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SUMMARY:Spin and valley control in semiconductor and carbon quantum dots -
  Prof Guido Burkard\, University of Konstanz\, Germany
DTSTART:20110228T141500Z
DTEND:20110228T160000Z
UID:TALK29626@talks.cam.ac.uk
CONTACT:afw24
DESCRIPTION:Coherent control of single electron spins in quantum dots has 
 been implemented in a variety of experimental demonstrations. However\, to
  use electron spins as qubits\, the spin coherence time should significant
 ly exceed the time for elementary operations. In typical semiconductor mat
 erials\, the hyperfine coupling to a large ensemble of nuclear spins limit
 s the electron coherence. This brings up interesting theoretical questions
 \, e.g.\, regarding the possibility of preparing a nuclear spin state that
  allows for prolonged electron coherence [1]. It turns out that the nuclea
 r spin ensemble in a can even be taken advantage of for spin qubit manipul
 ation using Landau-Zener transitions\, by using the so-called singlet-trip
 let T+ qubit [2]. An alternative strategy to achieve long spin coherence i
 s to use materials with lower nuclear spin content\, such as silicon or ca
 rbon.  Carbon has emerged as an interesting alternative material for spin 
 qubits\, due to both the low concentration of nuclear spins and relatively
  weak spin-orbit coupling. However\, the formation of quantum dots in grap
 hene is a non-trivial task due to the absence of a band gap and the relate
 d effect of Klein tunneling [3].  Interestingly\, electrons in some carbon
 -based quantum dots comprise a degree of freedom in addition to spin: The 
 existence of two Dirac cones in the graphene band structure leads to the v
 alley degree of freedom which can be coherently manipulated with oscillato
 ry fields in a similar way as the spin [4]. The valley degeneracy also ent
 ers the hyperfine interactions with remaining 13C nuclear spins and is man
 ifestated in the spin-valley blockade effect [5]. Finally\, spin-orbit cou
 pling in graphene and (more importantly) carbon nanotubes affects the spin
  blockade as well as the spin lifetime [6].\n\n[1] H. Ribeiro and G. Burka
 rd\, Phys. Rev. Lett. 102\, 216802 (2009).\n[2] H. Ribeiro\, J. R. Petta\,
  and G. Burkard\, Phys. Rev. B 82\, 115445 (2010).\n[3] B. Trauzettel\, D.
  Bulaev\, D. Loss\, and G. Burkard\, Nature Phys. 3\, 192 (2007).\n[4] A. 
 Pályi and G. Burkard\, Phys. Rev. Lett. 106\, 086801 (2011).\n[5] A. Pál
 yi and G. Burkard\, Phys. Rev. B 80\, 201404 (2009)\;  ibid. 82\, 155424 (
 2010).\n[6] P. R. Struck and G. Burkard\, Phys. Rev. B 82\, 125401 (2010).
  \n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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