Partly noiseless encoding of quantum information in quantum dot arrays against phonon-induced pure dephasing

We show that pure dephasing of a quantum dot charge (excitonic) qubit may be reduced for sufficiently slow gating by collectively encoding quantum information in an array of quantum dots. We study the role of the size and structure of the array and of the exciton lifetime for the resulting total error of a single-qubit operation.Comment: Final version; 10 pages, 8 figure

Phonon-assisted decoherence and tunneling in quantum dot molecules

We study the influence of the phonon environment on the electron dynamics in a doped quantum dot molecule. A non-perturbative quantum kinetic theory based on correlation expansion is used in order to describe both diagonal and off-diagonal electron-phonon couplings representing real and virtual processes with relevant acoustic phonons. We show that the relaxation is dominated by phonon-assisted electron tunneling between constituent quantum dots and occurs on a picosecond time scale. The dependence of the time evolution of the quantum dot occupation probabilities on the energy mismatch between the quantum dots is studied in detail.Comment: 4 pages, 2 figures, conference proceeding NOEKS10, to be published in Phys. Stat. So

Theory of phonon-mediated relaxation in doped quantum dot molecules

A quantum dot molecule doped with a single electron in the presence of diagonal and off-diagonal carrier-phonon couplings is studied by means of a non-perturbative quantum kinetic theory. The interaction with acoustic phonons by deformation potential and piezoelectric coupling is taken into account. We show that the phonon-mediated relaxation is fast on a picosecond timescale and is dominated by the usually neglected off-diagonal coupling to the lattice degrees of freedom leading to phonon-assisted electron tunneling. We show that in the parameter regime of current electrical and optical experiments, the microscopic non-Markovian theory has to be employed.Comment: Final extended version, 5 pages, 4 figure

Phonon-assisted tunneling between singlet states in two-electron quantum dot molecules

We study phonon-assisted electron tunneling in semiconductor quantum dot molecules. In particular, singlet-singlet relaxation in a two-electron doped structure is considered. The influence of Coulomb interaction is discussed via comparison with a single electron system. We find that the relaxation rate reaches similar values in the two cases but the Coulomb interaction shifts the maximum rates towards larger separations between the dots. The difference in electron-phonon interaction between deformation potential and piezoelectric coupling is investigated. We show that the phonon-induced tunneling between two-electron singlet states is a fast process, taking place on the time scales of the order of a few tens of picoseconds.Comment: Final extended version, 8 pages, 9 figure

Interplay and optimization of decoherence mechanisms in the optical control of spin quantum bits implemented on a semiconductor quantum dot

We study the influence of the environment on an optically induced rotation of a single electron spin in a charged semiconductor quantum dot. We analyze the decoherence mechanisms resulting from the dynamical lattice response to the charge evolution induced in a trion-based optical spin control scheme. Moreover, we study the effect of the finite trion lifetime and of the imperfections of the unitary evolution such as off-resonant excitations and the nonadiabaticity of the driving. We calculate the total error of the operation on a spin-based qubit in an InAs/GaAs quantum dot system and discuss possible optimization against the different contributions. We indicate the parameters which allow for coherent control of the spin with a single qubit gate error as low as $10^{-4}$.Comment: Final version, 14 pages, 11 figure

Heat pumping with optically driven excitons

We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible

Exciton spin-flip rate in quantum dots determined by a modified local density of optical states

The spin-flip rate that couples dark and bright excitons in self-assembled quantum dots is obtained from time-resolved spontaneous emission measurements in a modified local density of optical states. Employing this technique, we can separate effects due to non-radiative recombination and unambiguously record the spin-flip rate. The dependence of the spin-flip rate on emission energy is compared in detail to a recent model from the literature, where the spin flip is due to the combined action of short-range exchange interaction and acoustic phonons. We furthermore observe a surprising enhancement of the spin-flip rate close to a semiconductor-air interface, which illustrates the important role of interfaces for quantum dot based nanophotonic structures. Our work is an important step towards a full understanding of the complex dynamics of quantum dots in nanophotonic structures, such as photonic crystals, and dark excitons are potentially useful for long-lived coherent storage applications.Comment: 5 pages, 4 figure

Theory of Spin Relaxation in Two-Electron Lateral Coupled Si/SiGe Quantum Dots

Highly accurate numerical results of phonon-induced two-electron spin relaxation in silicon double quantum dots are presented. The relaxation, enabled by spin-orbit coupling and the nuclei of $^{29}$Si (natural or purified abundance), are investigated for experimentally relevant parameters, the interdot coupling, the magnetic field magnitude and orientation, and the detuning. We calculate relaxation rates for zero and finite temperatures (100 mK), concluding that our findings for zero temperature remain qualitatively valid also for 100 mK. We confirm the same anisotropic switch of the axis of prolonged spin lifetime with varying detuning as recently predicted in GaAs. Conditions for possibly hyperfine-dominated relaxation are much more stringent in Si than in GaAs. For experimentally relevant regimes, the spin-orbit coupling, although weak, is the dominant contribution, yielding anisotropic relaxation rates of at least two order of magnitude lower than in GaAs.Comment: 11 pages, 10 figure