arXiv subjects

Igor Bragar

Publications and source records attributed to Igor Bragar.

5 recordsLinked to original sources

arXiv2023

Limitations on the maximal level of entanglement of two singlet-triplet qubits in GaAs quantum dots

We analyze in detail a procedure of entangling of two singlet-triplet ($S$-$T_{0}$) qubits operated in a regime when energy associated with the magnetic field gradient, $ΔB_{z}$, is an order of magnitude smaller than the exchange energy, $J$, between singlet and triplet states [Shulman M. et al., Science 336, 202 (2012)]. We have studied theoretically a single $S$-$T_{0}$ qubit in free induction decay and spin echo experiments. We have obtained analytical expressions for time dependence of components of its Bloch vector for quasistatical fluctuations of $ΔB_{z}$ and quasistatical or dynamical $1/f^β$-type fluctuations of $J$. We have then considered the impact of fluctuations of these parameters on the efficiency of the entangling procedure which uses an Ising-type coupling between two $S$-$T_{0}$ qubits. Particularly, we have obtained an analytical expression for evolution of two qubits affected by $1/f^β$-type fluctuations of $J$. This expression indicates the maximal level of entanglement that can be generated by performing the entangling procedure. Our results deliver also an evidence that in the above-mentioned experiment, the $S$-$T_{0}$ qubits were affected by uncorrelated $1/f^β$ charge noises.

cond-mat.mes-hall↗

arXiv2021

Retardation of entanglement decay of two spin qubits by quantum measurements

We study a system of two electron spins each interacting with its small nuclear spin environment (NSE), which is a prototype system of two electron spin quantum dot (QD) qubits. We propose a way to counteract the decay of entanglement in two-electron spin subsystem (TESSS) by performing some manipulations on TESSS (the subsystem to which experimentalists have an access), e.g. repeatable quantum projective measurements of TESSS. Unlike in the quantum Zeno effect, the goal of the proposed manipulations is not to freeze TESSS in its initial state and to preclude any time evolution of the state by infinitely frequent quantum measurements. Instead of that, performing a few cycles of free evolution of the system for some time $τ$ followed by a quantum measurement of TESSS with subsequent postselection of TESSS state (the same as the initial one) produces quantum correlations in NSEs and also restores the quantum correlations in TESSS. By numerical calculation of the system evolution (the full density matrix $\hat ρ(t)$), we show that, in contrast to the fast decay of TESSS entanglement on timescale of the order of $T_2^*$, application of the proposed manipulation sequence gradually builds up coherences in the entire system and the rest decay of quantum correlations of TESSS may be significantly slowed down for specific cycle durations $τ$ and numbers of performed cycles.

cond-mat.mes-hall↗

arXiv2015

Dynamics of entanglement of two electron spins interacting with nuclear spin baths in quantum dots

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.

cond-mat.mes-hall↗

arXiv2012

Intraband absorption in finite, inhomogeneous quantum dot stacks for intermediate band solar cells: limitations and optimization

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.

cond-mat.mes-hall↗

arXiv2011

Intermediate band formation and intraband absorption for electrons in an inhomogeneous chain of quantum dots

We study the electron states of a chain of non-identical, vertically stacked quantum dots (QDs). We discuss how the pseudo-band formed of the ground states confined in the QDs 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 QD size.

cond-mat.mes-hall↗