Readout strength dependence of state projection in superconducting qubit

TL;DR

This paper investigates how varying measurement strength in superconducting qubits affects state projection during readout, revealing that projection occurs above a certain pulse threshold and persists even as visibility diminishes.

Contribution

It provides experimental insights into the conditions under which quantum state projection occurs in JBA-based readout of superconducting qubits, clarifying the measurement process.

Findings

Projection occurs above the pulse height where visibility begins.

Projection persists even as visibility vanishes with increasing readout strength.

The results enhance understanding of the JBA readout mechanism.

Abstract

We have controlled the measurement strength of the qubit readout process by using a Josephson bifurcation amplifier (JBA : an AC-driven SQUID). The optimum readout pulse changes the superposition state into an eigenstate. To clarify how a sufficiently strong measurement causes a projection phenomenon, we have studied the projection conditions in the JBA readout. We found that the projection occurs above the pulse height at which visibility starts to appear. Furthermore, the projection continues to occur as readout strength increases, even though the visibility vanishes. This result helps us understanding the JBA readout process.