Bipolar spin blockade and coherent state superpositions in a triple quantum dot

Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully(1,2). 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 current(3). At present, more complex spin qubit circuits including triple quantum dots are being developed(4). Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar(5) 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 nano-spintronic circuits.