Experimental analysis of the measurement strength dependence of superconducting qubit readout using a Josephson bifurcation readout method

Abstract

We succeeded in controlling the measurement strength of a macroscopic quantum system, namely a persistent current quantum bit (qubit), by using a transmission line type Josephson bifurcation amplifier (JBA: an ac-driven superconducting quantum interferometer device) as a probe. By employing a special pulse sequence, we found that the weighted average of the Ramsey fringe visibility ( α -value) can be used as an indicator of quantum state projection. The sudden change in the α -value magnitude at around the threshold suggests that an entangled state of the qubit–JBA composite system $ (\left |g \right >_{\mathrm { qubit}}\left |E \right >_{\mathrm { JBA}}+\left |e \right >_{\mathrm { qubit}}\left |G \right >_{\mathrm { JBA}})$ settles in one of the two possible classically correlated qubit–JBA states caused by strong decoherence. This is an important result in terms of understanding the mechanisms of quantum state measurement.