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Quantum dot (QD) spins are promising candidates for scalable quantum computation (QC). Electrical readout and control of single spins in QDs have proven challenging, yet GaAs double QDs, where spin blockade and charge sensors enable observation of single/twospin dynamics, have seen impressive experimental progress. Recent experiments [1] have demonstrated quantum coherence and the use of the twoelectron singlet and unpolarized triplet as a qubit, with reliable initialization, singlequbit rotation, and measurement. Yet the hyperne interaction provides a fundamental limit to spin lifetimes in GaAs, a fact which has spurred intense research into Si QDs. Silicon is often regarded as the best semiconducting host material for spin qubits because of its excellent spin coherence properties: spinorbit coupling is very small while the hyperne interaction can be reduced by isotopic enrichment. 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 two valleys are relevant to the ground orbital state, which introduces fundamental complications in distinguishing spin and orbital degrees of freedom. Scattering at the interface lifts the valley degeneracy by producing a valleyorbit coupling , yet the exact form and magnitude of is generally not known a priori and is sampledependent. With this in mind we have established the precise criteria for realizing spin qubits in Si QDs [2]. I will show that, for small , a singlettriplet qubit cannot be constructed since a number of dierent states may be initialized, leading to dierent experimental outcomes. For large valley splitting ( kBT) the experiment is analogous to GaAs. A Zeeman eld can be used to distinguish between dierent initialized states for any valley splitting, and sweeping a uniform magnetic eld provides a useful method for estimating , which is particularly useful when < kBT. This work is supported by LPSNSA. [1] J. R. Petta et al, Science 309, 2180 (2005). [2] Dimitrie Culcer, Lukasz Cywinski, Qiuzi Li, Xuedong Hu, and S. Das Sarma, arXiv:0903.0863. Host: John Singleton 