Dresden 2006 – scientific programme

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PV: Plenarvorträge

PV V

PV V: Plenary Talk

Tuesday, March 28, 2006, 08:30–09:15, HSZ 01

Spin Qubits in Nanostructures: Review and Outlook — •Daniel Loss — Department of Physics, University of Basel, Switzerland

I will give an overview of quantum computing in semiconducting nanostructures based on spin-qubits [1,2], and present the current status of theory and experiment. In GaAs quantum dots, single electrons can be confined, and the qubit is then represented by the spin of this electron. Although the quantum information is stored in the magnetic and not in the charge degrees of freedom of the electron, all basic operations of the qubits, can be implemented by electrical gates—an essential feature for scalability. These operations are single-spin gates, single-spin read out, and controlled spin-spin interactions (two-qubit gates). Interaction gates can be avoided altogether by partial Bell state measurements which can also be implemented by double dot structures [3]. One of the biggest challenges- and physically most interesting aspects- is decoherence. In GaAs structures the known sources for decoherence are spin-phonon processes [4] (via spin orbit interaction) and hyperfine interaction between the electron and about a million nuclear spins in a quantum dot [5]. The nuclear spins generate an extremely rich and complex spin dynamics which cannot be treated with standard rate equations. I will discuss the effects of nuclei in double quantum dots [6,7] where they not only cause decoherence but also provide a means to manipulate the electron spins and, in particular, allow to implement the square-root-of-swap operation needed for XOR, as recently demonstrated experimentally [6]. I will describe nuclear state narrowing, recently proposed [8], which is based on charge measurements forcing the nuclei in a quantum state that can be decoherence free in the ideal limit.

[1] D. Loss and D.P. DiVincenzo, Phys. Rev. A 57 (1998) 120.

[2] For a review, see V. Cerletti et al., Nanotechnology 16 , R27 (2005).

[3] H.-A. Engel and D. Loss, Science 309, 586 (2005).

[4] V. Golovach, A. Khaetskii, and D. Loss, Phys. Rev. Lett. 93, 016601 (2004).

[5] A. V. Khaetskii, D. Loss, and L. Glazman, Phys. Rev. Lett. 88, 186802 (2002); B. Coish and D. Loss, Phys. Rev. B 70, 195340 (2004).

[6] Petta et al., Science 309, 2180 (2005).

[7] B. Coish and D. Loss, Phys. Rev. B 72, 125337 (2005).

[8] D. Klauser, B. Coish, and D. Loss, cond-mat/0510177.