General method for extracting the quantum efficiency of dispersive qubit readout in circuit QED

  1. C. C. Bultink,
  2. B. Tarasinski,
  3. N. Haandbaek,
  4. S. Poletto,
  5. N. Haider,
  6. D. J. Michalak,
  7. A. Bruno,
  8. and L. DiCarlo
We present and demonstrate a general 3-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons
and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephon traveling-wave parametric amplifier and to quantify the different noise contributions in the readout amplification chain.

Scalable quantum circuit and control for a superconducting surface code

  1. R. Versluis,
  2. S. Poletto,
  3. N. Khammassi,
  4. N. Haider,
  5. D. J. Michalak,
  6. A. Bruno,
  7. K. Bertels,
  8. and L. DiCarlo
We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tuneable transmon qubits with nearest-neighbor coupling.
An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequency detuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based X- and Z-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.