Figure 4: Qubit readout using PPLO.
(a) Measurement set-up and pulse sequence. A 3-JJ superconducting flux qubit biased at Φq=Φ0/2 is capacitively (Cc=4 fF) coupled to a CPW resonator. Here 
and Nr represent the resonant frequency and the mean photon number of the readout resonator, respectively. The internal (
) and external (
) quality factors are 2.5 × 104 and 630, respectively, resulting in the loaded quality factor 
of 620. In the pulse sequence, Δt (=100 ns) represents the data acquisition time to extract the phase, and 
and Pc represent the power of the readout and the qubit control pulses, respectively. A short spike was added in the readout pulse to improve the readout fidelity. In c–f, we applied the pulse sequence 1 × 104 times to obtain the probability. (b) Frequency dependence of the phase of the reflection coefficient of the readout resonator with qubit control π pulse on (green) and off (blue). (c) Probability of the PPLO 0π state as a function of the readout-signal phase θs with the qubit control π pulse on (green) and off (blue). Here tc=10 ns, tp=50 ns, tr=50 ns and td=300 ns. (d) Rabi oscillations measured with tp=30 ns, tr=50 ns and td=300 ns. (e) Rabi oscillations along the dashed line in d. Red curve represents a fit with an exponentially damped sinusoidal function. (f) Visibility as a function of td. Here tc=10 ns, tp=30 ns and tr=50 ns. The error bars represents the standard deviation in five identical measurements.
