Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.

Among the theoretically predicted two-dimensional topological insulators, InAs=GaSb double quantum wells (DQWs) have a unique double-layered structure with electron and hole gases separated in two layers, which enables tuning of the band alignment via electric and magnetic fields. However, the rich trivialtopological phase diagram has yet to be experimentally explored. We present an in situ and continuous tuning between the trivial and topological insulating phases in InAs=GaSb DQWs through electrical dual gating. Furthermore, we show that an in-plane magnetic field shifts the electron and hole bands relatively to each other in momentum space, functioning as a powerful tool to discriminate between the topologically distinct states. DOI: 10.1103/PhysRevLett.115.036803 PACS numbers: 73.21.Fg, 71.30.+h, 72.80.Ey Two-dimensional topological insulators (2DTIs), known also as quantum spin Hall insulators, are a novel class of materials characterized by an insulating bulk and gapless helical edges [1][2][3][4]. Double quantum wells (DQWs) of indium arsenide and gallium antimonide (InAs=GaSb) have a unique type-II broken gap band alignment and are especially interesting since the electron and hole gases that form a topological band structure are spatially separated [5][6][7][8]. For the appropriate layer thicknesses, the top of the hole band in GaSb lies above the bottom of the electron band in InAs; hence, for small momentum (around k ¼ 0) the band structure is inverted. At the crossing point (k cross ) of the two bands, coupling of the electrons and holes opens up a bulk hybridization gap [9][10][11][12][13][14][15][16] with gapless helical edge modes [5]. The size of the gap is determined by both k cross and the overlap of the electron and hole wave functions [17]. Because of the spatial separation of the two gases, electric and magnetic fields can induce relative shifts of the bands in energy and momentum [10,18,19], respectively. By controlling such shifts, it is possible to in situ tune between the trivial and topological insulating phases, which is the key advantage of InAs=GaSb compared to the other known 2DTIs [5,[20][21][22].Here, for the first time, we map out the full phase diagram of the InAs=GaSb DQWs by independent control of the Fermi level and the band alignment through electric dual gating. In particular, we observe the phase transition between the trivial insulator (normal gap) and topological insulator (hybridization gap). Moreover, the evolution of the resistance for in-plane magnetic fields is different in the two distinct phases, consistent with the fact that one is trivial, and the other topological.In InAs=GaSb DQWs, the band alignment can be controlled by top and back gate electrodes [5,18] [see the structure shown in Fig. 1(a) ]. The two gates control the perpendicular electric field E z , which shifts the electron and the hole bands relatively to each other in energy by ΔE ¼ eE z hzi (hzi is the average separation of the electron and hole gases), and the position of the Fermi level E F . ...

We present transport and scanning SQUID measurements on InAs/GaSb double quantum wells, a system predicted to be a two-dimensional topological insulator. Top and back gates allow independent control of density and band offset, allowing tuning from the trivial to the topological regime. In the trivial regime, bulk conductivity is quenched but transport persists along the edges, superficially resembling the predicted helical edge-channels in the topological regime. We characterize edge conduction in the trivial regime in a wide variety of sample geometries and measurement configurations, as a function of temperature, magnetic field, and edge length. Despite similarities to studies claiming measurements of helical edge channels, our characterization points to a nontopological origin for these observations.

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.Comment: 4 pages, 4 figure

We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures.Comment: 3 pages, 3 figure