Artificial coupling of spin-resolved topologically protected states of a quantum Hall insulator

Topologically protected edge states are conducting surface states immune to impurity scattering and geometrical defects that occur in special materials characterized by a bulk insulating gap1.The prototypical example of such edge states is found in a two-dimensional electron gas (2DEG) in the quantum Hall (QH) regime2,3 where the bulk insulating condition is reached under the application of quantizing magnetic fields. Recently, however, a new class of topological insulators displaying dissipationless topologically protected surface states at room temperature without magnetic fields have been found.4,5,6.

The quantum information, such as spin or phase etc., can be encoded in the protected edge states and preserved over long distances. Based on these properties, there have been a few proposals on fault tolerant quantum information processing7,8, which requires control of the coupling between edge states. Eventually, a scalable architecture for quantum-information processing would be highly desirable9.
 
In this talk I experimentally demonstrate a new method to artificially couple spin resolved edge states of a QH insulator and induce inter-edge charge transfer associated to spin-flip scattering events10. The process exploits the coupling of the electron spin with a spatially-dependent periodic in-plane magnetic field that is created by an array of Cobalt nano-magnets placed at the boundary of the 2DEG realized in a GaAs/AlGaAs modulation doped heterostructure. The maximum charge/spin transfer of 28 ± 1 % is achieved at 250 mK by fine tuning the perpendicular magnetic field. Our result is the key step towards realization of scalable quantum information processing that exploits the spin degree of freedom of topologically-protected edge states. 
 
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2. K. v. Klitzing et al., Phys. Rev. Lett. 45 494 (1980) 
3. D. C. Tsui et al., Phy. Rev. Lett. 48 1559 (1982) 
4. B. A. Bernevig et al., Science 314 1757 (2006) 
5. D. Hsieh et al., Nature. 452 970 (2008) 
6. H. Zhang et al., Nat. Phys. 94 166802 (2005) 
7. S. D. Sarma et al., Phys. Rev. Lett. 94 166802 (2005) 
8. L. Fu et al., Phys. Rev. Lett. 100 096407 (2008) 
9. V. Giovannetti et al., Phys. Rev. B 77 155320 (2008) 
10. B. Karmakar et al., to be submitted.