Figure 1
Cavity transmission as a function of drive power and frequency, demonstrating the “bright” state at high incident power. (a) Dispersively shifted cavity response for excited (blue) and ground (red) states of the 8 GHz qubit with
photon mean cavity occupation. We reference this power to 0 dB. For all plots in this figure, the 8 GHz qubit is prepared, a measurement tone is pulsed on, and the responding homodyne amplitude is averaged for 400 ns to yield
. The mV scale used is arbitrary, but consistent to ease comparison. The
axis for (a)–(e) is frequency and covers the same range as for (f),(g). (b)–(d) Cavity response for increasing drive power, with data for previous power plotted with dashed lines. Transmission is inhibited by the cavity’s inherited nonlinearity, limiting dispersive measurement fidelity. Note, for example, that increasing drive 10 dB from (b) to (c) increases
by only a factor of
and complicates the frequency dependence. The emergence of a distinct resonance can be seen in (d). (e) High-transmission bright state. At large drive power, the Jaynes-Cummings cavity anharmonicity shrinks sufficiently to allow near-unity transmission at
. For this power the system only reaches its bright state when the qubit is excited due to the asymmetry of the dispersive cavity shift about
. This asymmetry is characteristic to the transmon qubit [
5] but might be possible to simulate for other designs [
17]. (f),(g) Cavity response (log magnitude) for qubit ground and excited states. The cavity continuously evolves from its low power linear behavior through the anharmonic bistable region and to the bright state. There are two peaks present in (g) due to qubit relaxation during measurement. The symbols (
) denote the optimal power and frequency for qubit readout, where the cavity response for the two qubit states is maximally different. (h) Response at
(arrows) versus input power, showing a steep jump in transmission corresponding to the onset of the bright state at a qubit-state-dependent power. Though transmission state dependence exists elsewhere, the behavior here is especially amenable for use as a qubit readout because the difference is large compared to amplifier noise.
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