Measuring charge based quantum bits by a superconducting single-electron transistor

TL;DR

This paper demonstrates that a superconducting single-electron transistor significantly improves the measurement of charge-based quantum bits by effectively blocking current in certain qubit states, enhancing read-out fidelity.

Contribution

It introduces a coupled superconducting transistor as a more effective read-out device for Cooper pair charge qubits compared to normal-metal transistors.

Findings

Superconducting gap blocks current when qubit is in logical state 1.

Superconducting transistor outperforms normal-metal transistor in measurement.

Numerical analysis confirms effectiveness with aluminium-based parameters.

Abstract

Single-electron transistors have been proposed to be used as a read-out device for Cooper pair charge qubits. Here we show that a coupled superconducting transistor at a threshold voltage is much more effective in measuring the state of a qubit than a normal-metal transistor at the same voltage range. The effect of the superconducting gap is to completely block the current through the transistor when the qubit is in the logical state 1, compared to the mere diminishment of the current in the normal-metal case. The time evolution of the system is solved when the measuring device is driven out of equilibrium and the setting is analysed numerically for parameters accessible by lithographic aluminium structures.