Electrical pulse measurement, inelastic relaxation, and non-equilibrium transport in a quantum dot

We review electrical pulse experiments carried out to probe inelastic energy relaxation processes and related non-equilibrium transport characteristics of quantum dots (QDs) in the Coulomb blockade (CB) regime. In contrast to the relatively short momentum relaxation time (∼10 ns) that can be understood on the basis of acoustic phonon emission, the spin-flip relaxation time is found to be extremely long (∼200 μs). The spin relaxation process in our QDs is actually dominated by a cotunnelling process, and thus the intrinsic spin relaxation should have a longer relaxation time. The long relaxation time is discussed in terms of potential applications to spin-based quantum information storage. On the other hand, the extremely long spin relaxation process can induce considerable fluctuation of the spin, charge, and total energy of the QD. The absence of efficient spin relaxation processes can cause highly non-equilibrium transport, which actually 'breaks down' the single-electron tunnelling scheme. The non-equilibrium effects must be considered when electrons and spins are manipulated in the CB regime.