There are various intrinsic device aspects that allow a clean spin transport signal in Ic2, and that make it immune to fringe field-induced magnetoresistance and Hall effects: 1. The exponential spin selective mean free path dependence in the ferromagnetic films create very large spin polarizations. In principle this can approach 100%, allowing effective injection and detection at cryogenic and room temperatures 11 ; 3 2. Because the spin filtering is caused by bulk scattering in the ferromagnetic films, they are easy to reproduce, since there is no interface sensitivity to the spin filtering (as there is, e.g., in magnetic tunnel junctions); 3. This device, like a spin-valve transistor, is a high impedance current source. 11,12 Since Ic2 is driven by Ic1, and Ic1 by Ie, Ic2 is virtually independent of Vc1, the applied voltage across the Si drift region. This also means that any generated Hall voltage in the FZ-Si has no effect on Ic2. The underlying background to the insensitivity to resistance and voltage of the FZ-Si is that the

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.

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500 ns at 60 K.

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time.

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