It has been suggested that triple quantum dots may provide additional tools and functionalities. These include encoding information either to obtain protection from decoherence or to permit all-electrical operation 5 , efficient spin busing across a quantum circuit 6 , and to enable quantum error correction using the three-spin Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate coherent manipulation of two interacting three-spin states. We employ the Landau-Zener-Stückelberg 7,8 approach for creating and manipulating coherent superpositions of quantum states 9 . We confirm that we are able to maintain coherence when decreasing the exchange coupling of one spin with another while simultaneously increasing its coupling with the third. Such control of pairwise exchange is a requirement of most spin qubit architectures 10 , but has not been previously demonstrated.Following the spin qubit proposal by Loss and DiVincenzo 10 and the electrostatic isolation of single spins in quantum dots (QDs) 11 and double quantum dots (DQDs) 12 , coherent manipulation was demonstrated in two-level systems based on single-spin up and down states 2 as well as two-spin singlet and triplet states 1 . Here we demonstrate coherent manipulation of a two-level system based on three-spin states. We employ the triple quantum dot (TQD) device layout shown in Fig. 1a, consisting of multiple metallic gates on a GaAs/AlGaAs heterostructure. The gates are used to electrostatically define three QDs in series within a two-dimensional electron gas 110 nm below the surface. The QDs are surrounded by two quantum point contact charge detectors (QPCs) 13 . The QPC conductance identifies the number of electrons in each QD and its derivative with respect to a relevant gate voltage maps out the device configuration stability diagram. We tune the device to the qubit operating electronic configuration, (N L ,N C ,N R ) = (1,1,1), between two spin-to-charge conversion regimes (1,0,2) and (2,0,1), where L, C and R refer to the left, centre and right QDs respectively. The detuning, ε, controls the energy difference between configurations (1,0,2), (1,1,1) and (2,0,1). The exchange coupling, J , depends on ε and the tunnel couplings.In this paper we concentrate on two scenarios. In the first scenario, at each point in the stability diagram the exchange coupling to the centre spin from one or both of the edge spins is minimal (that is, one edge spin resembles a passive spectator). This configuration is used as a control to confirm that our device maps onto two-spin results in this limit 9 . In the second scenario a true three-interacting-spin regime is achieved. (Results from a third intermediate regime are shown in the Supplementary Information.)

Individual and coupled quantum dots containing one or two electrons have been realized and are regarded as components for future quantum information circuits. In this Letter we map out experimentally the stability diagram of the few-electron triple dot system, the electron configuration map as a function of the external tuning parameters, and reveal experimentally for the first time the existence of quadruple points, a signature of the three dots being in resonance. In the vicinity of these quadruple points we observe a duplication of charge transfer transitions related to charge and spin reconfigurations triggered by changes in the total electron occupation number. The experimental results are largely reproduced by equivalent circuit analysis and Hubbard models. Our results are relevant for future quantum mechanical engineering applications within both quantum information and quantum cellular automata architectures.

A comparison is made between the conventional non-selective vapour-liquid-solid growth of InP nanowires and a novel selective-area growth process where the Au-seeded InP nanowires grow exclusively in the openings of a SiO(2) mask on an InP substrate. This new process allows the precise positioning and diameter control of the nanowires required for future advanced device fabrication. The growth temperature range is found to be extended for the selective-area growth technique due to removal of the competition between material incorporation at the Au/nanowire interface and the substrate. A model describing the growth mechanism is presented which successfully accounts for the nanoparticle size-dependent and time-dependent growth rate. The dominant indium collection process is found to be the scattering of the group III source material from the SiO(2) mask and subsequent capture by the nanowire, a process that had previously been ignored for selective-area growth by chemical beam epitaxy.

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