arXiv subjects

Karol Kawa

Publications and source records attributed to Karol Kawa.

7 recordsLinked to original sources

arXiv2026

A semiconductor photon-pair source based on a polariton cascade

We present a theoretical study of a semiconductor photon-pair source based on a radiative cascade enabled by a permanent exciton dipole. The source consists of a GaAs quantum dot placed between a metal nanoparticle and a mirror. The dot exciton and the localized electromagnetic mode mix to form upper and lower polaritons. Without a permanent dipole, a symmetry separating states with even and odd excitation numbers forbids photon emission between the polaritons. Separation of the mean electron and hole positions gives the exciton a permanent dipole and breaks this symmetry. The upper polariton can then emit an idler photon as it decays to the lower polariton. The lower polariton can emit a signal photon as the system returns to its ground state. Using an effective model, we calculate the probability that a prepared upper polariton emits both photons through the metal antenna. We also analyse fluctuations in the emitted photon counts and correlations between the two emission channels. The calculation establishes an operating principle for assumed emitter properties and optical loss rates. Whether the device can be built and its photon correlations measured remains open.

cond-mat.mes-hall↗

arXiv2026

Triplet-assisted leakage during singlet-triplet qubit readout with a quantum point contact

Quantum point contact readout theory for singlet-triplet qubits in a lateral double quantum dot is extended by including tunneling of triplet configurations into a higher-energy level of the neighboring dot. This additional channel creates energetically allowed leakage pathways that modify the branch-dependent charge and current-noise signatures, even when the Pauli blockade remains effective within the ground-state manifold. The model contains two single-particle levels in each dot. The resulting singlet and triplet block structure is derived together with a Lindblad master equation. Quantum-jump simulations are then used to resolve the dynamics of individual readout events. A complementary Liouvillian steady-state analysis identifies the regime in which tunneling to the excited level qualitatively changes the readout signatures, with the crossover determined by the level spacing.

cond-mat.mes-hall↗

arXiv2024

Exciton Diffusion in a Quantum Dot Ensemble

In this theoretical study, we explore Förster resonant energy transfer of a single exciton within a two-dimensional array of self-assembled quantum dots arranged randomly on a circular mesa. Employing the stochastic simulation method, we solve the equation of motion for the density matrix, considering a specified decay rate. Our analysis quantifies diffusion through the mean-square displacement from the initially excited quantum dot, revealing distinct temporal stages: ballistic, normal diffusion, and saturation. Furthermore, we observe power-law localization of the exciton. Complementing our numerical investigations, we develop approximate analytical expressions that closely align with the numerical findings.

cond-mat.mes-hall↗

arXiv2022

Coherence limitations in the optical control of the singlet-triplet qubit in a quantum dot molecule

We analyze the optically driven dynamics of a qubit implemented on a singlet-triplet subspace of two-electron states in a self-assembled quantum dot molecule. We study two possible control schemes based on the coupling to an excited (four-particle) state either by two spectrally separated laser pulses or by a single spectrally broad pulse. We quantitatively characterize the imperfections of the qubit operation resulting from non-adiabatic evolution and from limited spectral selectivity in a real system, as compared to the ideal adiabatic Raman transfer of occupation in the $Λ$-system. Next, we study the effects of decoherence induced by the coupling to the phonons of the surrounding crystal lattice and by radiative recombination. As a result, we are able to identify the optimization trade-offs between different sources of errors and indicate the most favorable conditions for quantum control of the singlet-triplet qubit in the two optical control schemes.

cond-mat.mes-hall↗

arXiv2021

Spread of Correlations in Strongly Disordered Lattice Systems with Long-Range Coupling

We investigate the spread of correlations carried by an excitation in a 1-dimensional lattice system with high on-site energy disorder and long-range couplings with a power-law dependence on the distance ($\propto r^{-μ}$). The increase in correlation between the initially quenched node and a given node exhibits three phases: quadratic in time, linear in time, and saturation. No further evolution is observed in the long time regime. We find an approximate solution of the model valid in the limit of strong disorder and reproduce the results of numerical simulations with analytical formulas. We also find the time needed to reach a given correlation value as a measure of the propagation speed. Because of the triple phase evolution of the correlation function the propagation changes its time dependence. In the particular case of $μ=1$, the propagation starts as a ballistic motion, then, at a certain crossover time, turns into standard diffusion.

cond-mat.dis-nn↗

arXiv2020

Diffusion of excitation and power-law localization in long-range-coupled strongly disordered systems

We investigate diffusion of excitation in one- and two-dimensional lattices with random on-site energies and deterministic long-range couplings (hopping) inversely proportional to the distance. Three regimes of diffusion are observed in strongly disordered systems: ballistic motion at short time, standard diffusion for intermediate times, and a stationary phase (saturation) at long times. We propose an analytical solution valid in the strong-coupling regime which explains the observed dynamics and relates the ballistic velocity, diffusion coefficient, and asymptotic diffusion range to the system size and disorder strength via simple formulas. We show also that in the long-time asymptotic limit of diffusion from a single site the occupations form a heavy-tailed power-law distribution.

cond-mat.dis-nn↗

arXiv2019

Spin-orbit-induced hole spin relaxation in a quantum dot molecule: the effect of $s$-$p$ coupling

We study the effect of the coupling between the hole $s$ shell of one quantum dot and the $p$ shell in the other dot forming a quantum dot molecule on the spin relaxation between the sublevels of the hole $s$ state. Using an effective model that captures the spin-orbit effects in the $p$ shell irrespective of their origin, we show that the strong spin mixing in the $p$ shell can be transferred to the $s$ shell of the other dot, leading to enhanced spin relaxation in a certain energy range around the $s$-$p$ resonance if the dots are misaligned and the magnetic field is tilted from the sample plane.

cond-mat.mes-hall↗