Disentanglement in bipartite continuous-variable systems

Abstract

Entanglement in bipartite continuous-variable systems is investigated in the presence of partial losses such as those introduced by a realistic quantum communication channel, e. g., by propagation in an optical fiber. We find that entanglement can vanish completely for partial losses, in a situation reminiscent of so-called entanglement sudden death. Even states with extreme squeezing may become separable after propagation in lossy channels. Having in mind the potential applications of such entangled light beams to optical communications, we investigate the conditions under which entanglement can survive for all partial losses. Different loss scenarios are examined, and we derive criteria to test the robustness of entangled states. These criteria are necessary and sufficient for Gaussian states. Our study provides a framework to investigate the robustness of continuous-variable entanglement in more complex multipartite systems

Entangled-coherent-state quantum key distribution with entanglement witnessing

“…This function W is similar in form to an entanglement witness W s introduced in [17] and studied in detail in [18], but due to the normalization differences mentioned in the introduction, it is not the same function and so here is not a true entanglement witness. W and W s are in fact related by a rescaling of the quadratures, but for the states considered in this paper W s vanishes identically.…”

“…It is because the witness S itself involves third-order correlation functions, which may be inconvenient to implement experimentally, that we also consider the second witness function W. W is related by rescaling of the quadratures to a well-known entanglement witness W s [17,18], but is not in the strict sense a true entanglement witness in the current context. Despite this, it gives eavesdropper-detection results that match well with those of S, and has the additional advantage that it is built from the covariance matrix of the system, which is easily accessible experimentally.…”

Robustez do emaranhamento em variáveis contínuas e fotodetecção de feixes intensos no domínio espectral

“…As três primeiras seções contém resultados já conhecidos na literatura e são revisados aqui, pois são parte importante da base teórica utilizada no nosso trabalho. Na quarta seção, discuto resultados originais sobre a investigação da dinâmica do emaranhamento frente a perdas por transmissão que estão publicados em [27,96] …”

“…Claramente, W REé modificada por uma transformação de squeezing local S local = S(r 1 ) ⊕ S(r 2 ), com S(r) = diag(e r , e −r ). De fato, squeezings locais podem transformar qualquer estado frágil em robusto [96].…”

Emaranhamento multicor para redes de informação quântica

“…Este estudo permite delimitar fronteiras entre estados robustos, frágeis e separáveis produzidos pelo OPO que podem ser cruzadas através de parâmetros físicos controláveis, tais como temperatura do cristal e potência de bombeio. Outro estudante do grupo, Felippe A. S. Barbosa, esteve diretamente ligado a esse trabalho e mais detalhes sobre a classificação desses estados poderão ser encontrados nas referências [Barbosa 2010a, Barbosa 2010b], e em sua tese de doutorado.…”