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This Week at Physics

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Events on Monday, December 1st, 2014

R. G. Herb Condensed Matter Seminar
Spin bath decoherence of mixed spin qubits
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Seto Balian, University College London
Abstract: The limiting decoherence mechanism for a wide variety of spin qubits (spin di usion) arises from
coupling to a bath of other spins; for example, 29Si impurities in natural silicon and 13C in diamond.
In addition to o ering 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 e ects. 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 di usion, 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 signi cant di erences
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].
Host: Sue Coppersmith
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Plasma Physics (Physics/ECE/NE 922) Seminar
Initial Computations of Vertical Displacement Events with NIMROD
Time: 12:00 pm
Place: 2535 Engineering Hall
Speaker: Carl Sovinec, UW
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