Qubit detection with a T-shaped double quantum dot detector

TL;DR

This paper investigates a T-shaped double quantum dot detector for continuous charge qubit measurement, revealing unique noise and dephasing behaviors, and demonstrating potential for quantum-limited measurement performance.

Contribution

It introduces a novel detector design and analyzes its measurement dynamics, including noise characteristics and conditions for quantum-limited efficiency.

Findings

Super-Poissonian noise due to dynamical blockade effects.

Inhibition of dephasing despite increased noise.

Suppressed noise pedestal leading to divergent signal-to-noise ratio.

Abstract

We propose to continuously monitor a charge qubit by utilizing a T-shaped double quantum dot detector, in which the qubit and double dot are arranged in such a unique way that the detector turns out to be particularly susceptible to the charge states of the qubit. Special attention is paid to the regime where acquisition of qubit information and backaction upon the measured system exhibit nontrivial correlation. The intrinsic dynamics of the qubit gives rise to dynamical blockade of tunneling events through the detector, resulting in a super-Poissonian noise. However, such a pronounced enhancement of detector's shot noise does not necessarily produce a rising dephasing rate. In contrast, an inhibition of dephasing is entailed by the reduction of information acquisition in the dynamically blockaded regimes. We further reveal the important impact of the charge fluctuations on the measurement characteristics. Noticeably, under the condition of symmetric junction capacitances the noise pedestal of circuit current is completely suppressed, leading to a divergent signal-to-noise ratio, and eventually to a violation of the Korotkov-Averin bound in quantum measurement. Our study offers the possibility for a double dot detector to reach the quantum limited effectiveness in a transparent manner.