Abstract
We propose a generalization of dispersive qubit readout that provides the time evolution of a flux qubit observable. Our proposal relies on the nonlinear coupling of the qubit to a harmonic oscillator with high frequency, representing a dc superconducting quantum interference device. Information about the qubit dynamics is obtained by recording the oscillator response to resonant driving and subsequent lock-in detection. The measurement process is simulated for the example of coherent qubit oscillations. This corroborates the underlying measurement relation and also reveals that the measurement scheme possesses low backaction and high fidelity.
Highlights
We propose a generalization of dispersive qubit readout that provides the time evolution of a flux qubit observable
We have generalized dispersive qubit readout to the time-resolved observation of the qubit dynamics
Concerning the setup, the main difference to dispersive readout is that in the present proposal, the oscillator frequency needs to exceed the qubit splitting by roughly one order of magnitude, and the oscillator bandwidth should be at least twice the qubit frequency
Summary
We consider a superconducting flux qubit coupled to a SQUID [7], as sketched in figure 1. The second coupling term proportional to g2, by contrast, is quadratic in the oscillator coordinate. Its physical origin is a nonlinear Josephson inductance, which depends on the magnetic flux, by which the SQUID is penetrated [7, 18]. This term provides a frequency shift already in first order of g2. We focus on setups of flux qubits coupled to SQUIDs possessing a sizable quadratic coupling, as described above. Q = a† + a is the oscillator coordinate, such that the interaction term represents the inductive coupling between the qubit and the flux degree of freedom of the SQUID.
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