ABSTRACT
A promising solution to the problem of nuclear decoherence is to use a sub-
strate consisting of spin-0 nuclei. This is available in isotopically purified 28Si, where the isotopic purification increases the coherence time of electron
spins bound to 31P impurities to an astounding value of 60 ms [35]. Even
longer coherence times of many hours may be expected for the 31P nuclear
spin itself. This time has yet to be measured, although nmr measurements
of 29Si nuclei isolated from dipolar noise via radio-frequency pulse sequences
puts an experimental lower bound of 25 seconds on this coherence time [36].
Such long coherence times may provide a boost to fault-tolerant quantum
computers, as the fidelity of transferring an electron spin coherence to a
nuclear spin coherence and back using pulsed electron-nuclear double res-
onance (endor) techniques may be higher than the fidelity of correcting
electron-spin decoherence for the equivalent number of error correcting cy-
cles. For quantum repeaters, in which error correction may not be imple-
mented at all and very small errors are tolerated rather than corrected, such
long decoherence times are critical, as qubit entanglement must remain co-
herent over the time-scale of generating long-distance entanglement, which
may easily be many seconds.
