Quantum Measurement with the Josephson Bifurcation Amplifier

Irfan A. Siddiqi, Rajamani Vijay, F. Pierre +5 more

Rajamani Vijayaraghavan

Hayato Nakano, Shiro Saito, Kouichi Semba +1 more

We analyzed the Josephson bifurcation amplifier (JBA) readout process of a superconducting qubit quantum mechanically by calculating the dynamics of the density operator of a driven nonlinear oscillator and a qubit coupled system during the measurement process. In purely quantum cases, bifurcation is impossible. Introducing decoherence enables us to reproduce the bifurcation with a finite hysteresis. When a qubit is initially in a superposition state, we have observed the qubit-probe (JBA) entangled state, and it is divided into two separable states at the moment the JBA transition begins. This corresponds to "projection." To readout the measurement result, however, we must wait until the two JBA states are macroscopically well separated. The waiting time is determined by the strength of the decoherence in the JBA.

Kosuke Kakuyanagi, Yuichiro Matsuzaki, Tatsuya Baba +3 more

Quantum measurements play a central role in quantum information processing. Understanding the mechanism of the quantum measurements is essential to both improve the visibility of the readout and decrease the projection error during measurement. Here, we describe and demonstrate an experimental protocol for quantifying a measurement backaction when we readout a superconducting flux qubit using a Josephson bifurcation amplifier. The coupling with the Josephson bifurcation amplifier induces a dephasing on the flux qubit during the readout. We have succeeded in estimating and controlling the strength of such a measurement-induced dephasing by changing the pulse height of the microwave driving. Our results are crucial for designing a suitable quantum readout technique for the realization of practical quantum devices.

Roger A. Hegstrom, Fernando Sols

Carmine Granata, CORATO V., Longobardi L. +3 more