Gambetta,
  3. and A. Blais
We study bifurcation measurement of a multi-level superconducting qubit using a nonlinear resonator biased in the straddling regime, where the resonator frequency sits between two qubit
transition frequencies. We find that high-fidelity bifurcation measurements are possible because of the enhanced qubit-state-dependent pull of the resonator frequency, the behavior of qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit that can be realized in this regime. Numerical simulations find up to a threefold improvement in qubit readout fidelity when operating in, rather than outside of, the straddling regime. High-fidelity measurements can be obtained at much smaller qubit-resonator couplings than current typical experimental realizations, reducing spectral crowding and potentially simplifying the implementation of multi-qubit devices.
arxiv pdf