Spin physics in quantum dots

spinphysics1
spinphysics2

In self-assembled InAs/GaAs quantum dots (QDs), level quantization and spatial confinement change dramatically the spin dynamics of electron by suppressing the spin relaxation due to  spin-orbit coupling and random scattering present in bulk semiconductors. On the opposite, the confinement enhances the interaction with other local spins, provided by additional charges or magnetic impurities, or by the many nuclear spins from the  host material itself. Since more than two decades, it has been clearly demonstrated that the hyperfine interaction with  the nuclear spins of InGaAs QDs produces an effective magnetic field (or Overhauser field)  experienced by a single electron confined in a quantum dot. This field is random in direction and strength (of the order of a few 10 mT)  when the nuclear spins  are disordered, and can rise up to ~4T when they are dynamically polarized,  notably under cw circularly polarized optical excitation in a longitudinal magnetic field.  The spin dynamics of an electron in an InGaAs QD  is thus strongly affected by this interaction with a  bath of  about 10^5 nuclear spins,  which gives rise to non-markovian  behavior and limits the effective electron spin coherence T2* time to about 2 ns. For application in photonic quantum  technologies with InGaAs QDs based on spin-photon entanglement in zero or weak external magnetic field (100mT), this limit is a severe constrain. Our current activities aim to implement  protocols enabling us to circumvent this constraint either by using dynamical decoupling techniques, or by  preparing the nuclear spin system in a lower state of entropy in a weak magnetic field.

References:

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