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arXiv:cond-mat/0606376 · Physical Review Letters

Qubit Measurements with a Double-Dot Detector

T. Gilad, S. A. Gurvitz

Published Sep 14, 2006• 42 citations
arXivHTMLPDF

Abstract

We propose to monitor a qubit with a double-dot (DD) resonant-tunneling detector, which can operate at higher temperatures than a single-dot detector. In order to assess the effectiveness of this device, we derive rate equations for the density matrix of the entire system. We show that the signal-to-noise ratio can be greatly improved by a proper choice of the parameters and location of the detector. We demonstrate that quantum interference effects within the DD detector play an important role in the measurement. Surprisingly, these effects produce a systematic measurement error, even when the entire system is in a stationary state.

DOI
10.1103/physrevlett.97.116806

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We propose to monitor a qubit with a double-dot (DD) resonant-tunneling detector, which can operate at higher temperatures than a single-dot detector. In order to assess the effectiveness of this device, we derive rate equations for the density matrix of the entire system. We show that the signal-to-noise ratio can be greatly improved by a proper choice of the parameters and location of the detector. We demonstrate that quantum interference effects within the DD detector play an important role in the measurement. Surprisingly, these effects produce a systematic measurement error, even when the entire system is in a stationary state.
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In this paper we propose a novel measurement scheme, based on a double quantum-dot detector, for reading out the qubit information sensed by the detector as a variation of the interdot coupling. By investigating the detector in the static qubit case, we find a sensitive qubit-detector response region determined by the detector geometry. Then we study quantum measurement of the information stored in a dynamic qubit. It is verified that whether the detector can perform well is closely dependent on the ratio of the right dot-lead coupling to interdot coupling in the detector, while the interdot Coulomb interaction only plays a trivial role. Furthermore, it is shown that the dephasing time of the qubit is strongly influenced by the difference between the interdot couplings of the detector corresponding to two qubit states. This work provides some helpful instructions on how to devise an effective qubit detector.
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We propose a quantum nondemolition measurement using a kicked two-state system (qubit). By tuning the waiting time between kicks to be the qubit oscillation period, the kicking apparatus performs a nondemolition measurement. While dephasing is unavoidable, the nondemolition measurement can (1) slow relaxation of diagonal density matrix elements, (2) avoid detector back-action, and (3) allow for a large signal-to-noise ratio. Deviations from the ideal behavior are investigated by allowing for detuning of the waiting time, as well as finite-time, noisy pulses. The scheme is illustrated with a double-dot qubit measured by a gate-pulsed quantum point contact.
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