BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-3530
DTSTART:20141201T160000Z
DURATION:PT1H0M0S
DTSTAMP:20260416T111512Z
LAST-MODIFIED:20141124T140503Z
LOCATION:5310 Chamberlin Hall
SUMMARY:Spin bath decoherence of mixed spin qubits\, R. G. Herb Conden
 sed Matter Seminar\, Seto Balian\, University College London
DESCRIPTION:The limiting decoherence mechanism for a wide variety of s
 pin qubits (spin diusion) arises from<br>
\ncoupling to a bath of oth
 er spins\; for example\, 29Si impurities in natural silicon and 13C in
  diamond.<br>
\nIn addition to oering fast manipulation times [1]\, m
 ixed spin systems such as electron-nuclear<br>
\nqubits (e.g. donors i
 n silicon) can possess `optimal working points' (OWPs) { magnetic eld
 s at<br>
\nwhich decoherence arising from both spin bath dynamics and 
 classical eld <br>
\nuctuations is strongly<br>
\nsuppressed [2{4].<b
 r>
\nIn this talk\, I will introduce mixed spin qubits and describe th
 e cluster correlation expansion<br>
\n(CCE) [5\, 6] for calculating de
 phasing times (T2) of qubits in spin baths associated with strong back
 -<br>
\naction and environment-memory eects. Using the pseudospin mod
 el and operating near OWPs\, I<br>
\nwill explain the suppression of d
 ecoherence driven by pairs of bath spins (lowest order CCE) and<br>
\n
 demonstrate enhancement of T2 in quantitative agreement with measureme
 nts [2]. Approaching<br>
\nthe OWP\, many-body correlations of increas
 ing order are isolated and begin contributing to the<br>
\nweakening d
 ecoherence mechanism. A simple closed-form T2 formula can be derived f
 or nuclear<br>
\nspin diusion\, predicting T2 in excellent agreement 
 with ESR and NMR measurements as well as<br>
\nCCE simulations across 
 a wide parameter range [2]. The formula also exposes signicant diere
 nces<br>
\nbetween spin bath decoherence and decoherence by classical 
 eld noise.<br>
\nFinally\, I will discuss a plausible decoherence mec
 hanism for 29Si nuclear impurity qubits in<br>
\nproximity to a donor\
 , based on equivalent atomic sites due to symmetries of the donor elec
 tron<br>
\nwavefunction. This `equivalent pairs' model predicts T2 in 
 the measured timescale of 1 second [7].
URL:https://www.physics.wisc.edu/events/?id=3530
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