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Journal of Physics Condensed Matter

Quantum measurement of dynamic qubit information by a double quantum-dot detector

Zhaotan Jiang, Jiajie Yang, Qingzhen Han

Published Jan 28, 2008• 4 citations
Publisher

Abstract

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.

DOI
10.1088/0953-8984/20/7/075210

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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 consider evolution of a double quantum dot (DQD) two-electron spin qubit which is continuously weakly measured with a linear charge detector (quantum point contact). Since the interaction between the spins of two electrons depends on their charge state, the charge measurement affects the state of two spins, and induces non-trivial spin dynamics. We consider the regimes of strong and weak coupling to the detector, and investigate the measurement-induced spin dynamics both analytically and numerically. We observe emergence of the negative-result evolution and the system stabilization due to an analog of quantum Zeno effect. Moreover, unitary evolution between the triplet and a singlet state is induced by the negative-result measurement. We demonstrate that these effects exist for both strong and weak coupling between the detector and the DQD system.
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We demonstrate one and two photoelectron trapping and the subsequent dynamics associated with interdot transfer in double quantum dots over a time scale much shorter than the typical spin lifetime. Identification of photoelectron trapping is achieved via resonant interdot tunneling of the photoelectrons in the excited states. The interdot transfer enables detection of single photoelectrons in a nondestructive manner. When two photoelectrons are trapped at almost the same time we observed that the interdot resonant tunneling is strongly affected by the Coulomb interaction between the electrons. Finally the influence of the two-electron singlet-triplet state hybridization has been detected using the interdot tunneling of a photoelectron.
Bound states in quantum dots coupled to superconductors can be in a coherent superposition of states with different electron number but with the same fermion parity. Electrostatic gating can tune this superposition to a sweet spot, where the quantum dot has the same mean electric charge independent of its electron-number parity. Here, we propose to encode quantum information in the local fermion parity of two tunnel-coupled quantum dots embedded in a Josephson junction. At the sweet spot, the qubit states have zero charge dipole moment. This protects the qubit from dephasing due to charge noise acting on the potential of each dot, as well as fluctuations of the (weak) interdot tunneling. At weak interdot tunneling, relaxation is suppressed because of disjoint qubit states. However, for strong interdot tunneling the system is protected against noise affecting each quantum dot separately (energy-level noise, dot-superconductor tunneling fluctuations, and hyperfine interactions). Finally, we describe initialization and readout as well as single-qubit and two-qubit gates by pulsing gate voltages. Published by the American Physical Society 2024