Katarzyna Roszak

Exciton spin decay is studied in a self-assembled InAs/GaAs quantum dot. The spin relaxation results from an interplay of two factors: the Bir-Pikus Hamiltonian and the short-range exchange interaction, leading to one and two phonon assisted transitions. We establish a hierarchy between the resulting transition rates and show the dominating role of transverse phonons for all the transitions.

We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield micro-second dephasing times at sub-Kelvin temperatures, which is consistent with experimental observations.

We study the evolution of a quantum state of a double quantum dot system interacting with the electromagnetic environment and with the lattice modes, in the presence of a coupling between the two dots. We propose a unified approach to the simulation of the system evolution under joint impact of the two reservoirs. We discuss the sub- and superradiant radiative decay of the system, the phonon-induced decay of entanglement between the dots, and the transfer of excitation between them.

This chapter is devoted to the recent theoretical results on the optical quantum control over charges confined in quantum dots under influence of phonons. We show that lattice relaxation processes lead to decoherence of the confined carrier states. The theoretical approach leading to a uniform, compact description of the phonon impact on carrier dynamics, perturbative in phonon couplings but applicable to arbitrary unperturbed evolution, is described in detail. Next, some applications are presented: phonon damping of Rabi oscillations in quantum dots and phonon-induced error of a single-qubit gate for an excitonic quantum dot qubit as well as for a semiconductor quantum dot spin qubit operated via a STIRAP transfer.

We study the evolution of entanglement between two excitons in a double quantum dot system coupled to a super-Ohmic reservoir. As expected entanglement is more fragile than local coherence, but, surprisingly, for a set of pure states disentanglement can be complete in a finite time under conditions that lead to the usual partial pure-dephasing.

We study the effect of pure dephasing on the entanglement of a pair of two-level subsystems (qubits). We show that partial dephasing induced by a super-Ohmic reservoir, corresponding to well-established properties of confined charge states and phonons in semiconductors, ...

We study single-qubit gates performed via stimulated Raman adiabatic passage (STIRAP) on a spin qubit implemented in a quantum dot system in the presence of phonons. We analyze the interplay of various kinds of errors resulting from the carrier-phonon interaction as well as from quantum jumps related to nonadiabaticity and calculate the fidelity as a function of the pulse parameters. We give quantitative estimates for an InAs/GaAs system and identify the parameter values for which the error is considerably minimized, even to values below $10^{-4}$ per operation.

We show that phonon-induced pure dephasing of excitons in quantum dots, which leads only to partial decay of system coherence, may completely erase entanglement between two excitons. We also discuss the role of the distance between the subsystems for this disentanglement process. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study the phonon influence on the spin of an exciton confined in a quantum dot. A process causing the transition of an initial bright heavy hole exciton spin state to dark states is identified. For a set of parameters typical for InAs/GaAs self-assembled quantum dots the corresponding decay times are over two orders of magnitude faster than for the

We discuss pure dephasing of singlet-triplet superpositions in two-electron double quantum dots due to elastic phonon scattering. We generalize our previous results to a system built of two non-identical dots. We show that the asymmetry must be very strong in order to considerably affect the dephasing rate.

We study phonon assisted transitions leading to exciton spin decay in quantum dots. We concentrate on couplings specific to the hole (Bir-Pikus Hamiltonian) and the exciton as a whole (short-range exchange interaction). A system strongly confined in a self-assembled quantum dot is considered.We discuss single-phonon assisted processes arising from the interplay of these interactions and show the dominating role of transverse phonons. We have also found a new two-phonon assisted transition, connected with the interplay of different parts of the Bir-Pikus Hamiltonian in the presence of the singlet-triplet splitting. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We show that the phonon-induced pure dephasing of excitons in quantum dots can be interpreted in terms of information leakage from the carrier subsystem to the lattice environment. We derive a quantitative relation between the coherence of the system, as manifested by the amplitude of the coherent optical polarization, and the amount of available which path information on the system state, quantified by the distinguishability of states.