RESUMO
Two-photon resonant excitation of the biexciton-exciton cascade in a quantum dot generates highly polarization-entangled photon pairs in a near-deterministic way. However, the ultimate level of achievable entanglement is still debated. Here, we observe the impact of the laser-induced ac-Stark effect on the quantum dot emission spectra and on entanglement. For increasing pulse-duration-to-lifetime ratios and pump powers, decreasing values of concurrence are recorded. Nonetheless, additional contributions are still required to fully account for the observed below-unity concurrence.
RESUMO
Entangled photon pairs are key to many novel applications in quantum technologies. Semiconductor quantum dots can be used as sources of on-demand, highly entangled photons. The fidelity to a fixed maximally entangled state is limited by the excitonic fine-structure splitting. This work demonstrates that, even if this splitting is absent, the degree of entanglement cannot reach unity when the excitation pulse in a two-photon resonance scheme has a finite duration. The degradation of the entanglement has its origin in a dynamically induced splitting of the exciton states caused by the laser pulse itself. Hence, in the setting explored here, the excitation process limits the achievable concurrence for entangled photons generated in an optically excited four-level quantum emitter.
RESUMO
Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of nonclassical light were used for seminal demonstration of entanglement swapping, but applications in quantum technologies demand push-button operation requiring single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent prerequisites on the efficiency and purity of the generation of entangled states. Here we show a proof-of-concept demonstration of all-photonic entanglement swapping with pairs of polarization-entangled photons generated on demand by a GaAs quantum dot without spectral and temporal filtering. Moreover, we develop a theoretical model that quantitatively reproduces the experimental data and provides insights on the critical figures of merit for the performance of the swapping operation. Our theoretical analysis also indicates how to improve state-of-the-art entangled-photon sources to meet the requirements needed for implementation of quantum dots in long-distance quantum communication protocols.
RESUMO
Self-assembly is an attractive phenomenon that, with proper handling, can enable the production of sophisticated hybrid nanostructures with sub-nm-scale precision. The importance of this phenomenon is particularly notable in the fabrication of metal-organic nanomaterials as promising substances for spintronic devices. The exploitation of self-assembly in nanofabrication requires a comprehension of atomic processes creating hybrid nanostructures. Here, we focus on the self-assembly processes in the vapour-deposited Au x C60 mixture films, revealing the exciting quantum plasmon effects. Through a systematic characterization of the Au x C60 films carried out using structure-sensitive techniques, we have established correlations between the film nanostructure and the Au concentration, x. The analysis of these correlations designates the Au intercalation into the C60 lattice and the Au clustering as the basic processes of the nanostructure self-assembly in the mixture films, the efficiency of which strongly depends on x. The evaluation of this dependence for the Au x C60 composite nanostructures formed in a certain composition interval allows us to control the size of the Au clusters and the intercluster spacing by adjusting the Au concentration only. This study represents the self-assembled Au x C60 mixtures as quantum materials with electronic functions tuneable by the Au concentration in the depositing mixture.