Relaxation of candidate electron spin qubits

We have recently measured pulsed electron spin resonance (ESR) from electrons bound to donors in silicon. Measurements made in the late 1950's showed that these spins were long-lived, but we find coherence times that are about two orders of magnitude longer than previously seen. We have also measured the spin-decoherence of free, 2-dimensional electrons in an ultra-high mobility Si/SiGe quantum well. The coherence time of the 2D electron spins is long in comparison to compound semiconductor systems, but several orders of magnitude shorter than that of the donor-bound electrons. Spin-orbit coupling in the form of the Structural Inversion Asymmetry (Rashba effect) appears to be the cause of the increased decoherence rate of the 2D electrons' spin, For architectures employing quantum dots at a heterointerface, the Rashba effect is not expected to cause a loss of spin coherence while the electron is in the ground state, but thermal excitation to upper dot levels could lead to decoherence. We discuss ways in which this Rashba term can be minimized in Si-based structures, as well as other physical systems (electrons on liquid helium, for example) in which much longer spin coherence times can be expected.