Control of Exchange Interaction in a Double Dot System

By Mike Stopa

Harvard University

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As Rolf Landauer observed in 1960, information is physical. As a consequence, the transport and processing of information must obey the laws of physics. It therefore makes sense to base the laws of information processing and computation on the laws of physics and in particular on quantum mechanics. This idea underlies the modern fields of Quantum Computation and Quantum Information Technology.

Electron spin is a particularly well-studied two level physical system that, in semiconductors, has been shown to remain coherent over remarkably long times [1]. This property, which results from the weakness of coupling of the environment to the magnetic moment of the electron, means that the information stored in a spin is preserved over relatively long times. This, combined with its simplicity, makes the electron spin potentially useful for quantum information processing in general and a good qubit candidate for quantum computing in particular [2]. The relative isolation of a spin from its environment, however, makes spin manipulation and especially single spin measurement very challenging. Long-lived information is of little use if we can't access it. The Pauli exclusion principle, however, causes the Coulomb interaction between electrons with parallel spins to be generally weaker than between electrons with anti-parallel spins. Since electrostatic forces are much stronger than magnetic ones, the prospect of controlling this exchange interaction has been envisioned as a basic element in many proposed applications in the emerging field of "spin-tronics."

In this talk, I will give a brief outline of some of the basic ideas of quantum information and quantum computing. I will describe our calculations of the electronic structure of artificial molecules or "double quantum dots," which are based on a unique hybrid of density functional and exact diagonalization techniques. I will attempt to show how basic quantum computing processes are proposed to be carried out within this system via manipulation of the exchange interaction between the electrons on the separate dots with either electrostatic gating or applied magnetic fields. I will finally attempt to indicate what the fundamental bottlenecks to future progress in the field are.

[1] T. Fujisawa et al., Science 282, 932 (1998)
[2] G. Burkard et al., Phys. Rev. B, 59, 2070 (1999).


Dr. Stopa receieved his B.A. for Astronomy from Wesleyan University in 1976. He was a Research and Teaching Assistant at Harvard University from 1977 to 1980. He completed his M.S. from the University of Maryland in 1982. He was a real time computer programmer at the System Development Corporation from 1982 to 1985. He worked with the Das Sarma Research Group at the University of Maryland from 1986 to 1990. He received his Ph. D. in 1990. From 1990 to 1992, he worked in the NTT Basic Research Laboratories, Tarucha group Riken. He collaborated with the Frontier Research Project from 1992 to 1997. He worked with the Walter Schottky Institute from 1998 to 1999. Since 2000, he has been with the Exploratory Research for Advanced Technologies (ERATO), Japan Science and Technology Corporation (JST), Tarucha Mesoscopic Correlaton Project.

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  • Mike Stopa (2004), "Control of Exchange Interaction in a Double Dot System,"

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