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UID:UW-Physics-Event-1532
DTSTART:20090813T150000Z
DURATION:PT1H0M0S
DTSTAMP:20260422T145318Z
LAST-MODIFIED:20090811T151954Z
LOCATION:5310 Chamberlin
SUMMARY:Singlet-triplet qubits in silicon quantum dots\, R. G. Herb Co
 ndensed Matter Seminar\, Dimi Culcer\, Dept. of Physics\, University o
 f Maryland
DESCRIPTION:Quantum dot (QD) spins are promising candidates for scalab
 le quantum computation (QC). Electrical readout and control of single 
 spins in QDs  have proven challenging\, yet GaAs double QDs\, where sp
 in blockade and charge sensors enable observation of single/two-spin d
 ynamics\, have seen impressive experimental progress. Recent experimen
 ts [1] have demonstrated quantum coherence and the distinct possibilit
 y of using the two-electron singlet and unpolarized triplet as the two
  states of a logical qubit\, with reliable initialization\, single-qub
 it rotation\, and measurement. Yet the hyperfine interaction with the 
 nuclei provides a fundamental limit to spin lifetimes in GaAs\, a fact
  which has spurred intense research into Si QDs. Silicon is often rega
 rded as the best semiconducting host material for spin qubits because 
 of its excellent spin coherence properties: spin-orbit coupling is ver
 y small while the hyperfine interaction can be reduced by isotopic enr
 ichment. The biggest obstacle to spin QC in Si is the valley degree of
  freedom\, which I will discuss at length. At the Si/SiO2 interface tw
 o valleys are relevant to the ground orbital state\, which introduces 
 fundamental complications in distinguishing spin and orbital degrees o
 f freedom. Scattering at the interface lifts the valley degeneracy by 
 producing a valley-orbit coupling Δ\, yet the exact form and magnitud
 e of Δ is generally not known a priori and is sample-dependent. With 
 this in mind we have established the precise criteria for realizing sp
 in qubits in Si QDs [2]. I will show that\, for small Δ\, a singlet-t
 riplet qubit cannot be constructed since a number of different states 
 may be initialized\, leading to different experimental outcomes. For l
 arge valley splitting (Δ >> kT) the experiment is analogous to GaAs. 
 A Zeeman field can be used to distinguish between different initialize
 d states for any valley splitting\, and sweeping a uniform magnetic fi
 eld provides a useful method for estimating Δ. An important consequen
 ce of our work is the proposed new experimental method for estimating 
 the valley splitting Δ in Si QDs\, particularly when Δ < kT. This wo
 rk is supported by LPS-NSA.<br>
\n<br>
\n[1] J. R. Petta\, A. C. Johns
 on\, J. M. Taylor\, E. A. Laird\, A. Yacoby\, M. D. Lukin\, C. M. Marc
 us\, M. P. Hanson\, and A. C. Gossard\, Science 309\, 2180 (2005).<br>
 
\n[2] Dimitrie Culcer\, Lukasz Cywinski\, Qiuzi Li\, Xuedong Hu\, and
  S. Das Sarma\, arXiv:0903.0863\, submitted to Phys. Rev. Lett.
\n
URL:https://www.physics.wisc.edu/events/?id=1532
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