Physik seit Einstein
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Deutsche Physikalische Gesellschaft Humboldt-Universität zu Berlin Technische Universtiät Berlin
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Spin Qubits with Quantum Dots

Leo Kouwenhoven
Kavli Institute of NanoScience, Delft University of Technology,
POB 5046, 2600GA Delft, The Netherlands

Quantum dots are nano-scale field-effect transistor devices, which only contain a small number of electrons. This number can be changed with a gate voltage such that one can create a box containing exactly one, two, three, etc. electrons. This artificial, human-fabricated system has many similarities with atoms: the electron energy spectrum is discrete with a shell structure, which is filled with electrons according to Hund's rules. We exploit the ability to tune in-situ the quantum dot properties for a controlled study of quantum mechanical interactions for a specific number of electrons. These interactions lead to phenomena such as two-electron singlet and triplet states and the Kondo effect. We will further discuss various quantum dot systems (semiconductor, nanocrystals and carbon nanotubes) including some of the fabrication procedures. Although these studies are presently pure scientific we will speculate on electronic applications.

One such speculation concerns the possible development of quantum computers. We will outline the basic principles of a quantum computer as well as the foreseen advantages. The realization of a quantum computer contains many difficulties. We discuss these hurdles using qu-bits based on quantum dots as an example. We particularly focus on the quantum information contained in the spin degree of freedom of individual electrons. Our experimental efforts focus on realizing spin-qubit circuits. These little circuits have to include a double quantum dot with controllable tunnel coupling between the dots; electron-spin resonance loop for performing single spin rotations; and a non-invasive read-out system. Our read-out is performed by a quantum point contact detector. Parts of this little qubit circuit are now being tested, some parts are already working.

[1] See for a review on quantum dots: Few-electron quantum dots,
L.P. Kouwenhoven, D.G. Austing and S. Tarucha, Rep. Prog. Phys. 64, 701-736 (2001). This review and other papers can be found at
[2] Double transport through double quantum dots,
W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, T. Fujisawa, S. Tarucha and L. P. Kouwenhoven, Reviews of Modern Physics 75, No.1, 1-22 (2003)
[3] Single shot read-out of an individual electron spin in a quantum dot,
J.M. Elzerman, R. Hanson, L.H. Willems van Beveren, B. Witkamp, L.M.K. Vandersypen and L.P. Kouwenhoven, Nature 430, 4310435 (2004).

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