  Transition metal dichalcogenides (TMDs) have valley degree of freedom, which
features optical selection rule and spin-valley locking, making them promising
for valleytronics devices and quantum computation. For either application, a
long valley polarization lifetime is crucial. Previous results showed that it
is around picosecond in monolayer excitons, nanosecond for electrons, holes or
local excitons and tens of nanosecond for interlayer excitons. Here we show
that dark excitons in 2D heterostructures provide a microsecond valley
polarization memory thanks to the magnetic field induced suppression of valley
mixing. The lifetime of the dark excitons shows magnetic field and temperature
dependence which is consistent with the theoretical prediction. The long dark
exciton lifetime and valley polarization lifetime in 2D heterostructures make
them promising for long-distance exciton transport and macroscopic quantum
state generations.
