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Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena

Controlled decoherence of a charge qubit in a double quantum dot

T. Fujisawa, T. Hayashi, Y. Hirayama

Published Jul 1, 2004• 36 citations
Publisher

Abstract

We investigate coherent time evolution of a charge qubit in a semiconductor double quantum dot. A high-speed voltage pulse controls the energy bias and strength of the decoherence of the system. The qubit is effectively isolated from the electrodes in the Coulomb blockade regime, while it is affected by dissipative tunneling processes in the single-electron tunneling regime. We discuss the importance of the controlled decoherence by means of density matrix simulations.

DOI
10.1116/1.1771679

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We investigate coherent time evolution of a charge qubit in a semiconductor double quantum dot. A high-speed voltage pulse controls the energy bias and strength of the decoherence of the system. The qubit is effectively isolated from the electrodes in the Coulomb blockade regime, while it is affected by dissipative tunneling processes in the single-electron tunneling regime. We discuss the importance of the controlled decoherence by means of density matrix simulations.
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The coherent time evolution of electrons in double quantum dots (DQDs) induced by fast bias-voltage switches is studied theoretically. As it was shown experimentally, such driven DQDs are potential devices for controlled manipulation of charge qubits. By numerically solving a quantum master equation, we obtain the energy- and time-resolved electron transfer through the device that resembles the measured data. The observed oscillations are found to depend on the level offset of the two dots during the manipulation and, most surprisingly, also the initialization stage. By means of an analytical expression, obtained from a large-bias model, we can understand the prominent features of these oscillations seen in both the experimental data and the numerical results. These findings strengthen the common interpretation in terms of a coherent transfer of electrons between the dots.
We study the short time dynamics of a charge qubit encoded in two coherently coupled quantum dots connected to external electrodes.In the regime of sequential tunneling the waiting time distribution is used to investigate in details statistics of the tunneling events and their correlations.We show that the coherent rotation of the qubit state on the Bloch sphere can be seen in tunneling events when an electron is injected and leave the system.Decoherence, relaxation of the qubit state as well as leakage processes are studied by means of transient currents.
Lateral semiconductor quantum dot structures have been proposed as an effective quantum bit (qubit) for quantum computation. A single excess electron with the freedom to move between two capacitively coupled quantum dots creates a `pseudo'-spin system with the same qubit behavior as the more natural two level system of a single electron spin. The excess electron in the double dot system is restricted to one of the two dots, thereby creating two separate and distinct states (usually referred to as |L> and |R>). The benefit of these charge qubits lie in the relative ease with which they can be manipulated and created. Experiments have been performed in this area and have shown controllable coherent oscillations and thus efficient single-qubit operations. However, the decoherence rates observed in the experiments is still quite high, making double dot charge qubits not very appealing for large-scale implementations. The following work describes the effect of the electromagnetic (EM) environment of the double quantum dot system on the decoherence of the charge state. Sources of decoherence in similar systems have been extensively investigated before and this paper follows a close theoretical framework to previous work done in the area. The effect of the EM environment can been seen in the calculations discussed below, although it is clear that the decoherence seen in experiments cannot be fully explained by the voltage fluctuations as they are investigated here. The limitations of the calculations are discussed and improvements are suggested.