Analyses of the runs (i.e., uninterrupted sequences of identical outcomes) were made of the temporal succession of decays from three nuclear processes: (1) \ensuremath{\beta} decay of
137Cs, (2) electron capture decay of
54Mn, and (3) \ensuremath{\beta} decay of
214Bi followed by \ensuremath{\alpha} decay of
214Po. Sample half lives vary widely from about 30 yr
(137Cs) to 20 min
(214Bi). For each process, the numbers of runs of length k
(1< k< 20) were determined from three distinct types of generating procedures: (a) runs with respect to a target value, (b) runs with respect to parity, and (c) runs with respect to sequential differences. Comparison of experimental results with those predicted by the theory of runs (on the assumption that every distinguishable arrangement of a temporal sequence of disintegrations is equally likely) provides comprehensive support of the hypothesis that the spontaneous decay of a nucleus is a random occurrence unaffected by past history.
We consider collective spontaneous emission from an ensemble of N identical two-level atoms prepared by absorption of a single photon–a.k.a. single photon Dicke superradiance. We discuss dynamical properties of superradiance for small (R ≪ λ) and large (R ≫ λ) atomic cloud. Moreover, we address the effects of virtual processes on collective decay rate and Lamb shift. It turns out that virtual processes lead to relatively small yet interesting effects on the time evolution of a rapidly decaying state. However, such processes substantially modify the dynamics of trapped states by bringing in new channels of decay.
Spontaneous emission of atoms in free space is modified by the presence of other atoms in close vicinity inducing collective super- and subradiance. For two nearby atoms with a single decay channel the entangled antisymmetric superposition state of the two single excited states will not decay spontaneously. No such excited two-atom dark state exists, if the excited state has two independent optical decay channels of different frequencies or polarizations. However, we show that for an excited atomic state with N-1 independent spontaneous decay channels one can find a highly entangled N-particle dark state, which completely decouples from the vacuum radiation field. It does not decay spontaneously, nor will it absorb resonant laser light. Mathematically, we see that this state is the only such state orthogonal to the subspace spanned by the atomic ground states. Moreover, by means of generic numerical examples we demonstrate that the subradiant behavior largely survives at finite atomic distances including dipole-dipole interactions.