Long-lived Spin States in a Quantum Dot
Physical Science Laboratory
A semiconductor quantum dot, which is often referred to as an artificial atom, accommodates tunable number of electrons that occupy well-defined orbitals. Dynamical behaviors of electrons and electron spins in a quantum dot are growing interests for novel spintronics devices and quantum computing applications, as well as for fundamental characteristics of electrons and spins in nanostructures. The energy relaxation process, which is one of the most fundamental dynamical properties, is very important for spin based information storage (quantum bit). We successfully measured the energy relaxation time in a quantum dot and compared to the theories. The results indicate that the spin and orbital degrees of freedom are well separated in our system. This is desirable for potential applications to spin based information storage.
We employ novel electrical pump-and-probe measurements to investigate the relaxation time from an excited state to the ground state in one- and two-electron semiconductor quantum dot artificial atoms . We repeatedly applied voltage pulses to push the system out of equilibrium, and measure the time-integrated non-equilibrium transient current to determine the energy relaxation time. We find that the relaxation time of the one-electron artificial hydrogen atom is 3 - 10 ns, which is understood by spontaneous emission of acoustic phonons. This process can be regarded as an allowed transition in artificial atoms. However, the relaxation time of the two-electron artificial helium atom can be longer than 200 ms, which is 4 or 5 orders of magnitude longer than that for the allowed transition. This transition is forbidden by total-spin conservation. Although the relaxation is actually dominated by cotunneling process in our sample, the spin relaxation time is comparable to the theoretical predictions based on spin-orbit coupling, indicating the high quality of our sample. The large ratio of the allowed and forbidden transition rates indicates that the total spin is an excellent quantum number in artificial atoms.
For the application to a spin quantum bit, in which just a single-electron spin occupy the lowest orbit in a magnetic field, the spin-orbit interactions can determine the energy relaxation time of a spin quantum bit. We estimate from our observation that this spin relaxation time can be longer than 1ms, which is extremely (about 9 orders of magnitude) longer than the time required for typical one- and two-qubit manipulations (a few ps) . Our results therefore encourage further research in the use of the spin degree of freedom in QDs.
 T. Fujisawa et al., Nature 419 (2002) 278.
 J. Gupta et al., Science 292 (2001) 2458.
Fig. 1. Energy relaxation processes in a semiconductor quantum dot.
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