Spin Manipulation of Free 2-Dimensional Electrons in Si/SiGe Quantum Wells

An important requirement for a physical embodiment of a quantum computer is that arbitrary single-qubit operations can be performed. In the case of spin-qubits, this means that arbitrary spin rotations must be possible. Here we demonstrate spin rotations of an ensemble of free 2-dimensional electrons confined to a silicon quantum well embedded in a silicon-germanium alloy host. The spins are manipulated by resonant microwave pulses and an applied magnetic field in a pulsed electron paramagnetic resonance spectrometer. From the pulsed measurements we deduce a spin coherence time in this system of about 3 microsec, allowing at least 100 elementary operations before decoherence destroys the spin state. These measurements represent an important step towards the realization of quantum computation using electron spins in semiconductors, but at the same time establish some constraints on the design of such a system.