Transport signatures of long-range nuclear-spin coherence in a quantum-dot spin valve (Prof. S. Chesi)

Flip-flop processes between electron and nuclear spins in the quantum dot are allowed by the hyperfine interaction. Long-range nuclear-spin coherence can induce a strong enhancement of such spin-flip transition rates, by an amount proportional to the number of nuclear spins. Under a finite voltage bias, the enhancement is revealed by an intense current burst analogous to superradiant light emission. Instead, fast local dephasing for the nuclear spins leads to an incoherent evolution analogous to spontaneous emission. Through a combination of simple rate equations and a more general master equation we have characterized these two regimes and the crossover between them. We also discuss our ongoing work about related schemes, but with unpolarized contacts. We generally assume uniform hyperfine couplings, which yield the strongest coherent enhancement. We propose realistic strategies, based on isotopic modulation and wavefunction engineering in core-shell nanowires, to realize this analytically solvable “box-model" of hyperfine couplings.