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arXiv:1707.06155 · Physical review. B./Physical review. B

Controllable electron spin dephasing due to phonon state distinguishability in a coupled quantum dot system

Michał Gawełczyk, Mateusz Krzykowski, Krzysztof Gawarecki, Paweł Machnikowski

Published Aug 3, 2018• 11 citations
arXivHTMLPDF

Abstract

We predict a spin pure dephasing channel in electron relaxation between states with unequal Zeeman splittings, exemplified by a spin-preserving electron tunneling between quantum dots in a magnetic field. The dephasing is caused by a mismatch in electron $g$ factors in the dots leading to distinguishability of phonons emitted during tunneling with opposite spins. Combining multiband $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ modeling and dynamical simulations via a master equation, we show that this fundamental effect of spin measurement effected by the phonon bath may be widely controlled by the size and composition of the dots or on demand via tuning of external fields. By comparing the numerically simulated degree of dephasing with the predictions of general theory based on distinguishability of environment states, we show that the proposed mechanism is the dominant phonon-related spin dephasing channel and may limit spin coherence time in tunnel-coupled structures at cryogenic temperatures.

DOI
10.1103/physrevb.98.075403

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We predict a spin pure dephasing channel in electron relaxation between states with unequal Zeeman splittings, exemplified by a spin-preserving electron tunneling between quantum dots in a magnetic field. The dephasing is caused by a mismatch in electron ggg factors in the dots leading to distinguishability of phonons emitted during tunneling with opposite spins. Combining multiband \mathbitk\ifmmode⋅\else\textperiodcentered\fi\mathbitp\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}\mathbitk\ifmmode⋅\else\textperiodcentered\fi\mathbitp modeling and dynamical simulations via a master equation, we show that this fundamental effect of spin measurement effected by the phonon bath may be widely controlled by the size and composition of the dots or on demand via tuning of external fields. By comparing the numerically simulated degree of dephasing with the predictions of general theory based on distinguishability of environment states, we show that the proposed mechanism is the dominant phonon-related spin dephasing channel and may limit spin coherence time in tunnel-coupled structures at cryogenic temperatures.
In this contribution we study carrier tunneling in a system consisting of a pair of coupled quantum dots.We predict the presence of a spin dephasing channel in such a system, which is associated with a "welcher-weg" type of decoherence process occurring during carrier tunneling.In our model such a process is caused by a mismatch of g-factor values in two quantum dots in the presence of external magnetic field.This leads to a mismatch in spin Zeeman splitting between the dots and, in consequence, to the distinguishability of phonons emitted during the tunneling of carriers with opposite spins.Thus we demonstrate a process of spin dephasing without any direct spin-environment coupling present in the model.
Phonon-assisted tunneling of electrons between nonidentical quantum dots in a magnetic field has been recently shown to be affected by a spin pure dephasing.The process is due to the mismatch of the Zeeman splittings caused by the difference of effective electron g-factors in the two dots.Based on both analytical estimation and an accurate numerical modeling of a double quantum dot system, we show that this dephasing can be canceled by applying an appropriately tuned magnetic-field gradient that compensates for the built-in inequality of the Zeeman splittings.
Phonon-induced dephasing of two-electron spin states in two vertically stacked self-assembled GaAs/InGaAs quantum dots is studied.A pure dephasing process due to elastic phonon scattering is found to dominate at low temperatures.This process is independent of the spin-orbit coupling and does not require the presence of a magnetic field.It relies on interdot tunneling and the Pauli principle, which make the double quantum dot gate for spin based quantum computing possible, and therefore cannot be avoided.
We study theoretically phonon-induced spin dynamics of two electrons confined in a self-assembled double quantum dot. We calculate the transition rates and time evolution of occupations for the spin-triplet and spin-singlet states. We characterize the relative importance of various relaxation channels, including two-phonon processes, as a function of the electric and magnetic fields. The simulations are based on a model combining the eight-band [Formula: see text] method and configuration-interaction approach. We show that the electron g-factor mismatch between the Zeeman doublets localized on different dots opens a relatively fast triplet-singlet phonon-assisted relaxation channel. We also demonstrate, that the relaxation near the triplet-singlet anticrossing is slowed down up to several orders of magnitude due to vanishing of some relaxation channels.
We study electron-phonon interaction induced decoherence between two-electron singlet and triplet states in a semiconductor double quantum dot using a spin-boson model. We investigate the onset and time evolution of this dephasing, and study its dependence on quantum dot parameters such as dot size and double dot separations, as well as the host materials (GaAs and Si). At the short time limit, electron-phonon interaction only causes an incomplete initial Gaussian decay of the off-diagonal density matrix element in the singlet-triplet Hilbert space; a complete long-time exponential decay due to phonon relaxation would eventually dominate over two-spin decoherence. We analyze two-spin decoherence in both symmetric and biased double quantum dots, identifying their difference in electron-phonon coupling and the relevant consequences.
Spin-phonon interactions are known to drive magnetic relaxation in solid-state systems but are generally overlooked as a contribution to spin decoherence through dephasing. Here, we extend quantum master equations to account for coherence terms and describe the full effect of up to two-phonon processes on spin dynamics. We implement this method fully ab initio for a molecule with large magnetization blocking temperature and show that, although strong axial magnetic anisotropy ensures slow magnetic relaxation approaching seconds at 77 kelvins, the superposition of Kramers doublets is coherent for less than 10 nanoseconds due to a two-phonon pure dephasing mechanism. This process, in principle, applies to any quantum system interacting with a thermal bath of phonons, advancing our understanding of quantum decoherence in solid-state systems.