Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully1,2. Readout of the qubit state involves a conversion of spin to charge information, which is universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current3. At present, more complex spin qubit circuits including triple quantum dots are being developed4. Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar5 with current strongly suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In this mechanism, charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other, without involving the centre site. Our results have implications for future complex nanospintronic circuits.
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The authors thank P. Hawrylak and C.Y. Hsieh for discussions. M.B., R.S. and G.P. acknowledge financial support from the Spanish Ministry of Education (grant no. MAT2011-24331, MEC) and from the Marie Curie Initial Training Network (grant no. 234970, EU). M.B. and R.S. were supported by the Consejo Superior de Investigaciones Científicas through the JAE and JAE-Doc programmes, co-financed by the Fondo Social Europeo. G.G. acknowledges funding from the National Research Council Canada – Centre national de la recherche scientifique collaboration and Canadian Institute for Advanced Research. A.S.S. acknowledges funding from the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research.
The authors declare no competing financial interests.
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Busl, M., Granger, G., Gaudreau, L. et al. Bipolar spin blockade and coherent state superpositions in a triple quantum dot. Nature Nanotech 8, 261–265 (2013). https://doi.org/10.1038/nnano.2013.7
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