coupling to a bath of other spins; for example\, 29Si impurities in natural silicon and 13C in diamond.

In addition to oering fast manipulation times [1]\, mixed spin systems such as electron-nuclear

qubits (e.g. donors in silicon) can possess `optimal working points' (OWPs) { magnetic elds at

which decoherence arising from both spin bath dynamics and classical eld

uctuations is strongly

suppressed [2{4].

In this talk\, I will introduce mixed spin qubits and describe the cluster correlation expansion

(CCE) [5\, 6] for calculating dephasing times (T2) of qubits in spin baths associated with strong back-

action and environment-memory eects. Using the pseudospin model and operating near OWPs\, I

will explain the suppression of decoherence driven by pairs of bath spins (lowest order CCE) and

demonstrate enhancement of T2 in quantitative agreement with measurements [2]. Approaching

the OWP\, many-body correlations of increasing order are isolated and begin contributing to the

weakening decoherence mechanism. A simple closed-form T2 formula can be derived for nuclear

spin diusion\, predicting T2 in excellent agreement with ESR and NMR measurements as well as

CCE simulations across a wide parameter range [2]. The formula also exposes signicant dierences

between spin bath decoherence and decoherence by classical eld noise.

Finally\, I will discuss a plausible decoherence mechanism for 29Si nuclear impurity qubits in

proximity to a donor\, based on equivalent atomic sites due to symmetries of the donor electron

wavefunction. This `equivalent pairs' model predicts T2 in the measured timescale of 1 second [7]. URL:http://www.physics.wisc.edu/twap/view.php?id=3530 END:VEVENT END:VCALENDAR