Quantum Efficiency of Charge Qubit Measurements Using a Single Electron Transistor

TL;DR

This paper investigates the quantum efficiency of charge qubit measurements using a single-electron transistor, analyzing models with different response strengths to determine conditions for quantum-limited measurement performance.

Contribution

It introduces a detailed analysis of SET-based charge qubit measurement models, identifying response strength bounds and conditions for achieving quantum-limited measurements.

Findings

Response strength bounds quantum efficiency.

Quantum-limited measurement possible with strong response and asymmetry.

Clarifies measurement and dephasing rate evaluations.

Abstract

The quantum efficiency, which characterizes the quality of information gain against information loss, is an important figure of merit for any realistic quantum detectors in the gradual process of collapsing the state being measured. In this work we consider the problem of solid-state charge qubit measurements with a single-electron-transistor (SET). We analyze two models: one corresponds to a strong response SET, and the other is a tunable one in response strength. We find that the response strength would essentially bound the quantum efficiency, making the detector non-quantum-limited. Quantum limited measurements, however, can be achieved in the limits of strong response and asymmetric tunneling. The present study is also associated with appropriate justifications for the measurement and backaction-dephasing rates, which were usually evaluated in controversial methods.