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arXiv:1106.2667 · Physical Review A

Comment on “Effect of entanglement on the decay dynamics of a pair of H(2 p ) atoms due to spontaneous emission”

Pedro Sancho, Luis Plaja

Published Jun 13, 2011• 8 citations
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Abstract

T. Tanabe et al. [Phys. Rev. A 82, 040101(R) (2010)] have experimentally demonstrated that the emission properties of unstable atoms in entangled and product states are different. The authors define an apparent decay time as a fitting parameter which falls below the lifetime of the single atom for entangled pairs. We argue that their results about coincidence time spectra are correct, but those concerning lifetimes cannot be considered conclusive because they assume the emission of photons by the two atoms to be independent processes, a doubtful hypothesis for entangled states. We suggest an improved evaluation of the lifetimes based on a rigorous approach, which demands some modifications of the experimental procedure.

DOI
10.1103/physreva.83.066101

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Effect of entanglement on the decay dynamics of a pair of H( 2p ) atoms due to spontaneous emission

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We have measured the coincidence time spectra of two Lyman-$\ensuremath{\alpha}$ photons emitted by a pair of H($2p$) atoms in the photodissociation of ${\text{H}}_{2}$ at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 and 0.02 Pa. The decay time constant at 0.02 Pa is approximately half the lifetime of a single H($2p$) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H($2p$) atom. It turns out that the decay faster than the lifetime of a single H($2p$) atom originates from the entanglement in the pair of H($2p$) atoms. We have demonstrated an effect of entanglement on atomic decay.

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Reply to “Comment on ‘Effect of entanglement on the decay dynamics of a pair of H( 2p ) atoms due to spontaneous emission’ ”

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In this Reply to the preceding Comment by Sancho and Plaja, we point out the difference between the coincidence time spectrum of two Lyman-$\ensuremath{\alpha}$ photons measured by us and their theoretical one.

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Physical Review A•Mar 15, 2018•4 citations
T. Tanabe et al. [Phys. Rev. A 82, 040101(R) (2010)] have experimentally demonstrated that the emission properties of unstable atoms in entangled and product states are different. The authors define an apparent decay time as a fitting parameter which falls below the lifetime of the single atom for entangled pairs. We argue that their results about coincidence time spectra are correct, but those concerning lifetimes cannot be considered conclusive because they assume the emission of photons by the two atoms to be independent processes, a doubtful hypothesis for entangled states. We suggest an improved evaluation of the lifetimes based on a rigorous approach, which demands some modifications of the experimental procedure.
We have measured the coincidence time spectra of two Lyman-$\ensuremath{\alpha}$ photons emitted by a pair of H(2p2p2p) atoms in the photodissociation of H2{\text{H}}_{2}H2​ at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 and 0.02 Pa. The decay time constant at 0.02 Pa is approximately half the lifetime of a single H(2p2p2p) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H(2p2p2p) atom. It turns out that the decay faster than the lifetime of a single H(2p2p2p) atom originates from the entanglement in the pair of H(2p2p2p) atoms. We have demonstrated an effect of entanglement on atomic decay.
We analyse the influence of entanglement on the emission properties of atoms. To this end, first, we propose a scheme for the preparation of a pair of entangled Helium atoms, one in the ortho and the other in the para spin configuration. We discuss a realistic scenario for this process, based in the double ionization of He by intense laser fields. These states are used to analyse disentanglement and the role of entanglement in the spontaneous emission from the pair. In particular, we show that the decaying rate of an entangled atom is different from that in a product state, modifying the temporal emission distribution and lifetime of the atoms.
We have measured the coincidence time spectra of two Lyman-α photons emitted by a pair of H(2p) atoms in the photodissociation of H 2 at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 Pa and 0.02 Pa, from which the angular distributions of two Lyman-α photons have been obtained. The experimental angular distributions indicate the generation of the entangled pair of H(2p) atoms as predicted by the theory of our group (Miyagi et al 2007 J. Phys. B 40 617) and the role of a new kind of reaction, i.e. the reaction of the entangled pair of H(2p) atoms with an H 2 molecule that efficiently changes the entanglement. It has turned out that more entangled pairs of H(2p) atoms survive at 0.02 Pa than at 0.40 Pa. The decay time constant in the coincidence time spectrum at 0.02 Pa is approximately half the lifetime of a single H(2p) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H(2p) atom. It follows that the decay faster than the lifetime of a single H(2p) atom originates from the entanglement in the pair of H(2p) atoms.
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\mathsf{V}V-type three-level atoms and showed that more than 5050%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.