  Spin-blockaded quantum dots provide a unique setting for studying
nuclear-spin dynamics in a nanoscale system. Despite recent experimental
progress, observing phase-sensitive phenomena in nuclear spin dynamics remains
challenging. Here we point out that such a possibility opens up in the regime
where hyperfine exchange directly competes with a purely electronic spin-flip
mechanism such as the spin-orbital interaction. Interference between the two
spin-flip processes, resulting from long-lived coherence of the nuclear-spin
bath, modulates the electron-spin-flip rate, making it sensitive to the
transverse component of nuclear polarization. In a system repeatedly swept
through a singlet-triplet avoided crossing, nuclear precession is manifested in
oscillations and sign reversal of the nuclear-spin pumping rate as a function
of the waiting time between sweeps. This constitutes a purely electrical method
for the detection of coherent nuclear-spin dynamics.
