In an interesting article [Phys. Rev. A 81, 052341 (2010)], Das and Agarwal have discussed the preservation of bipartite entanglement in three-level atoms employing the coherences induced by spontaneous emission. The authors considered various initially entangled qubits prepared from two
V-type three-level atoms and showed that more than
50 of the initial (bipartite) entanglement can be preserved in steady state due to vacuum-induced coherence. In this Comment, we point out that their analytical formulas for the entanglement measure contain errors affecting all the numerical results of that article. We substantiate our claim by giving correct analytical results for the time evolution of the two-atom system.
In contrast to the general argument that the spontaneous decay is intrinsically incoherent in nature and harmful to entanglement, here, we show that nearly perfect entanglement between two bright pump fields can be realized via spontaneous decay-induced destructive quantum interference in a closed double \ensuremath{\Lambda}-type four-level system with the energy separation of the excited doublet comparable to their decay rates. The high degree of bipartite entanglement results from the cancellation of spontaneous emission and subsequent elimination of the associated noise due to the destructive quantum interference between the two spontaneous emission pathways from the upper doublet to each of the two lower levels when the two strong pump fields are tuned to the particular frequencies where quantum interference takes place. This scheme is particularly suitable for the generation of entanglement between two bright light fields, and may find potential applications in realistic quantum information processing.