Confluence of resonant laser excitation and bidirectional quantum-dot nuclear-spin polarization

Abstract

Resonant laser scattering along with photon correlation measurements have established the atomlike character of quantum dots. Here, we present measurements which challenge this identification for a wide range of experimental parameters: the absorption lineshapes that we measure at magnetic fields exceeding 1 Tesla indicate that the nuclear spins polarize by an amount that ensures locking of the quantum dot resonances to the incident laser frequency. In contrast to earlier experiments, this nuclear spin polarization is bi-directional, allowing the electron+nuclear spin system to track the changes in laser frequency dynamically on both sides of the quantum dot resonance. Our measurements reveal that the confluence of the laser excitation and nuclear spin polarization suppresses the fluctuations in the resonant absorption signal. A master equation analysis shows narrowing of the nuclear Overhauser field variance, pointing to potential applications in quantum information processing. PACS numbers:A number of ground-breaking experiments have demonstrated fundamental atom-like properties of quantum dots (QD), such as photon antibunching [1] and radiative lifetime limited Lorentzian absorption lineshape [2] of optical transitions. Successive experiments using transport [3] as well as optical spectroscopy [4] however, revealed that the nature of hyperfine interactions in QDs is qualitatively different than that of atoms: coupling of a single electron spin to the mesoscopic ensemble of ∼ 10 5 QD nuclear spins results in non-Markovian electron spin decoherence [5] and presents a major drawback for applications in quantum information science. Nevertheless, it is still customary to refer to QDs as artificial atoms; i.e. two level emitters with an unconventional dephasing mechanism. Here, we present resonant absorption experiments demonstrating that for a wide range of system parameters, such as the gate voltage, the length of the tunnel barrier that separates the QDs from the back contact and the external magnetic field, it is impossible to isolate the optical excitations of QD electronic states from a strong influence of nuclear spin physics. We determine that the striking locking effect of any QD transition to an incident near-resonant laser, which we refer to as dragging, is associated with dynamic nuclear spin polarization (DNSP); in stark contrast to previous experiments [6,7,8,9,10,11,12] the relevant nuclear spin polarization is bi-directional and its orientation is determined simply by the sign of the excitation laser detuning. We find that fluctuations in the QD transition energy, either naturally occurring [2] or introduced by externally modulating the Stark field, are suppressed when the laser and the QD resonances are locked. We also find that when the exchange interaction between the confined QD electron and the nearby electron Fermi-sea that leads to spin-flip co-tunneling [13] is sufficiently strong, it can suppress the confluence of laser and QD transition energies by inducing fast nuclear spin depolarization...

Collective nuclear stabilization in single quantum dots by noncollinear hyperfine interaction

“…This resonant nature makes the strength of our mechanism comparable with the detrimental nuclear spin depolarization even if the non-collinear HI is weak 6,11 . The physics of our mechanism depicted above differs qualitatively from previous theories [6][7][8][9][10][11] . First, the nuclear spin flip is not accompanied by any change of the hole state, in contrast to the pair-wise electron-nuclear spin flip-flop [7][8][9][10] .…”

“…The physics of our mechanism depicted above differs qualitatively from previous theories [6][7][8][9][10][11] . First, the nuclear spin flip is not accompanied by any change of the hole state, in contrast to the pair-wise electron-nuclear spin flip-flop [7][8][9][10] . Second, the nuclear spin is flipped by a virtual hole in a coherent channel (resembling co-tunneling 15 ).…”

“…Recently, three groups [6][7][8] reported significant NS in single QDs. Xu et al 6 observed prolongation of electron spin coherence time by NS upon optical pumping of trion in the Voigt geometry.…”

“…Xu et al 6 observed prolongation of electron spin coherence time by NS upon optical pumping of trion in the Voigt geometry. Latta et al 7 observed NS upon optical pumping of trion and blue exciton in the Faraday geometry. Vink et al 8 theoretically deduced NS upon microwave pumping of electron spin resonance.…”

“…The key process of this mechanism is the flip of the nuclear spins through their non-collinear HI with the hole excited by nonresonant pumping (instead of resonant pumping 6,11 or through the isotropic contact HI with the ground state electron under resonant pumping [7][8][9][10] ). This process has a preferential direction giving rise to an antisymmetric steady-state nuclear field, which in turn drives a nonlinear feedback loop leading to efficient NS.…”