W.G. van der Wiel1,2,3 *, T. Kodera3, W. Naber2, K. Ono3, T. Fujisawa4 and S. Tarucha3,4,5
1PRESTO-JST, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Kavli Institute of NanoScience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
3University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
4NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi,Kanagawa 243-0198, Japan
5ERATO/SORST-JST, 7-3-1, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan
* E-mail: wilfred@qt.tn.tudelft.nl
We discuss our experimental approach for realizing single-electron spin qubits in semiconductor quantum dots [1]. We use photon assisted tunneling (PAT) spectroscopy [2] to study the high-frequency response of well-defined one and two-electron states in a vertically coupled double dot system [3]. Clear PAT associated with two-electron spin states is observed, indicating the robustness of our electron spin device under strong microwave irradiation. We study few-electron GaAs quantum single-dot devices designed for generating a local AC magnetic field (~mT) in order to realize single-electron spin resonance [4]. The AC magnetic field is induced by an AC current driven through a metal line in the vicinity of the dot. The LandŽ g-factor in the quantum dot is derived from the large (~T) in-plane magnetic field evolution of the Zeeman splitting observed in both Coulomb peak spacings and Coulomb diamonds. The obtained g-factor (|g| = 0.23) is significantly smaller than that for bulk GaAs. We measure electron transport through the dot for various AC currents and find evidence for the presence of an AC electric field in the form of photon assisted tunneling and current rectification. So far, we cannot confirm any effect of the AC magnetic field.
In addition to single-qubit gates, two-qubit gates are indispensable for creating a universal set of quantum gates. Two electron spins in a double quantum dot [5] with a high-speed tunability of the exchange coupling can be applied for this purpose. Bosonic degrees of freedom in the double dot environment influence its transport and coherence properties. In order to study the effect of phonons on the double dot performance, we develop double quantum dot devices with integrated on-chip interdigital transducers to generate surface acoustic waves in the dot environment. The surface acoustic waves are expected to influence the electron transport through the double dot (in the form of phonon assisted tunneling) and to affect the spin relaxation times.
[2] W.G. van der Wiel, T.H. Oosterkamp, S. de Franceschi, C.J.P.M. Harmans, and L.P. Kouwenhoven in Strongly Correlated Fermions and Bosons in Low-Dimensional Disordered Systems, I.V. Lerner et al. (eds.), pp.43-68, Kluwer Academic Publishers, Boston/Dordrecht/London, ISBN 1-4020-0748-5 (2002).
[3] T. Kodera, W.G. van der Wiel, K. Ono, S. Sasaki and S. Tarucha, Physica E 22, 518 (2004).
[4] T. Kodera, W.G. van der Wiel, T. Maruyama, Y. Hirayama and S. Tarucha, World Scientific Publishing, in press.
[5] W.G. van der Wiel, S. De Franceschi, J.M. Elzerman, T. Fujisawa, S. Tarucha and L.P. Kouwenhoven, Rev. Mod. Phys. 75, 1-22 (2003).