We study the dynamics of entanglement of two electron spins in two quantum dots, in which each electron is interacting with its nuclear spin environment. Focusing on the case of uncoupled dots, and starting from either Bell or Werner states of two qubits, we calculate the decay of entanglement due to the hyperfine interaction with the nuclei. We mostly focus on the regime of magnetic fields in which the bath-induced electron spin flips play a role, for example their presence leads to the appearance of entanglement sudden death at finite time for two qubits initialized in a Bell state. For these fields the intrabath dipolar interactions and spatial inhomogeneity of hyperfine couplings are irrelevant on the time scale of coherence (and entanglement) decay, and most of the presented calculations are performed using the uniform-coupling approximation to the exact hyperfine Hamiltonian. We provide a comprehensive overview of entanglement decay in this regime, considering both free evolution of the qubits, and an echo protocol with simultaneous application of π pulses to the two spins. All the currently relevant for experiments bath states are considered: the thermal state, narrowed states (characterized by diminished uncertainty of one of the components of the Overhauser field) of two uncorrelated baths, and a correlated narrowed state with a well-defined value of the z component of the Overhauser field interdot gradient. While we mostly use concurrence to quantify the amount of entanglement in a mixed state of the two electron spins, we also show that their entanglement dynamics can be reconstructed from measurements of the currently relevant for experiments entanglement witnesses, and the fidelity of quantum teleportation performed using a partially disentangled state as a resource.

We study the electron states of a chain of non-identical, vertically stacked quantum dots. We discuss how the pseudo-band formed of the ground states confined in the quantum dots disintegrates upon increasing the inhomogeneity of the electron energies and analyze the impact of localization on the intraband absorption from the pseudo-band to extended (bulk) states. We describe also the dependence of the intraband absorption spectrum on the quantum dot size.

We present a theoretical analysis of intraband optical transitions from the intermediate pseudo-band of confined states to the conduction band in a finite, inhomogeneous stack of self-assembled semiconductor quantum dots. The chain is modeled with an effective Hamiltonian including nearest-neighbor tunnel couplings and the absorption under illumination with both coherent (laser) and thermal radiation is discussed. We show that the absorption spectrum already for a few coupled dots differs from that of a single dot and develops a structure with additional maxima at higher energies. We find out that this leads to an enhancement of the overall transition rate under solar illumination by up to several per cent which grows with the number of QDs but saturates already for a few QDs in the chain. The decisive role of the strength of inter-dot coupling for the stability of this enhancement against QD stack inhomogeneity and temperature is revealed.One of the ways to improve the efficiency of solar cells is to introduce an intermediate band in the energy spectrum of a photovoltaic structure 1,2 . In this way, electrons can be sequentially promoted from the valence band to the intermediate band and then to the conduction band by absorbing photons with energies below the band gap which are not converted into useful electrochemical energy in a standard structure. As an implementation of this concept, a stack of quantum dots (QDs) in the intrinsic region of a p-i-n junction solar cell has been proposed 3 . This idea has indeed gained some experimental support in recent years 4-12 . Quantum-dot-embedded p-i-n solar cells show higher quantum efficiency in near infrared range but their overall efficiency still is lower than the efficiency of similar devices without QDs 4-12 . On the theory side, models involving a single QD were formulated to describe the kinetics of transitions from and into the intermediate levels 13,14 . On the other hand, modeling of the electron states and optical absorption in chains and arrays of QDs has been mostly limited to infinite, periodic superlattices of identical dots [15][16][17][18][19][20] . As we have shown recently 21 , enhanced absorption can appear also in finite chains of non-identical QDs but it is suppressed if the inhomogeneity of the QD chain (leading to non-identical electron ground state energies in the individual QDs) becomes too large. Since the actual QD chains are always finite (usually built of several to a few tens of QDs)4-12 and unavoidably inhomogeneous it is of large practical importance for the optimal design of intermediate band photovoltaic devices to extend the theoretical analysis to such more realistic structures.In this paper, we study the intraband optical absorption associated with the electron transition from the states confined in a finite stack of quantum dots to the conduction band. (Fig. 1) We propose a relatively simple and computation-effective model which, however, includes all the essential features of the system, in particular the inhomogeneity of the e...