Quantum estimation of parameter in circuit QED by continuous quantum measurement

  1. Cheng Zhang,
  2. Kai Zhou,
  3. Wei Feng,
  4. and Xin-Qi Li
Designing high-precision and efficient schemes is of crucial importance for quantum parameter estimation in practice. The estimation scheme based on continuous quantum measurement is
one possible type of this, which looks also the most natural choice in case such as continuous dynamical process. In this work we specify the study to the stat-of-the-art superconducting circuit quantum-electrodynamics (QED) system, where the high-quality continuous measurement has been extensively exploited in the past decade. Within the framework of Bayesian estimation and particularly using the quantum Bayesian rule in circuit QED, we numerically simulate the likelihood function as estimator for the Rabi frequency of qubit oscillation. We find that, by proper design of the interaction strength of measurement, the estimate precision can scale with the measurement time beyond the standard quantum limit, which is usually assumed for this type of continuous measurement since no more special quantum resource is involved. We understand this remarkable result by quantum correlation in time between the output signals, and simulate the effect of quantum efficiency of the measurement on the precision scaling behavior.

Gradual partial-collapse theory for ideal nondemolition measurements of qubits in circuit QED

  1. Wei Feng,
  2. Cheng Zhang,
  3. Zhong Wang,
  4. Lupei Qin,
  5. and Xin-Qi Li
The conventional method of qubit measurements in circuit QED is employing the dispersive regime of qubit-cavity coupling, which results in an approximated scheme of quantum nondemolition
(QND) readout. However, this scheme breaks down owing to the Purcell effect in the case of strong coupling and/or strong measurement drive. To remove the drawbacks of the dispersive readout, a recent proposal by virtue of longitudinal coupling suggests a new scheme to realize fast, high-fidelity and ideal QND readout of qubit state. In the present work, following dispersive readout, we construct the gradual partial-collapse theory for this new measurement scheme, in terms of both the quantum trajectory equation and quantum Bayesian approach. The longitudinal coupling provides as well a convenient method of cavity reset. In combination with the reset procedure, the established theory is expected to be useful for such as measurement-based feedback control and many other quantum applications associated with partial-collapse weak measurements.